path: root/man5/proc.5

.\" Copyright (C) 1994, 1995 by Daniel Quinlan (quinlan@yggdrasil.com)
.\" and Copyright (C) 2002-2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\" with networking additions from Alan Cox (A.Cox@swansea.ac.uk)
.\" and scsi additions from Michael Neuffer (neuffer@mail.uni-mainz.de)
.\" and sysctl additions from Andries Brouwer (aeb@cwi.nl)
.\" and System V IPC (as well as various other) additions from
.\" Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\"
.\" Modified 1995-05-17 by faith@cs.unc.edu
.\" Minor changes by aeb and Marty Leisner (leisner@sdsp.mc.xerox.com).
.\" Modified 1996-04-13, 1996-07-22 by aeb@cwi.nl
.\" Modified 2001-12-16 by rwhron@earthlink.net
.\" Modified 2002-07-13 by jbelton@shaw.ca
.\" Modified 2002-07-22, 2003-05-27, 2004-04-06, 2004-05-25
.\"    by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2004-11-17, mtk -- updated notes on /proc/loadavg
.\" 2004-12-01, mtk, rtsig-max and rtsig-nr went away in 2.6.8
.\" 2004-12-14, mtk, updated 'statm', and fixed error in order of list
.\" 2005-05-12, mtk, updated 'stat'
.\" 2005-07-13, mtk, added /proc/sys/fs/mqueue/*
.\" 2005-09-16, mtk, Added /proc/sys/fs/suid_dumpable
.\" 2005-09-19, mtk, added /proc/zoneinfo
.\" 2005-03-01, mtk, moved /proc/sys/fs/mqueue/* material to mq_overview.7.
.\" 2008-06-05, mtk, Added /proc/[pid]/oom_score, /proc/[pid]/oom_adj,
.\"     /proc/[pid]/limits, /proc/[pid]/mountinfo, /proc/[pid]/mountstats,
.\"     and /proc/[pid]/fdinfo/*.
.\" 2008-06-19, mtk, Documented /proc/[pid]/status.
.\" 2008-07-15, mtk, added /proc/config.gz
.\"
.\" FIXME 2.6.13 seems to have /proc/vmcore implemented
.\" in the source code, but there is no option available under
.\" 'make xconfig'; eventually this should be fixed, and then info
.\" from the patch-2.6.13 and change log could be used to write an
.\" entry in this man page.
.\" Needs CONFIG_VMCORE
.\"
.\" FIXME cross check against Documentation/filesystems/proc.txt
.\" to see what information could be imported from that file
.\" into this file.
.\"
.TH PROC 5 2013-09-04 "Linux" "Linux Programmer's Manual"
.SH NAME
proc \- process information pseudo-filesystem
.SH DESCRIPTION
The
.I proc
filesystem is a pseudo-filesystem which provides an interface to
kernel data structures.
It is commonly mounted at
.IR /proc .
Most of it is read-only, but some files allow kernel variables to be
changed.
.LP
The following list describes many of the files and directories under the
.I /proc
hierarchy.
.PD 1
.TP
.I /proc/[pid]
There is a numerical subdirectory for each running process; the
subdirectory is named by the process ID.
Each such subdirectory contains the following
pseudo-files and directories.
.\" FIXME Describe /proc/[pid]/attr and
.\"       /proc/[pid]/task/[tid]/attr
.\"	  This is a directory
.\"       Added in ???
.\"	  CONFIG_SECURITY
.\"
.TP
.IR /proc/[pid]/auxv " (since 2.6.0-test7)"
This contains the contents of the ELF interpreter information passed
to the process at exec time.
The format is one \fIunsigned long\fP ID
plus one \fIunsigned long\fP value for each entry.
The last entry contains two zeros.
.TP
.IR /proc/[pid]/cgroup " (since Linux 2.6.24)"
.\" 	  Info in Documentation/cgroups/cgroups.txt
This file describes control groups to which the process/task belongs.
For each cgroup hierarchy there is one entry containing
colon-separated fields of the form:
.nf
.ft CW

    5:cpuacct,cpu,cpuset:/daemons
.ft
.fi
.IP
The colon-separated fields are, from left to right:
.RS 11
.IP 1. 3
hierarchy ID number
.IP 2.
set of subsystems bound to the hierarchy
.IP 3.
control group in the hierarchy to which the process belongs
.RE
.IP
This file is present only if the
.B CONFIG_CGROUPS
kernel configuration option is enabled.
.\"
.\" FIXME Describe /proc/[pid]/clear_refs
.\"       Added in 2.6.22
.\"       "Clears page referenced bits shown in smaps output"
.\"       write-only
.\"       CONFIG_PROC_PAGE_MONITOR
.TP
.I /proc/[pid]/cmdline
This holds the complete command line for the process,
unless the process is a zombie.
.\" In 2.3.26, this also used to be true if the process was swapped out.
In the latter case, there is nothing in this file:
that is, a read on this file will return 0 characters.
The command-line arguments appear in this file as a set of
strings separated by null bytes (\(aq\\0\(aq),
with a further null byte after the last string.
.TP
.IR /proc/[pid]/coredump_filter " (since kernel 2.6.23)"
See
.BR core (5).
.TP
.IR /proc/[pid]/cpuset " (since kernel 2.6.12)"
.\" and/proc/[pid]/task/[tid]/cpuset
See
.BR cpuset (7).
.TP
.I /proc/[pid]/cwd
This is a symbolic link to the current working directory of the process.
To find out the current working directory of process 20,
for instance, you can do this:

.in +4n
.nf
.RB "$" " cd /proc/20/cwd; /bin/pwd"
.fi
.in

Note that the
.I pwd
command is often a shell built-in, and might
not work properly.
In
.BR bash (1),
you may use
.IR "pwd\ \-P" .

.\" The following was still true as at kernel 2.6.13
In a multithreaded process, the contents of this symbolic link
are not available if the main thread has already terminated
(typically by calling
.BR pthread_exit (3)).
.TP
.I /proc/[pid]/environ
This file contains the environment for the process.
The entries are separated by null bytes (\(aq\\0\(aq),
and there may be a null byte at the end.
Thus, to print out the environment of process 1, you would do:
.in +4n
.nf

.ft CW
.RB "$" " strings /proc/1/environ"
.fi
.ft P
.in
.TP
.I /proc/[pid]/exe
Under Linux 2.2 and later, this file is a symbolic link
containing the actual pathname of the executed command.
This symbolic link can be dereferenced normally; attempting to open
it will open the executable.
You can even type
.I /proc/[pid]/exe
to run another copy of the same executable as is being run by
process [pid].
.\" The following was still true as at kernel 2.6.13
In a multithreaded process, the contents of this symbolic link
are not available if the main thread has already terminated
(typically by calling
.BR pthread_exit (3)).

Under Linux 2.0 and earlier
.I /proc/[pid]/exe
is a pointer to the binary which was executed,
and appears as a symbolic link.
A
.BR readlink (2)
call on this file under Linux 2.0 returns a string in the format:

    [device]:inode

For example, [0301]:1502 would be inode 1502 on device major 03 (IDE,
MFM, etc. drives) minor 01 (first partition on the first drive).

.BR find (1)
with the
.I \-inum
option can be used to locate the file.
.TP
.I /proc/[pid]/fd/
This is a subdirectory containing one entry for each file which the
process has open, named by its file descriptor, and which is a
symbolic link to the actual file.
Thus, 0 is standard input, 1 standard output, 2 standard error, etc.

For file descriptors for pipes and sockets,
the entries will be symbolic links whose content is the
file type with the inode.
A
.BR readlink (2)
call on this file returns a string in the format:

    type:[inode]

For example,
.I socket:[2248868]
will be a socket and its inode is 2248868.
For sockets, that inode can be used to find more information
in one of the files under
.IR /proc/net/ .

For file descriptors that have no corresponding inode
(e.g., file descriptors produced by
.BR epoll_create (2),
.BR eventfd (2),
.BR inotify_init (2),
.BR signalfd (2),
and
.BR timerfd (2)),
the entry will be a symbolic link with contents of the form

    anon_inode:<file-type>

In some cases, the
.I file-type
is surrounded by square brackets.

For example, an epoll file descriptor will have a symbolic link
whose content is the string
.IR "anon_inode:[eventpoll]" .

.\"The following was still true as at kernel 2.6.13
In a multithreaded process, the contents of this directory
are not available if the main thread has already terminated
(typically by calling
.BR pthread_exit (3)).

Programs that will take a filename as a command-line argument,
but will not take input from standard input if no argument is supplied,
or that write to a file named as a command-line argument,
but will not send their output to standard output
if no argument is supplied, can nevertheless be made to use
standard input or standard out using
.IR /proc/[pid]/fd .
For example, assuming that
.I \-i
is the flag designating an input file and
.I \-o
is the flag designating an output file:
.in +4n
.nf

.RB "$" " foobar \-i /proc/self/fd/0 \-o /proc/self/fd/1 ..."
.fi
.in

and you have a working filter.
.\" The following is not true in my tests (MTK):
.\" Note that this will not work for
.\" programs that seek on their files, as the files in the fd directory
.\" are not seekable.

.I /proc/self/fd/N
is approximately the same as
.I /dev/fd/N
in some UNIX and UNIX-like systems.
Most Linux MAKEDEV scripts symbolically link
.I /dev/fd
to
.IR /proc/self/fd ,
in fact.

Most systems provide symbolic links
.IR /dev/stdin ,
.IR /dev/stdout ,
and
.IR /dev/stderr ,
which respectively link to the files
.IR 0 ,
.IR 1 ,
and
.IR 2
in
.IR /proc/self/fd .
Thus the example command above could be written as:
.in +4n
.nf

.RB "$" " foobar \-i /dev/stdin \-o /dev/stdout ..."
.fi
.in
.\" FIXME Describe /proc/[pid]/loginuid
.\"       Added in 2.6.11; updating requires CAP_AUDIT_CONTROL
.\"       CONFIG_AUDITSYSCALL
.TP
.IR /proc/[pid]/fdinfo/ " (since kernel 2.6.22)"
This is a subdirectory containing one entry for each file which the
process has open, named by its file descriptor.
The contents of each file can be read to obtain information
about the corresponding file descriptor, for example:
.in +4n
.nf

.RB "$" " cat /proc/12015/fdinfo/4"
pos:    1000
flags:  01002002
.fi
.in

The
.I pos
field is a decimal number showing the current file offset.
The
.I flags
field is an octal number that displays the
file access mode and file status flags (see
.BR open (2)).

The files in this directory are readable only by the owner of the process.
.TP
.IR /proc/[pid]/io " (since kernel 2.6.20)"
.\" commit 7c3ab7381e79dfc7db14a67c6f4f3285664e1ec2
This file contains I/O statistics for the process, for example:
.in +4n
.nf

.RB "#" " cat /proc/3828/io"
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0
.fi
.in

The fields are as follows:
.RS
.TP
.IR rchar ": characters read"
The number of bytes which this task has caused to be read from storage.
This is simply the sum of bytes which this process passed to
.BR read (2)
and similar system calls.
It includes things such as terminal I/O and 
is unaffected by whether or not actual
physical disk I/O was required (the read might have been satisfied from
pagecache).
.TP
.IR wchar ": characters written"
The number of bytes which this task has caused, or shall cause to be written
to disk.
Similar caveats apply here as with
.IR rchar .
.TP
.IR syscr ": read syscalls"
Attempt to count the number of read I/O operations\(emthat is,
system calls such as
.BR read (2)
and
.BR pread (2).
.TP
.IR syscw ": write syscalls"
Attempt to count the number of write I/O operations\(emthat is,
system calls such as
.BR write (2)
and
.BR pwrite (2).
.TP
.IR read_bytes ": bytes read"
Attempt to count the number of bytes which this process really did cause to
be fetched from the storage layer.
This is accurate for block-backed filesystems.
.TP
.IR write_bytes ": bytes written"
Attempt to count the number of bytes which this process caused to be sent to
the storage layer.
.TP
.IR cancelled_write_bytes :
The big inaccuracy here is truncate.
If a process writes 1MB to a file and then deletes the file,
it will in fact perform no writeout.
But it will have been accounted as having caused 1MB of write.
In other words: this field represents the number of bytes which this process
caused to not happen, by truncating pagecache.
A task can cause "negative" I/O too.
If this task truncates some dirty pagecache,
some I/O which another task has been accounted for
(in its
.IR write_bytes )
will not be happening.
.RE
.IP
.IR Note :
In the current implementation, things are a bit racy on 32-bit systems:
if process A reads process B's
.I /proc/[pid]/io
while process B is updating one of these 64-bit counters,
process A could see an intermediate result.
.RE
.TP
.IR /proc/[pid]/limits " (since kernel 2.6.24)"
This file displays the soft limit, hard limit, and units of measurement
for each of the process's resource limits (see
.BR getrlimit (2)).
Up to and including Linux 2.6.35,
this file is protected to allow reading only by the real UID of the process.
Since Linux 2.6.36,
.\" commit 3036e7b490bf7878c6dae952eec5fb87b1106589
this file is readable by all users on the system.
.TP
.IR /proc/[pid]/map_files/ " (since kernel 3.3)
.\" commit 640708a2cff7f81e246243b0073c66e6ece7e53e
This subdirectory contains entries corresponding to memory-mapped
files (see
.BR mmap (2)).
Entries are named by memory region start and end
address pair (expressed as hexadecimal numbers),
and are symbolic links to the mapped files themselves.
Here is an example, with the output wrapped and reformatted to fit on an 80-column display:
.in +4n
.nf

.RB "$" " ls -l /proc/self/map_files/"
lr\-\-\-\-\-\-\-\-. 1 root root 64 Apr 16 21:31
            3252e00000\-3252e20000 \-> /usr/lib64/ld-2.15.so
\&...
.fi
.in

Although these entries are present for memory regions that were
mapped with  the
.BR MAP_FILE
flag, the way anonymous shared memory (regions created with the
.B MAP_ANON | MAP_SHARED
flags)
is implemented in Linux
means that such regions also appear on this directory.
Here is an example where the target file is the deleted
.I /dev/zero
one:
.in +4n
.nf

.RB
lrw\-\-\-\-\-\-\-. 1 root root 64 Apr 16 21:33
            7fc075d2f000\-7fc075e6f000 \-> /dev/zero (deleted)
.fi
.in

This directory appears only if the
.B CONFIG_CHECKPOINT_RESTORE
kernel configuration option is enabled.
.TP
.I /proc/[pid]/maps
A file containing the currently mapped memory regions and their access
permissions.
See
.BR mmap (2)
for some further information about memory mappings.

The format of the file is:

.in -7n
.nf
.ft CW
.ft
.I "address           perms offset  dev   inode       pathname"
00400000-00452000 r-xp 00000000 08:02 173521      /usr/bin/dbus-daemon
00651000-00652000 r--p 00051000 08:02 173521      /usr/bin/dbus-daemon
00652000-00655000 rw-p 00052000 08:02 173521      /usr/bin/dbus-daemon
00e03000-00e24000 rw-p 00000000 00:00 0           [heap]
00e24000-011f7000 rw-p 00000000 00:00 0           [heap]
\&...
35b1800000-35b1820000 r-xp 00000000 08:02 135522  /usr/lib64/ld-2.15.so
35b1a1f000-35b1a20000 r--p 0001f000 08:02 135522  /usr/lib64/ld-2.15.so
35b1a20000-35b1a21000 rw-p 00020000 08:02 135522  /usr/lib64/ld-2.15.so
35b1a21000-35b1a22000 rw-p 00000000 00:00 0
35b1c00000-35b1dac000 r-xp 00000000 08:02 135870  /usr/lib64/libc-2.15.so
35b1dac000-35b1fac000 ---p 001ac000 08:02 135870  /usr/lib64/libc-2.15.so
35b1fac000-35b1fb0000 r--p 001ac000 08:02 135870  /usr/lib64/libc-2.15.so
35b1fb0000-35b1fb2000 rw-p 001b0000 08:02 135870  /usr/lib64/libc-2.15.so
\&...
f2c6ff8c000-7f2c7078c000 rw-p 00000000 00:00 0    [stack:986]
\&...
7fffb2c0d000-7fffb2c2e000 rw-p 00000000 00:00 0   [stack]
7fffb2d48000-7fffb2d49000 r-xp 00000000 00:00 0   [vdso]
.fi
.in

The
.I address
field is the address space in the process that the mapping occupies.
The
.I perms
field is a set of permissions:

.nf
.in +5
r = read
w = write
x = execute
s = shared
p = private (copy on write)
.fi
.in

The
.I offset
field is the offset into the file/whatever;
.I dev
is the device
(major:minor);
.I inode
is the inode on that device.
0 indicates that no inode is associated with the memory region,
as would be the case with BSS (uninitialized data).

The
.I pathname
field will usually be the file that is backing the mapping.
For ELF files,
you can easily coordinate with the
.I offset
field by looking at the
Offset field in the ELF program headers
.RI ( "readelf\ \-l" ).

There are additional helpful pseudo-paths:
.RS 12
.TP
.IR [stack]
The initial process's (also known as the main thread's) stack.
.TP
.IR [stack:<tid>] " (since Linux 3.4)"
.\" commit b76437579d1344b612cf1851ae610c636cec7db0
A thread's stack (where the
.IR <tid>
is a thread ID).
It corresponds to the
.IR /proc/[pid]/task/[tid]/
path.
.TP
.IR [vdso]
The virtual dynamically linked shared object.
.TP
.IR [heap]
The process's heap.
.in
.fi
.RE
.IP
If the
.I pathname
field is blank,
this is an anonymous mapping as obtained via the
.BR mmap (2)
function.
There is no easy way to coordinate this back to a process's source,
short of running it through
.BR gdb (1),
.BR strace (1),
or similar.

Under Linux 2.0 there is no field giving pathname.
.TP
.I /proc/[pid]/mem
This file can be used to access the pages of a process's memory through
.BR open (2),
.BR read (2),
and
.BR lseek (2).
.TP
.IR /proc/[pid]/mountinfo " (since Linux 2.6.26)"
.\" This info adapted from Documentation/filesystems/proc.txt
This file contains information about mount points.
It contains lines of the form:
.nf
.ft CW

36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
(1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)
.ft
.fi
.IP
The numbers in parentheses are labels for the descriptions below:
.RS 7
.TP 5
(1)
mount ID: unique identifier of the mount (may be reused after
.BR umount (2)).
.TP
(2)
parent ID: ID of parent mount (or of self for the top of the mount tree).
.TP
(3)
major:minor: value of
.I st_dev
for files on filesystem (see
.BR stat (2)).
.TP
(4)
root: root of the mount within the filesystem.
.TP
(5)
mount point: mount point relative to the process's root.
.TP
(6)
mount options: per-mount options.
.TP
(7)
optional fields: zero or more fields of the form "tag[:value]".
.TP
(8)
separator: marks the end of the optional fields.
.TP
(9)
filesystem type: name of filesystem in the form "type[.subtype]".
.TP
(10)
mount source: filesystem-specific information or "none".
.TP
(11)
super options: per-super block options.
.RE
.IP
Parsers should ignore all unrecognized optional fields.
Currently the possible optional fields are:
.RS 12
.TP 18
shared:X
mount is shared in peer group X
.TP
master:X
mount is slave to peer group X
.TP
propagate_from:X
mount is slave and receives propagation from peer group X (*)
.TP
unbindable
mount is unbindable
.RE
.IP
(*) X is the closest dominant peer group under the process's root.
If X is the immediate master of the mount,
or if there is no dominant peer group under the same root,
then only the "master:X" field is present
and not the "propagate_from:X" field.

For more information on mount propagation see:
.I Documentation/filesystems/sharedsubtree.txt
in the Linux kernel source tree.
.TP
.IR /proc/[pid]/mounts " (since Linux 2.4.19)"
This is a list of all the filesystems currently mounted in the
process's mount namespace.
The format of this file is documented in
.BR fstab (5).
Since kernel version 2.6.15, this file is pollable:
after opening the file for reading, a change in this file
(i.e., a filesystem mount or unmount) causes
.BR select (2)
to mark the file descriptor as readable, and
.BR poll (2)
and
.BR epoll_wait (2)
mark the file as having an error condition.
.TP
.IR /proc/[pid]/mountstats " (since Linux 2.6.17)"
This file exports information (statistics, configuration information)
about the mount points in the process's name space.
Lines in this file have the form:
.nf

device /dev/sda7 mounted on /home with fstype ext3 [statistics]
(       1      )            ( 2 )             (3 ) (4)
.fi
.IP
The fields in each line are:
.RS 7
.TP 5
(1)
The name of the mounted device
(or "nodevice" if there is no corresponding device).
.TP
(2)
The mount point within the filesystem tree.
.TP
(3)
The filesystem type.
.TP
(4)
Optional statistics and configuration information.
Currently (as at Linux 2.6.26), only NFS filesystems export
information via this field.
.RE
.IP
This file is readable only by the owner of the process.
.TP
.IR /proc/[pid]/ns/ " (since Linux 3.0)"
.\" See commit 6b4e306aa3dc94a0545eb9279475b1ab6209a31f
This is a subdirectory containing one entry for each namespace that
supports being manipulated by
.BR setns (2).
For information about namespaces, see
.BR clone (2).
.TP
.IR /proc/[pid]/ns/ipc " (since Linux 3.0)"
Bind mounting this file (see
.BR mount (2))
to somewhere else in the filesystem keeps
the IPC namespace of the process specified by
.I pid
alive even if all processes currently in the namespace terminate.

Opening this file returns a file handle for the IPC namespace
of the process specified by
.IR pid .
As long as this file descriptor remains open,
the IPC namespace will remain alive,
even if all processes in the namespace terminate.
The file descriptor can be passed to
.BR setns (2).
.TP
.IR /proc/[pid]/ns/net " (since Linux 3.0)"
Bind mounting this file (see
.BR mount (2))
to somewhere else in the filesystem keeps
the network namespace of the process specified by
.I pid
alive even if all processes in the namespace terminate.

Opening this file returns a file handle for the network namespace
of the process specified by
.IR pid .
As long as this file descriptor remains open,
the network namespace will remain alive,
even if all processes in the namespace terminate.
The file descriptor can be passed to
.BR setns (2).
.TP
.IR /proc/[pid]/ns/uts " (since Linux 3.0)"
Bind mounting this file (see
.BR mount (2))
to somewhere else in the filesystem keeps
the UTS namespace of the process specified by
.I pid
alive even if all processes currently in the namespace terminate.

Opening this file returns a file handle for the UTS namespace
of the process specified by
.IR pid .
As long as this file descriptor remains open,
the UTS namespace will remain alive,
even if all processes in the namespace terminate.
The file descriptor can be passed to
.BR setns (2).
.TP
.IR /proc/[pid]/numa_maps " (since Linux 2.6.14)"
See
.BR numa (7).
.TP
.IR /proc/[pid]/oom_adj " (since Linux 2.6.11)"
This file can be used to adjust the score used to select which process
should be killed in an out-of-memory (OOM) situation.
The kernel uses this value for a bit-shift operation of the process's
.IR oom_score
value:
valid values are in the range \-16 to +15,
plus the special value \-17,
which disables OOM-killing altogether for this process.
A positive score increases the likelihood of this
process being killed by the OOM-killer;
a negative score decreases the likelihood.
.IP
The default value for this file is 0;
a new process inherits its parent's
.I oom_adj
setting.
A process must be privileged
.RB ( CAP_SYS_RESOURCE )
to update this file.
.IP
Since Linux 2.6.36, use of this file is deprecated in favor of
.IR /proc/[pid]/oom_score_adj .
.TP
.IR /proc/[pid]/oom_score " (since Linux 2.6.11)"
.\" See mm/oom_kill.c::badness() in the 2.6.25 sources
This file displays the current score that the kernel gives to
this process for the purpose of selecting a process
for the OOM-killer.
A higher score means that the process is more likely to be
selected by the OOM-killer.
The basis for this score is the amount of memory used by the process,
with increases (+) or decreases (\-) for factors including:
.\" See mm/oom_kill.c::badness() in the 2.6.25 sources
.RS
.IP * 2
whether the process creates a lot of children using
.BR fork (2)
(+);
.IP *
whether the process has been running a long time,
or has used a lot of CPU time (\-);
.IP *
whether the process has a low nice value (i.e., > 0) (+);
.IP *
whether the process is privileged (\-); and
.\" More precisely, if it has CAP_SYS_ADMIN or CAP_SYS_RESOURCE
.IP *
whether the process is making direct hardware access (\-).
.\" More precisely, if it has CAP_SYS_RAWIO
.RE
.IP
The
.I oom_score
also reflects the adjustment specified by the
.I oom_score_adj
or
.I oom_adj
setting for the process.
.TP
.IR /proc/[pid]/oom_score_adj " (since Linux 2.6.36)"
.\" Text taken from 3.7 Documentation/filesystems/proc.txt
This file can be used to adjust the badness heuristic used to select which
process gets killed in out-of-memory conditions.

The badness heuristic assigns a value to each candidate task ranging from 0
(never kill) to 1000 (always kill) to determine which process is targeted.
The units are roughly a proportion along that range of
allowed memory the process may allocate from,
based on an estimation of its current memory and swap use.
For example, if a task is using all allowed memory,
its badness score will be 1000.
If it is using half of its allowed memory, its score will be 500.

There is an additional factor included in the badness score: root
processes are given 3% extra memory over other tasks.

The amount of "allowed" memory depends on the context
in which the OOM-killer was called.
If it is due to the memory assigned to the allocating task's cpuset
being exhausted,
the allowed memory represents the set of mems assigned to that
cpuset (see
.BR cpuset (7)).
If it is due to a mempolicy's node(s) being exhausted,
the allowed memory represents the set of mempolicy nodes.
If it is due to a memory limit (or swap limit) being reached,
the allowed memory is that configured limit.
Finally, if it is due to the entire system being out of memory, the
allowed memory represents all allocatable resources.

The value of
.I oom_score_adj
is added to the badness score before it
is used to determine which task to kill.
Acceptable values range from \-1000
(OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX).
This allows user space to control the preference for OOM-killing,
ranging from always preferring a certain
task or completely disabling it from OOM-killing.
The lowest possible value, \-1000, is
equivalent to disabling OOM-killing entirely for that task,
since it will always report a badness score of 0.

Consequently, it is very simple for user space to define
the amount of memory to consider for each task.
Setting a
.I oom_score_adj
value of +500, for example,
is roughly equivalent to allowing the remainder of tasks sharing the
same system, cpuset, mempolicy, or memory controller resources
to use at least 50% more memory.
A value of \-500, on the other hand, would be roughly
equivalent to discounting 50% of the task's
allowed memory from being considered as scoring against the task.

For backward compatibility with previous kernels,
.I /proc/[pid]/oom_adj
can still be used to tune the badness score.
Its value is
scaled linearly with
.IR oom_score_adj .

Writing to
.IR /proc/[pid]/oom_score_adj
or
.IR /proc/[pid]/oom_adj
will change the other with its scaled value.
.\" FIXME Describe /proc/[pid]/pagemap
.\"       Added in 2.6.25
.\"       CONFIG_PROC_PAGE_MONITOR
.TP
.I /proc/[pid]/root
UNIX and Linux support the idea of a per-process root of the
filesystem, set by the
.BR chroot (2)
system call.
This file is a symbolic link that points to the process's
root directory, and behaves as exe, fd/*, etc. do.

.\" The following was still true as at kernel 2.6.13
In a multithreaded process, the contents of this symbolic link
are not available if the main thread has already terminated
(typically by calling
.BR pthread_exit (3)).
.\" FIXME Describe /proc/[pid]/seccomp
.\"       Added in 2.6.12
.\" FIXME Describe /proc/[pid]/sessionid
.\"       Added in 2.6.25; read-only; only readable by real UID
.\"       CONFIG_AUDITSYSCALL
.\" FIXME Describe /proc/[pid]/sched
.\"       Added in 2.6.23
.\"       CONFIG_SCHED_DEBUG, and additional fields if CONFIG_SCHEDSTATS
.\"       Displays various scheduling parameters
.\"       This file can be written, to reset stats
.\" FIXME Describe /proc/[pid]/schedstats and
.\"       /proc/[pid]/task/[tid]/schedstats
.\"       Added in 2.6.9
.\"       CONFIG_SCHEDSTATS
.TP
.IR /proc/[pid]/smaps " (since Linux 2.6.14)"
.\"       CONFIG_PROC_PAGE_MONITOR
This file shows memory consumption for each of the process's mappings.
For each of mappings there is a series of lines such as the following:
.in +4n
.nf

08048000-080bc000 r-xp 00000000 03:02 13130      /bin/bash
Size:               464 kB
Rss:                424 kB
Shared_Clean:       424 kB
Shared_Dirty:         0 kB
Private_Clean:        0 kB
Private_Dirty:        0 kB

.fi
.in
The first of these lines shows the same information as is displayed
for the mapping in
.IR /proc/[pid]/maps .
The remaining lines show the size of the mapping,
the amount of the mapping that is currently resident in RAM,
the number of clean and dirty shared pages in the mapping,
and the number of clean and dirty private pages in the mapping.

This file is present only if the
.B CONFIG_MMU
kernel configuration
option is enabled.
.TP
.I /proc/[pid]/stat
Status information about the process.
This is used by
.BR ps (1).
It is defined in
.IR /usr/src/linux/fs/proc/array.c "."

The fields, in order, with their proper
.BR scanf (3)
format specifiers, are:
.RS
.TP 12
\fIpid\fP %d
(1) The process ID.
.TP
\fIcomm\fP %s
(2) The filename of the executable, in parentheses.
This is visible whether or not the executable is swapped out.
.TP
\fIstate\fP %c
(3) One character from the string "RSDZTW" where R is running, S is
sleeping in an interruptible wait, D is waiting in uninterruptible
disk sleep, Z is zombie, T is traced or stopped (on a signal),
and W is paging.
.TP
\fIppid\fP %d
(4) The PID of the parent.
.TP
\fIpgrp\fP %d
(5) The process group ID of the process.
.TP
\fIsession\fP %d
(6) The session ID of the process.
.TP
\fItty_nr\fP %d
(7) The controlling terminal of the process.
(The minor device number is contained in the combination of bits
31 to 20 and 7 to 0;
the major device number is in bits 15 to 8.)
.TP
\fItpgid\fP %d
.\" This field and following, up to and including wchan added 0.99.1
(8) The ID of the foreground process group of the controlling
terminal of the process.
.TP
\fIflags\fP %u (%lu before Linux 2.6.22)
(9) The kernel flags word of the process.
For bit meanings,
see the PF_* defines in the Linux kernel source file
.IR include/linux/sched.h .
Details depend on the kernel version.
.TP
\fIminflt\fP %lu
(10) The number of minor faults the process has made which have not
required loading a memory page from disk.
.TP
\fIcminflt\fP %lu
(11) The number of minor faults that the process's
waited-for children have made.
.TP
\fImajflt\fP %lu
(12) The number of major faults the process has made which have
required loading a memory page from disk.
.TP
\fIcmajflt\fP %lu
(13) The number of major faults that the process's
waited-for children have made.
.TP
\fIutime\fP %lu
(14) Amount of time that this process has been scheduled in user mode,
measured in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
This includes guest time, \fIguest_time\fP
(time spent running a virtual CPU, see below),
so that applications that are not aware of the guest time field
do not lose that time from their calculations.
.TP
\fIstime\fP %lu
(15) Amount of time that this process has been scheduled in kernel mode,
measured in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
.TP
\fIcutime\fP %ld
(16) Amount of time that this process's
waited-for children have been scheduled in user mode,
measured in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
(See also
.BR times (2).)
This includes guest time, \fIcguest_time\fP
(time spent running a virtual CPU, see below).
.TP
\fIcstime\fP %ld
(17) Amount of time that this process's
waited-for children have been scheduled in kernel mode,
measured in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
.TP
\fIpriority\fP %ld
(18) (Explanation for Linux 2.6)
For processes running a real-time scheduling policy
.RI ( policy
below; see
.BR sched_setscheduler (2)),
this is the negated scheduling priority, minus one;
that is, a number in the range \-2 to \-100,
corresponding to real-time priorities 1 to 99.
For processes running under a non-real-time scheduling policy,
this is the raw nice value
.RB ( setpriority (2))
as represented in the kernel.
The kernel stores nice values as numbers
in the range 0 (high) to 39 (low),
corresponding to the user-visible nice range of \-20 to 19.

Before Linux 2.6, this was a scaled value based on
the scheduler weighting given to this process.
.\" And back in kernel 1.2 days things were different again.
.TP
\fInice\fP %ld
(19) The nice value (see
.BR setpriority (2)),
a value in the range 19 (low priority) to \-20 (high priority).
.\" Back in kernel 1.2 days things were different.
.\" .TP
.\" \fIcounter\fP %ld
.\" The current maximum size in jiffies of the process's next timeslice,
.\" or what is currently left of its current timeslice, if it is the
.\" currently running process.
.\" .TP
.\" \fItimeout\fP %u
.\" The time in jiffies of the process's next timeout.
.\" timeout was removed sometime around 2.1/2.2
.TP
\fInum_threads\fP %ld
(20) Number of threads in this process (since Linux 2.6).
Before kernel 2.6, this field was hard coded to 0 as a placeholder
for an earlier removed field.
.TP
\fIitrealvalue\fP %ld
(21) The time in jiffies before the next
.B SIGALRM
is sent to the process due to an interval timer.
Since kernel 2.6.17, this field is no longer maintained,
and is hard coded as 0.
.TP
\fIstarttime\fP %llu (was %lu before Linux 2.6)
(22) The time the process started after system boot.
In kernels before Linux 2.6, this value was expressed in jiffies.
Since Linux 2.6, the value is expressed in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
.TP
\fIvsize\fP %lu
(23) Virtual memory size in bytes.
.TP
\fIrss\fP %ld
(24) Resident Set Size: number of pages the process has in real memory.
This is just the pages which
count toward text, data, or stack space.
This does not include pages
which have not been demand-loaded in, or which are swapped out.
.TP
\fIrsslim\fP %lu
(25) Current soft limit in bytes on the rss of the process;
see the description of
.B RLIMIT_RSS
in
.BR getrlimit (2).
.TP
\fIstartcode\fP %lu
(26) The address above which program text can run.
.TP
\fIendcode\fP %lu
(27) The address below which program text can run.
.TP
\fIstartstack\fP %lu
(28) The address of the start (i.e., bottom) of the stack.
.TP
\fIkstkesp\fP %lu
(29) The current value of ESP (stack pointer), as found in the
kernel stack page for the process.
.TP
\fIkstkeip\fP %lu
(30) The current EIP (instruction pointer).
.TP
\fIsignal\fP %lu
(31) The bitmap of pending signals, displayed as a decimal number.
Obsolete, because it does not provide information on real-time signals; use
.I /proc/[pid]/status
instead.
.TP
\fIblocked\fP %lu
(32) The bitmap of blocked signals, displayed as a decimal number.
Obsolete, because it does not provide information on real-time signals; use
.I /proc/[pid]/status
instead.
.TP
\fIsigignore\fP %lu
(33) The bitmap of ignored signals, displayed as a decimal number.
Obsolete, because it does not provide information on real-time signals; use
.I /proc/[pid]/status
instead.
.TP
\fIsigcatch\fP %lu
(34) The bitmap of caught signals, displayed as a decimal number.
Obsolete, because it does not provide information on real-time signals; use
.I /proc/[pid]/status
instead.
.TP
\fIwchan\fP %lu
(35) This is the "channel" in which the process is waiting.
It is the
address of a system call, and can be looked up in a namelist if you
need a textual name.
(If you have an up-to-date
.IR /etc/psdatabase ,
then
try \fIps \-l\fP to see the WCHAN field in action.)
.TP
\fInswap\fP %lu
(36)
.\" nswap was added in 2.0
Number of pages swapped (not maintained).
.TP
\fIcnswap\fP %lu
(37)
.\" cnswap was added in 2.0
Cumulative \fInswap\fP for child processes (not maintained).
.TP
\fIexit_signal\fP %d (since Linux 2.1.22)
(38) Signal to be sent to parent when we die.
.TP
\fIprocessor\fP %d (since Linux 2.2.8)
(39) CPU number last executed on.
.TP
\fIrt_priority\fP %u (since Linux 2.5.19; was %lu before Linux 2.6.22)
(40) Real-time scheduling priority, a number in the range 1 to 99 for
processes scheduled under a real-time policy,
or 0, for non-real-time processes (see
.BR sched_setscheduler (2)).
.TP
\fIpolicy\fP %u (since Linux 2.5.19; was %lu before Linux 2.6.22)
(41) Scheduling policy (see
.BR sched_setscheduler (2)).
Decode using the SCHED_* constants in
.IR linux/sched.h .
.TP
\fIdelayacct_blkio_ticks\fP %llu (since Linux 2.6.18)
(42) Aggregated block I/O delays, measured in clock ticks (centiseconds).
.TP
\fIguest_time\fP %lu (since Linux 2.6.24)
(43) Guest time of the process (time spent running a virtual CPU
for a guest operating system), measured in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
.TP
\fIcguest_time\fP %ld (since Linux 2.6.24)
(44) Guest time of the process's children, measured in clock ticks (divide by
.IR sysconf(_SC_CLK_TCK) ).
.RE
.TP
.I /proc/[pid]/statm
Provides information about memory usage, measured in pages.
The columns are:
.in +4n
.nf

size       (1) total program size
           (same as VmSize in \fI/proc/[pid]/status\fP)
resident   (2) resident set size
           (same as VmRSS in \fI/proc/[pid]/status\fP)
share      (3) shared pages (i.e., backed by a file)
text       (4) text (code)
.\" (not including libs; broken, includes data segment)
lib        (5) library (unused in Linux 2.6)
data       (6) data + stack
.\" (including libs; broken, includes library text)
dt         (7) dirty pages (unused in Linux 2.6)
.fi
.in
.TP
.I /proc/[pid]/status
Provides much of the information in
.I /proc/[pid]/stat
and
.I /proc/[pid]/statm
in a format that's easier for humans to parse.
Here's an example:
.in +4n
.nf

.RB "$" " cat /proc/$$/status"
Name:   bash
State:  S (sleeping)
Tgid:   3515
Pid:    3515
PPid:   3452
TracerPid:      0
Uid:    1000    1000    1000    1000
Gid:    100     100     100     100
FDSize: 256
Groups: 16 33 100
VmPeak:     9136 kB
VmSize:     7896 kB
VmLck:         0 kB
VmHWM:      7572 kB
VmRSS:      6316 kB
VmData:     5224 kB
VmStk:        88 kB
VmExe:       572 kB
VmLib:      1708 kB
VmPTE:        20 kB
Threads:        1
SigQ:   0/3067
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000010000
SigIgn: 0000000000384004
SigCgt: 000000004b813efb
CapInh: 0000000000000000
CapPrm: 0000000000000000
CapEff: 0000000000000000
CapBnd: ffffffffffffffff
Cpus_allowed:   00000001
Cpus_allowed_list:      0
Mems_allowed:   1
Mems_allowed_list:      0
voluntary_ctxt_switches:        150
nonvoluntary_ctxt_switches:     545
.fi
.in
.IP
The fields are as follows:
.RS
.IP * 2
.IR Name :
Command run by this process.
.IP *
.IR State :
Current state of the process.
One of
"R (running)",
"S (sleeping)",
"D (disk sleep)",
"T (stopped)",
"T (tracing stop)",
"Z (zombie)",
or
"X (dead)".
.IP *
.IR Tgid :
Thread group ID (i.e., Process ID).
.IP *
.IR Pid :
Thread ID (see
.BR gettid (2)).
.IP *
.IR PPid :
PID of parent process.
.IP *
.IR TracerPid :
PID of process tracing this process (0 if not being traced).
.IP *
.IR Uid ", " Gid :
Real, effective, saved set, and filesystem UIDs (GIDs).
.IP *
.IR FDSize :
Number of file descriptor slots currently allocated.
.IP *
.IR Groups :
Supplementary group list.
.IP *
.IR VmPeak :
Peak virtual memory size.
.IP *
.IR VmSize :
Virtual memory size.
.IP *
.IR VmLck :
Locked memory size (see
.BR mlock (3)).
.IP *
.IR VmHWM :
Peak resident set size ("high water mark").
.IP *
.IR VmRSS :
Resident set size.
.IP *
.IR VmData ", " VmStk ", " VmExe :
Size of data, stack, and text segments.
.IP *
.IR VmLib :
Shared library code size.
.IP *
.IR VmPTE :
Page table entries size (since Linux 2.6.10).
.IP *
.IR Threads :
Number of threads in process containing this thread.
.IP *
.IR SigQ :
This field contains two slash-separated numbers that relate to
queued signals for the real user ID of this process.
The first of these is the number of currently queued
signals for this real user ID, and the second is the
resource limit on the number of queued signals for this process
(see the description of
.BR RLIMIT_SIGPENDING
in
.BR getrlimit (2)).
.IP *
.IR SigPnd ", " ShdPnd :
Number of signals pending for thread and for process as a whole (see
.BR pthreads (7)
and
.BR signal (7)).
.IP *
.IR SigBlk ", " SigIgn ", " SigCgt :
Masks indicating signals being blocked, ignored, and caught (see
.BR signal (7)).
.IP *
.IR CapInh ", " CapPrm ", " CapEff :
Masks of capabilities enabled in inheritable, permitted, and effective sets
(see
.BR capabilities (7)).
.IP *
.IR CapBnd :
Capability Bounding set
(since kernel 2.6.26, see
.BR capabilities (7)).
.IP *
.IR Cpus_allowed :
Mask of CPUs on which this process may run
(since Linux 2.6.24, see
.BR cpuset (7)).
.IP *
.IR Cpus_allowed_list :
Same as previous, but in "list format"
(since Linux 2.6.26, see
.BR cpuset (7)).
.IP *
.IR Mems_allowed :
Mask of memory nodes allowed to this process
(since Linux 2.6.24, see
.BR cpuset (7)).
.IP *
.IR Mems_allowed_list :
Same as previous, but in "list format"
(since Linux 2.6.26, see
.BR cpuset (7)).
.IP *
.IR voluntary_context_switches ", " nonvoluntary_context_switches :
Number of voluntary and involuntary context switches (since Linux 2.6.23).
.RE
.TP
.IR /proc/[pid]/task " (since Linux 2.6.0-test6)"
This is a directory that contains one subdirectory
for each thread in the process.
The name of each subdirectory is the numerical thread ID
.RI ( [tid] )
of the thread (see
.BR gettid (2)).
Within each of these subdirectories, there is a set of
files with the same names and contents as under the
.I /proc/[pid]
directories.
For attributes that are shared by all threads, the contents for
each of the files under the
.I task/[tid]
subdirectories will be the same as in the corresponding
file in the parent
.I /proc/[pid]
directory
(e.g., in a multithreaded process, all of the
.I task/[tid]/cwd
files will have the same value as the
.I /proc/[pid]/cwd
file in the parent directory, since all of the threads in a process
share a working directory).
For attributes that are distinct for each thread,
the corresponding files under
.I task/[tid]
may have different values (e.g., various fields in each of the
.I task/[tid]/status
files may be different for each thread).

.\" The following was still true as at kernel 2.6.13
In a multithreaded process, the contents of the
.I /proc/[pid]/task
directory are not available if the main thread has already terminated
(typically by calling
.BR pthread_exit (3)).
.TP
.I /proc/apm
Advanced power management version and battery information when
.B CONFIG_APM
is defined at kernel compilation time.
.TP
.I /proc/bus
Contains subdirectories for installed busses.
.TP
.I /proc/bus/pccard
Subdirectory for PCMCIA devices when
.B CONFIG_PCMCIA
is set at kernel compilation time.
.TP
.I /proc/bus/pccard/drivers
.TP
.I /proc/bus/pci
Contains various bus subdirectories and pseudo-files containing
information about PCI busses, installed devices, and device
drivers.
Some of these files are not ASCII.
.TP
.I /proc/bus/pci/devices
Information about PCI devices.
They may be accessed through
.BR lspci (8)
and
.BR setpci (8).
.TP
.I /proc/cmdline
Arguments passed to the Linux kernel at boot time.
Often done via a boot manager such as
.BR lilo (8)
or
.BR grub (8).
.TP
.IR /proc/config.gz " (since Linux 2.6)"
This file exposes the configuration options that were used
to build the currently running kernel,
in the same format as they would be shown in the
.I .config
file that resulted when configuring the kernel (using
.IR "make xconfig" ,
.IR "make config" ,
or similar).
The file contents are compressed; view or search them using
.BR zcat (1),
.BR zgrep (1),
etc.
As long as no changes have been made to the following file,
the contents of
.I /proc/config.gz
are the same as those provided by :
.in +4n
.nf

cat /lib/modules/$(uname \-r)/build/.config
.fi
.in
.IP
.I /proc/config.gz
is provided only if the kernel is configured with
.BR CONFIG_IKCONFIG_PROC .
.TP
.I /proc/cpuinfo
This is a collection of CPU and system architecture dependent items,
for each supported architecture a different list.
Two common entries are \fIprocessor\fP which gives CPU number and
\fIbogomips\fP; a system constant that is calculated
during kernel initialization.
SMP machines have information for
each CPU.
The
.BR lscpu (1)
command gathers its information from this file.
.TP
.I /proc/devices
Text listing of major numbers and device groups.
This can be used by MAKEDEV scripts for consistency with the kernel.
.TP
.IR /proc/diskstats " (since Linux 2.5.69)"
This file contains disk I/O statistics for each disk device.
See the Linux kernel source file
.I Documentation/iostats.txt
for further information.
.TP
.I /proc/dma
This is a list of the registered \fIISA\fP DMA (direct memory access)
channels in use.
.TP
.I /proc/driver
Empty subdirectory.
.TP
.I /proc/execdomains
List of the execution domains (ABI personalities).
.TP
.I /proc/fb
Frame buffer information when
.B CONFIG_FB
is defined during kernel compilation.
.TP
.I /proc/filesystems
A text listing of the filesystems which are supported by the kernel,
namely filesystems which were compiled into the kernel or whose kernel
modules are currently loaded.
(See also
.BR filesystems (5).)
If a filesystem is marked with "nodev",
this means that it does not require a block device to be mounted
(e.g., virtual filesystem, network filesystem).

Incidentally, this file may be used by
.BR mount (8)
when no filesystem is specified and it didn't manage to determine the
filesystem type.
Then filesystems contained in this file are tried
(excepted those that are marked with "nodev").
.TP
.I /proc/fs
Empty subdirectory.
.TP
.I /proc/ide
This directory
exists on systems with the IDE bus.
There are directories for each IDE channel and attached device.
Files include:

.in +4n
.nf
cache              buffer size in KB
capacity           number of sectors
driver             driver version
geometry           physical and logical geometry
identify           in hexadecimal
media              media type
model              manufacturer's model number
settings           drive settings
smart_thresholds   in hexadecimal
smart_values       in hexadecimal
.fi
.in

The
.BR hdparm (8)
utility provides access to this information in a friendly format.
.TP
.I /proc/interrupts
This is used to record the number of interrupts per CPU per IO device.
Since Linux 2.6.24,
for the i386 and x86_64 architectures, at least, this also includes
interrupts internal to the system (that is, not associated with a device
as such), such as NMI (nonmaskable interrupt), LOC (local timer interrupt),
and for SMP systems, TLB (TLB flush interrupt), RES (rescheduling
interrupt), CAL (remote function call interrupt), and possibly others.
Very easy to read formatting, done in ASCII.
.TP
.I /proc/iomem
I/O memory map in Linux 2.4.
.TP
.I /proc/ioports
This is a list of currently registered Input-Output port regions that
are in use.
.TP
.IR /proc/kallsyms " (since Linux 2.5.71)"
This holds the kernel exported symbol definitions used by the
.BR modules (X)
tools to dynamically link and bind loadable modules.
In Linux 2.5.47 and earlier, a similar file with slightly different syntax
was named
.IR ksyms .
.TP
.I /proc/kcore
This file represents the physical memory of the system and is stored
in the ELF core file format.
With this pseudo-file, and an unstripped
kernel
.RI ( /usr/src/linux/vmlinux )
binary, GDB can be used to
examine the current state of any kernel data structures.

The total length of the file is the size of physical memory (RAM) plus
4KB.
.TP
.I /proc/kmsg
This file can be used instead of the
.BR syslog (2)
system call to read kernel messages.
A process must have superuser
privileges to read this file, and only one process should read this
file.
This file should not be read if a syslog process is running
which uses the
.BR syslog (2)
system call facility to log kernel messages.

Information in this file is retrieved with the
.BR dmesg (1)
program.
.TP
.IR /proc/ksyms " (Linux 1.1.23-2.5.47)"
See
.IR /proc/kallsyms .
.TP
.I /proc/loadavg
The first three fields in this file are load average figures
giving the number of jobs in the run queue (state R)
or waiting for disk I/O (state D) averaged over 1, 5, and 15 minutes.
They are the same as the load average numbers given by
.BR uptime (1)
and other programs.
The fourth field consists of two numbers separated by a slash (/).
The first of these is the number of currently runnable kernel
scheduling entities (processes, threads).
The value after the slash is the number of kernel scheduling entities
that currently exist on the system.
The fifth field is the PID of the process that was most
recently created on the system.
.TP
.I /proc/locks
This file shows current file locks
.RB ( flock "(2) and " fcntl (2))
and leases
.RB ( fcntl (2)).
.TP
.IR /proc/malloc " (only up to and including Linux 2.2)"
.\" It looks like this only ever did something back in 1.0 days
This file is present only if
.B CONFIG_DEBUG_MALLOC
was defined during compilation.
.TP
.I /proc/meminfo
This file reports statistics about memory usage on the system.
It is used by
.BR free (1)
to report the amount of free and used memory (both physical and swap)
on the system as well as the shared memory and buffers used by the
kernel.
Each line of the file consists of a parameter name, followed by a colon,
the value of the parameter, and an option unit of measurement (e.g., "kB").
The list below describes the parameter names and
the format specifier required to read the field value.
Except as noted below,
all of the fields have been present since at least Linux 2.6.0.
Some fileds are displayed only if the kernel was configured
with various options; those dependencies are noted in the list.
.RS
.TP
.IR MemTotal " %lu"
Total usable RAM (i.e. physical RAM minus a few reserved
bits and the kernel binary code).
.TP
.IR MemFree " %lu"
The sum of
.IR LowFree + HighFree .
.TP
.IR Buffers " %lu"
Relatively temporary storage for raw disk blocks that
shouldn't get tremendously large (20MB or so).
.TP
.IR Cached " %lu"
In-memory cache for files read from the disk (the page cache).
Doesn't include
.IR SwapCached .
.TP
.IR SwapCached " %lu"
Memory that once was swapped out, is swapped back in but
still also is in the swap file.
(If memory pressure is high, these pages
don't need to be swapped out again because they are already
in the swap file.
This saves I/O.)
.TP
.IR Active " %lu"
Memory that has been used more recently and usually not
reclaimed unless absolutely necessary.
.TP
.IR Inactive " %lu"
Memory which has been less recently used.
It is more eligible to be reclaimed for other purposes.
.TP
.IR Active(anon) " %lu (since Linux 2.6.28)"
[To be documented.]
.TP
.IR Inactive(anon) " %lu (since Linux 2.6.28)"
[To be documented.]
.TP
.IR Active(file) " %lu (since Linux 2.6.28)"
[To be documented.]
.TP
.IR Inactive(file) " %lu (since Linux 2.6.28)"
[To be documented.]
.TP
.IR Unevictable " %lu (since Linux 2.6.28)"
(From Linux 2.6.28 to 2.6.30,
\fBCONFIG_UNEVICTABLE_LRU\fP was required.)
[To be documented.]
.TP
.IR Mlocked " %lu (since Linux 2.6.28)"
(From Linux 2.6.28 to 2.6.30,
\fBCONFIG_UNEVICTABLE_LRU\fP was required.)
[To be documented.]
.TP
.IR HighTotal " %lu"
(Starting with Linux 2.6.19, \fBCONFIG_HIGHMEM\fP is required.)
Total amount of highmem.
Highmem is all memory above ~860MB of physical memory.
Highmem areas are for use by user-space programs,
or for the page cache.
The kernel must use tricks to access
this memory, making it slower to access than lowmem.
.TP
.IR HighFree " %lu
(Starting with Linux 2.6.19, \fBCONFIG_HIGHMEM\fP is required.)
Amount of free highmem.
.TP
.IR LowTotal " %lu
(Starting with Linux 2.6.19, \fBCONFIG_HIGHMEM\fP is required.)
Total amount of lowmem.
Lowmem is memory which can be used for everything that
highmem can be used for, but it is also available for the
kernel's use for its own data structures.
Among many other things,
it is where everything from
.I Slab
is allocated.
Bad things happen when you're out of lowmem.
.TP
.IR LowFree " %lu
(Starting with Linux 2.6.19, \fBCONFIG_HIGHMEM\fP is required.)
Amount of free lowmem.
.TP
.IR MmapCopy " %lu (since Linux 2.6.29)"
.RB ( CONFIG_MMU
is required.)
[To be documented.]
.TP
.IR SwapTotal " %lu"
Total amount of swap space available.
.TP
.IR SwapFree " %lu"
Amount of swap space that is currently unused.
.TP
.IR Dirty " %lu"
Memory which is waiting to get written back to the disk.
.TP
.IR Writeback " %lu"
Memory which is actively being written back to the disk.
.TP
.IR AnonPages " %lu (since Linux 2.6.18)"
Non-file backed pages mapped into user-space page tables.
.TP
.IR Mapped " %lu"
Files which have been mmaped, such as libraries.
.TP
.IR Shmem " %lu (since Linux 2.6.32)"
[To be documented.]
.TP
.IR Slab " %lu"
In-kernel data structures cache.
.TP
.IR SReclaimable " %lu (since Linux 2.6.19)"
Part of
.IR Slab ,
that might be reclaimed, such as caches.
.TP
.IR SUnreclaim " %lu (since Linux 2.6.19)"
Part of
.IR Slab ,
that cannot be reclaimed on memory pressure.
.TP
.IR KernelStack " %lu (since Linux 2.6.32)"
Amount of memory allocated to kernel stacks.
.TP
.IR PageTables " %lu (since Linux 2.6.18)"
Amount of memory dedicated to the lowest level of page tables.
.TP
.IR Quicklists " %lu (since Linux 2.6.27)"
(\fBCONFIG_QUICKLIST\fP is required.)
[To be documented.]
.TP
.IR NFS_Unstable " %lu (since Linux 2.6.18)"
NFS pages sent to the server, but not yet committed to stable storage.
.TP
.IR Bounce " %lu (since Linux 2.6.18)"
Memory used for block device "bounce buffers".
.TP
.IR WritebackTmp " %lu (since Linux 2.6.26)"
Memory used by FUSE for temporary writeback buffers.
.TP
.IR CommitLimit " %lu (since Linux 2.6.10)"
Based on the overcommit ratio ('vm.overcommit_ratio'),
this is the total amount of  memory currently available to
be allocated on the system.
This limit is adhered to
only if strict overcommit accounting is enabled (mode 2 in
.IR /proc/sys/vm/overcommit_ratio ).
The
.I CommitLimit
is calculated using the following formula:

    CommitLimit = (overcommit_ratio * Physical RAM) + Swap

For example, on a system with 1GB of physical RAM and 7GB
of swap with a
.I overcommit_ratio
of 30, this formula yields a
.I CommitLimit
of 7.3GB.
For more details, see the memory overcommit documentation
in the kernel source file
.IR Documentation/vm/overcommit-accounting .
.TP
.IR Committed_AS " %lu"
The amount of memory presently allocated on the system.
The committed memory is a sum of all of the memory which
has been allocated by processes, even if it has not been
"used" by them as of yet.
A process which allocates 1GB of memory (using
.BR malloc (3)
or similar), but touches only 300MB of that memory will show up
as using only 300MB of memory even if it has the address space
allocated for the entire 1GB.
This 1GB is memory which has been "committed" to by the VM
and can be used at any time by the allocating application.
With strict overcommit enabled on the system (mode 2
.IR /proc/sys/vm/overcommit_memory ),
allocations which would exceed the
.I CommitLimit
(detailed above) will not be permitted.
This is useful if one needs to guarantee that processes will not
fail due to lack of memory once that memory has been successfully allocated.
.TP
.IR VmallocTotal " %lu"
Total size of vmalloc memory area.
.TP
.IR VmallocUsed " %lu"
Amount of vmalloc area which is used.
.TP
.IR VmallocChunk " %lu"
Largest contiguous block of vmalloc area which is free.
.TP
.IR HardwareCorrupted " %lu (since Linux 2.6.32)"
(\fBCONFIG_MEMORY_FAILURE\fP is required.)
[To be documented.]
.TP
.IR AnonHugePages " %lu (since Linux 2.6.38)"
(\fBCONFIG_TRANSPARENT_HUGEPAGE\fP is required.)
Non-file backed huge pages mapped into user-space page tables.
.TP
.IR HugePages_Total " %lu"
(\fBCONFIG_HUGETLB_PAGE\fP is required.)
The size of the pool of huge pages.
.TP
.IR HugePages_Free " %lu"
(\fBCONFIG_HUGETLB_PAGE\fP is required.)
The number of huge pages in the pool that are not yet allocated.
.TP
.IR HugePages_Rsvd " %lu (since Linux 2.6.17)"
(\fBCONFIG_HUGETLB_PAGE\fP is required.)
This is the number of huge pages for
which a commitment to allocate from the pool has been made,
but no allocation has yet been made.
These reserved huge pages
guarantee that an application will be able to allocate a
huge page from the pool of huge pages at fault time.
.TP
.IR HugePages_Surp " %lu (since Linux 2.6.24)"
(\fBCONFIG_HUGETLB_PAGE\fP is required.)
This is the number of huge pages in
the pool above the value in
.IR /proc/sys/vm/nr_hugepages .
The maximum number of surplus huge pages is controlled by
.IR /proc/sys/vm/nr_overcommit_hugepages .
.TP
.IR Hugepagesize " %lu"
(\fBCONFIG_HUGETLB_PAGE\fP is required.)
The size of huge pages.
.RE
.TP
.I /proc/modules
A text list of the modules that have been loaded by the system.
See also
.BR lsmod (8).
.TP
.I /proc/mounts
Before kernel 2.4.19, this file was a list
of all the filesystems currently mounted on the system.
With the introduction of per-process mount namespaces in
Linux 2.4.19, this file became a link to
.IR /proc/self/mounts ,
which lists the mount points of the process's own mount namespace.
The format of this file is documented in
.BR fstab (5).
.TP
.I /proc/mtrr
Memory Type Range Registers.
See the Linux kernel source file
.I Documentation/mtrr.txt
for details.
.TP
.I /proc/net
various net pseudo-files, all of which give the status of some part of
the networking layer.
These files contain ASCII structures and are,
therefore, readable with
.BR cat (1).
However, the standard
.BR netstat (8)
suite provides much cleaner access to these files.
.TP
.I /proc/net/arp
This holds an ASCII readable dump of the kernel ARP table used for
address resolutions.
It will show both dynamically learned and preprogrammed ARP entries.
The format is:

.nf
.ft CW
.in 8n
IP address     HW type   Flags     HW address          Mask   Device
192.168.0.50   0x1       0x2       00:50:BF:25:68:F3   *      eth0
192.168.0.250  0x1       0xc       00:00:00:00:00:00   *      eth0
.ft
.fi
.in

Here "IP address" is the IPv4 address of the machine and the "HW type"
is the hardware type of the address from RFC\ 826.
The flags are the internal
flags of the ARP structure (as defined in
.IR /usr/include/linux/if_arp.h )
and
the "HW address" is the data link layer mapping for that IP address if
it is known.
.TP
.I /proc/net/dev
The dev pseudo-file contains network device status information.
This gives
the number of received and sent packets, the number of errors and
collisions
and other basic statistics.
These are used by the
.BR ifconfig (8)
program to report device status.
The format is:

.nf
.ft CW
.in 1n
Inter-|   Receive                                                |  Transmit
 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
    lo: 2776770   11307    0    0    0     0          0         0  2776770   11307    0    0    0     0       0          0
  eth0: 1215645    2751    0    0    0     0          0         0  1782404    4324    0    0    0   427       0          0
  ppp0: 1622270    5552    1    0    0     0          0         0   354130    5669    0    0    0     0       0          0
  tap0:    7714      81    0    0    0     0          0         0     7714      81    0    0    0     0       0          0
.in
.ft
.fi
.\" .TP
.\" .I /proc/net/ipx
.\" No information.
.\" .TP
.\" .I /proc/net/ipx_route
.\" No information.
.TP
.I /proc/net/dev_mcast
Defined in
.IR /usr/src/linux/net/core/dev_mcast.c :
.nf
.in +5
indx interface_name  dmi_u dmi_g dmi_address
2    eth0            1     0     01005e000001
3    eth1            1     0     01005e000001
4    eth2            1     0     01005e000001
.in
.fi
.TP
.I /proc/net/igmp
Internet Group Management Protocol.
Defined in
.IR /usr/src/linux/net/core/igmp.c .
.TP
.I /proc/net/rarp
This file uses the same format as the
.I arp
file and contains the current reverse mapping database used to provide
.BR rarp (8)
reverse address lookup services.
If RARP is not configured into the
kernel,
this file will not be present.
.TP
.I /proc/net/raw
Holds a dump of the RAW socket table.
Much of the information is not of
use
apart from debugging.
The "sl" value is the kernel hash slot for the
socket,
the "local_address" is the local address and protocol number pair.
\&"St" is
the internal status of the socket.
The "tx_queue" and "rx_queue" are the
outgoing and incoming data queue in terms of kernel memory usage.
The "tr", "tm\->when", and "rexmits" fields are not used by RAW.
The "uid"
field holds the effective UID of the creator of the socket.
.\" .TP
.\" .I /proc/net/route
.\" No information, but looks similar to
.\" .BR route (8).
.TP
.I /proc/net/snmp
This file holds the ASCII data needed for the IP, ICMP, TCP, and UDP
management
information bases for an SNMP agent.
.TP
.I /proc/net/tcp
Holds a dump of the TCP socket table.
Much of the information is not
of use apart from debugging.
The "sl" value is the kernel hash slot
for the socket, the "local_address" is the local address and port number pair.
The "rem_address" is the remote address and port number pair
(if connected).
\&"St" is the internal status of the socket.
The "tx_queue" and "rx_queue" are the
outgoing and incoming data queue in terms of kernel memory usage.
The "tr", "tm\->when", and "rexmits" fields hold internal information of
the kernel socket state and are only useful for debugging.
The "uid"
field holds the effective UID of the creator of the socket.
.TP
.I /proc/net/udp
Holds a dump of the UDP socket table.
Much of the information is not of
use apart from debugging.
The "sl" value is the kernel hash slot for the
socket, the "local_address" is the local address and port number pair.
The "rem_address" is the remote address and port number pair
(if connected). "St" is the internal status of the socket.
The "tx_queue" and "rx_queue" are the outgoing and incoming data queue
in terms of kernel memory usage.
The "tr", "tm\->when", and "rexmits" fields
are not used by UDP.
The "uid"
field holds the effective UID of the creator of the socket.
The format is:

.nf
.ft CW
.in 1n
sl  local_address rem_address   st tx_queue rx_queue tr rexmits  tm\->when uid
 1: 01642C89:0201 0C642C89:03FF 01 00000000:00000001 01:000071BA 00000000 0
 1: 00000000:0801 00000000:0000 0A 00000000:00000000 00:00000000 6F000100 0
 1: 00000000:0201 00000000:0000 0A 00000000:00000000 00:00000000 00000000 0
.in
.ft
.fi
.TP
.I /proc/net/unix
Lists the UNIX domain sockets present within the system and their
status.
The format is:
.nf
.sp .5
.ft CW
Num RefCount Protocol Flags    Type St Path
 0: 00000002 00000000 00000000 0001 03
 1: 00000001 00000000 00010000 0001 01 /dev/printer
.ft
.sp .5
.fi

Here "Num" is the kernel table slot number, "RefCount" is the number
of users of the socket, "Protocol" is currently always 0, "Flags"
represent the internal kernel flags holding the status of the
socket.
Currently, type is always "1" (UNIX domain datagram sockets are
not yet supported in the kernel).
\&"St" is the internal state of the
socket and Path is the bound path (if any) of the socket.
.TP
.I /proc/partitions
Contains major and minor numbers of each partition as well as number
of blocks and partition name.
.TP
.I /proc/pci
This is a listing of all PCI devices found during kernel initialization
and their configuration.

This file has been deprecated in favor of a new
.I /proc
interface for PCI
.RI ( /proc/bus/pci ).
It became optional in Linux 2.2 (available with
.B CONFIG_PCI_OLD_PROC
set at kernel compilation).
It became once more nonoptionally enabled in Linux 2.4.
Next, it was deprecated in Linux 2.6 (still available with
.B CONFIG_PCI_LEGACY_PROC
set), and finally removed altogether since Linux 2.6.17.
.\" FIXME /proc/sched_debug
.\" .TP
.\" .IR /proc/sched_debug " (since Linux 2.6.23)"
.\" See also /proc/[pid]/sched
.TP
.IR /proc/profile " (since Linux 2.4)"
This file is present only if the kernel was booted with the
.I profile=1
command-line option.
It exposes kernel profiling information in a binary format for use by
.BR readprofile (1).
Writing (e.g., an empty string) to this file resets the profiling counters;
on some architectures,
writing a binary integer "profiling multiplier" of size
.IR sizeof(int)
sets the profiling interrupt frequency.
.TP
.I /proc/scsi
A directory with the
.I scsi
mid-level pseudo-file and various SCSI low-level
driver directories,
which contain a file for each SCSI host in this system, all of
which give the status of some part of the SCSI IO subsystem.
These files contain ASCII structures and are, therefore, readable with
.BR cat (1).

You can also write to some of the files to reconfigure the subsystem or
switch certain features on or off.
.TP
.I /proc/scsi/scsi
This is a listing of all SCSI devices known to the kernel.
The listing is similar to the one seen during bootup.
scsi currently supports only the \fIadd-single-device\fP command which
allows root to add a hotplugged device to the list of known devices.

The command
.in +4n
.nf

echo \(aqscsi add-single-device 1 0 5 0\(aq > /proc/scsi/scsi

.fi
.in
will cause
host scsi1 to scan on SCSI channel 0 for a device on ID 5 LUN 0.
If there
is already a device known on this address or the address is invalid, an
error will be returned.
.TP
.I /proc/scsi/[drivername]
\fI[drivername]\fP can currently be NCR53c7xx, aha152x, aha1542, aha1740,
aic7xxx, buslogic, eata_dma, eata_pio, fdomain, in2000, pas16, qlogic,
scsi_debug, seagate, t128, u15-24f, ultrastore, or wd7000.
These directories show up for all drivers that registered at least one
SCSI HBA.
Every directory contains one file per registered host.
Every host-file is named after the number the host was assigned during
initialization.

Reading these files will usually show driver and host configuration,
statistics, etc.

Writing to these files allows different things on different hosts.
For example, with the \fIlatency\fP and \fInolatency\fP commands,
root can switch on and off command latency measurement code in the
eata_dma driver.
With the \fIlockup\fP and \fIunlock\fP commands,
root can control bus lockups simulated by the scsi_debug driver.
.TP
.I /proc/self
This directory refers to the process accessing the
.I /proc
filesystem,
and is identical to the
.I /proc
directory named by the process ID of the same process.
.TP
.I /proc/slabinfo
Information about kernel caches.
Since Linux 2.6.16 this file is present only if the
.B CONFIG_SLAB
kernel configuration option is enabled.
The columns in
.I /proc/slabinfo
are:
.in +4n
.nf

cache-name
num-active-objs
total-objs
object-size
num-active-slabs
total-slabs
num-pages-per-slab
.fi
.in

See
.BR slabinfo (5)
for details.
.TP
.I /proc/stat
kernel/system statistics.
Varies with architecture.
Common
entries include:
.RS
.TP
\fIcpu  3357 0 4313 1362393\fP
The amount of time, measured in units of
USER_HZ (1/100ths of a second on most architectures, use
.IR sysconf(_SC_CLK_TCK)
to obtain the right value),
.\" 1024 on Alpha and ia64
that the system spent in various states:
.RS
.TP
.I user
(1) Time spent in user mode.
.TP
.I nice
(2) Time spent in user mode with low priority (nice).
.TP
.I system
(3) Time spent in system mode.
.TP
.I idle
(4) Time spent in the idle task.
.\" FIXME Actually, the following info about the /proc/stat 'cpu' field
.\"       does not seem to be quite right (at least in 2.6.12 or 3.6):
.\"       the idle time in /proc/uptime does not quite match this value
This value should be USER_HZ times the
second entry in the
.I /proc/uptime
pseudo-file.
.TP
.IR iowait " (since Linux 2.5.41)"
(5) Time waiting for I/O to complete.
.TP
.IR irq " (since Linux 2.6.0-test4)"
(6) Time servicing interrupts.
.TP
.IR softirq " (since Linux 2.6.0-test4)"
(7) Time servicing softirqs.
.TP
.IR steal " (since Linux 2.6.11)"
(8) Stolen time, which is the time spent in other operating systems when
running in a virtualized environment
.TP
.IR guest " (since Linux 2.6.24)"
(9) Time spent running a virtual CPU for guest
operating systems under the control of the Linux kernel.
.\" See Changelog entry for 5e84cfde51cf303d368fcb48f22059f37b3872de
.TP
.IR guest_nice " (since Linux 2.6.33)"
.\" commit ce0e7b28fb75cb003cfc8d0238613aaf1c55e797
(10) Time spent running a niced guest (virtual CPU for guest
operating systems under the control of the Linux kernel).
.RE
.TP
\fIpage 5741 1808\fP
The number of pages the system paged in and the number that were paged
out (from disk).
.TP
\fIswap 1 0\fP
The number of swap pages that have been brought in and out.
.TP
.\" FIXME The following is not the full picture for the 'intr' of
.\"       /proc/stat on 2.6:
\fIintr 1462898\fP
This line shows counts of interrupts serviced since boot time,
for each of the possible system interrupts.
The first column is the total of all interrupts serviced;
each subsequent column is the total for a particular interrupt.
.TP
\fIdisk_io: (2,0):(31,30,5764,1,2) (3,0):\fP...
(major,disk_idx):(noinfo, read_io_ops, blks_read, write_io_ops, blks_written)
.br
(Linux 2.4 only)
.TP
\fIctxt 115315\fP
The number of context switches that the system underwent.
.TP
\fIbtime 769041601\fP
boot time, in seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
.TP
\fIprocesses 86031\fP
Number of forks since boot.
.TP
\fIprocs_running 6\fP
Number of processes in runnable state.
(Linux 2.5.45 onward.)
.TP
\fIprocs_blocked 2\fP
Number of processes blocked waiting for I/O to complete.
(Linux 2.5.45 onward.)
.RE
.TP
.I /proc/swaps
Swap areas in use.
See also
.BR swapon (8).
.TP
.I /proc/sys
This directory (present since 1.3.57) contains a number of files
and subdirectories corresponding to kernel variables.
These variables can be read and sometimes modified using
the \fI/proc\fP filesystem, and the (deprecated)
.BR sysctl (2)
system call.
.TP
.IR /proc/sys/abi " (since Linux 2.4.10)"
This directory may contain files with application binary information.
.\" On some systems, it is not present.
See the Linux kernel source file
.I Documentation/sysctl/abi.txt
for more information.
.TP
.I /proc/sys/debug
This directory may be empty.
.TP
.I /proc/sys/dev
This directory contains device-specific information (e.g.,
.IR dev/cdrom/info ).
On
some systems, it may be empty.
.TP
.I /proc/sys/fs
This directory contains the files and subdirectories for kernel variables
related to filesystems.
.TP
.I /proc/sys/fs/binfmt_misc
Documentation for files in this directory can be found
in the Linux kernel sources in
.IR Documentation/binfmt_misc.txt .
.TP
.IR /proc/sys/fs/dentry-state " (since Linux 2.2)"
This file contains information about the status of the
directory cache (dcache).
The file contains six numbers,
.IR nr_dentry ", " nr_unused ", " age_limit " (age in seconds), "
.I want_pages
(pages requested by system) and two dummy values.
.RS
.IP * 2
.I nr_dentry
is the number of allocated dentries (dcache entries).
This field is unused in Linux 2.2.
.IP *
.I nr_unused
is the number of unused dentries.
.IP *
.I age_limit
.\" looks like this is unused in kernels 2.2 to 2.6
is the age in seconds after which dcache entries
can be reclaimed when memory is short.
.IP *
.I want_pages
.\" looks like this is unused in kernels 2.2 to 2.6
is nonzero when the kernel has called shrink_dcache_pages() and the
dcache isn't pruned yet.
.RE
.TP
.I /proc/sys/fs/dir-notify-enable
This file can be used to disable or enable the
.I dnotify
interface described in
.BR fcntl (2)
on a system-wide basis.
A value of 0 in this file disables the interface,
and a value of 1 enables it.
.TP
.I /proc/sys/fs/dquot-max
This file shows the maximum number of cached disk quota entries.
On some (2.4) systems, it is not present.
If the number of free cached disk quota entries is very low and
you have some awesome number of simultaneous system users,
you might want to raise the limit.
.TP
.I /proc/sys/fs/dquot-nr
This file shows the number of allocated disk quota
entries and the number of free disk quota entries.
.TP
.IR /proc/sys/fs/epoll " (since Linux 2.6.28)"
This directory contains the file
.IR max_user_watches ,
which can be used to limit the amount of kernel memory consumed by the
.I epoll
interface.
For further details, see
.BR epoll (7).
.TP
.I /proc/sys/fs/file-max
This file defines
a system-wide limit on the number of open files for all processes.
(See also
.BR setrlimit (2),
which can be used by a process to set the per-process limit,
.BR RLIMIT_NOFILE ,
on the number of files it may open.)
If you get lots
of error messages in the kernel log about running out of file handles
(look for "VFS: file-max limit <number> reached"),
try increasing this value:
.br

.br
.nf
.ft CW
    echo 100000 > /proc/sys/fs/file-max
.fi
.ft

The kernel constant
.B NR_OPEN
imposes an upper limit on the value that may be placed in
.IR file-max .

If you increase
.IR /proc/sys/fs/file-max ","
be sure to increase
.I /proc/sys/fs/inode-max
to 3-4 times the new
value of
.IR /proc/sys/fs/file-max ","
or you will run out of inodes.

Privileged processes
.RB ( CAP_SYS_ADMIN )
can override the
.I file-max
limit.
.TP
.I /proc/sys/fs/file-nr
This (read-only) file contains three numbers:
the number of allocated file handles
(i.e., the number of files presently opened);
the number of free file handles;
and the maximum number of file handles (i.e., the same value as
.IR /proc/sys/fs/file-max ).
If the number of allocated file handles is close to the
maximum, you should consider increasing the maximum.
Before Linux 2.6,
the kernel allocated file handles dynamically,
but it didn't free them again.
Instead the free file handles were kept in a list for reallocation;
the "free file handles" value indicates the size of that list.
A large number of free file handles indicates that there was
a past peak in the usage of open file handles.
Since Linux 2.6, the kernel does deallocate freed file handles,
and the "free file handles" value is always zero.
.TP
.I /proc/sys/fs/inode-max
This file contains the maximum number of in-memory inodes.
On some (2.4) systems, it may not be present.
This value should be 3-4 times larger
than the value in
.IR file-max ,
since \fIstdin\fP, \fIstdout\fP
and network sockets also need an inode to handle them.
When you regularly run out of inodes, you need to increase this value.
.TP
.I /proc/sys/fs/inode-nr
This file contains the first two values from
.IR inode-state .
.TP
.I /proc/sys/fs/inode-state
This file
contains seven numbers:
.IR nr_inodes ,
.IR nr_free_inodes ,
.IR preshrink ,
and four dummy values.
.I nr_inodes
is the number of inodes the system has allocated.
This can be slightly more than
.I inode-max
because Linux allocates them one page full at a time.
.I nr_free_inodes
represents the number of free inodes.
.I preshrink
is nonzero when the
.I nr_inodes
>
.I inode-max
and the system needs to prune the inode list instead of allocating more.
.TP
.IR /proc/sys/fs/inotify " (since Linux 2.6.13)"
This directory contains files
.IR max_queued_events ", " max_user_instances ", and " max_user_watches ,
that can be used to limit the amount of kernel memory consumed by the
.I inotify
interface.
For further details, see
.BR inotify (7).
.TP
.I /proc/sys/fs/lease-break-time
This file specifies the grace period that the kernel grants to a process
holding a file lease
.RB ( fcntl (2))
after it has sent a signal to that process notifying it
that another process is waiting to open the file.
If the lease holder does not remove or downgrade the lease within
this grace period, the kernel forcibly breaks the lease.
.TP
.I /proc/sys/fs/leases-enable
This file can be used to enable or disable file leases
.RB ( fcntl (2))
on a system-wide basis.
If this file contains the value 0, leases are disabled.
A nonzero value enables leases.
.TP
.IR /proc/sys/fs/mqueue " (since Linux 2.6.6)"
This directory contains files
.IR msg_max ", " msgsize_max ", and " queues_max ,
controlling the resources used by POSIX message queues.
See
.BR mq_overview (7)
for details.
.TP
.IR /proc/sys/fs/overflowgid " and " /proc/sys/fs/overflowuid
These files
allow you to change the value of the fixed UID and GID.
The default is 65534.
Some filesystems support only 16-bit UIDs and GIDs, although in Linux
UIDs and GIDs are 32 bits.
When one of these filesystems is mounted
with writes enabled, any UID or GID that would exceed 65535 is translated
to the overflow value before being written to disk.
.TP
.IR /proc/sys/fs/pipe-max-size " (since Linux 2.6.35)"
The value in this file defines an upper limit for raising the capacity
of a pipe using the
.BR fcntl (2)
.B F_SETPIPE_SZ
operation.
This limit applies only to unprivileged processes.
The default value for this file is 1,048,576.
The value assigned to this file may be rounded upward,
to reflect the value actually employed for a convenient implementation.
To determine the rounded-up value,
display the contents of this file after assigning a value to it.
The minimum value that can be assigned to this file is the system page size.
.TP
.IR /proc/sys/fs/protected_hardlinks " (since Linux 3.6)"
.\" commit 800179c9b8a1e796e441674776d11cd4c05d61d7
When the value in this file is 0,
no restrictions are placed on the creation of hard links
(i.e., this is the historical behaviour before Linux 3.6).
When the value in this file is 1,
a hard link can be created to a target file
only if one of the following conditions is true:
.RS
.IP * 3
The caller has the
.BR CAP_FOWNER
capability.
.IP *
The filesystem UID of the process creating the link matches
the owner (UID) of the target file
(as described in
.BR credentials (7),
a process's filesystem UID is normally the same as its effective UID).
.IP *
All of the following conditions are true:
.RS 4
.IP \(bu 3
the target is a regular file;
.IP \(bu
the target file does not have its set-user-ID permission bit enabled;
.IP \(bu
the target file does not have both its set-group-ID and
group-executable permission bits enabled; and
.IP \(bu
the caller has permission to read and write the target file
(either via the file's permissions mask or because it has
suitable capabilities).
.RE
.RE
.IP
The default value in this file is 0.
Setting the value to 1
prevents a longstanding class of security issues caused by
hard-link-based time-of-check, time-of-use races,
most commonly seen in world-writable directories such as
.IR /tmp .
The common method of exploiting this flaw
is to cross privilege boundaries when following a given hard link
(i.e., a root process follows a hard link created by another user).
Additionally, on systems without separated partitions,
this stops unauthorized users from "pinning" vulnerable set-user-ID and
set-group-ID files against being upgraded by
the administrator, or linking to special files.
.TP
.IR /proc/sys/fs/protected_symlinks " (since Linux 3.6)"
.\" commit 800179c9b8a1e796e441674776d11cd4c05d61d7
When the value in this file is 0,
no restrictions are placed on following symbolic links
(i.e., this is the historical behaviour before Linux 3.6).
When the value in this file is 1, symbolic links are followed only
in the following circumstances:
.RS
.IP * 3
the filesystem UID of the process following the link matches
the owner (UID) of the symbolic link
(as described in
.BR credentials (7),
a process's filesystem UID is normally the same as its effective UID);
.IP *
the link is not in a sticky world-writable directory; or
.IP *
the symbolic link and and its parent directory have the same owner (UID)
.RE
.IP
A system call that fails to follow a symbolic link
because of the above restrictions returns the error
.BR EACCES
in
.IR errno .
.IP
The default value in this file is 0.
Setting the value to 1 avoids a longstanding class of security issues
based on time-of-check, time-of-use races when accessing symbolic links.
.TP
.IR /proc/sys/fs/suid_dumpable " (since Linux 2.6.13)"
.\" The following is based on text from Documentation/sysctl/kernel.txt
The value in this file determines whether core dump files are
produced for set-user-ID or otherwise protected/tainted binaries.
Three different integer values can be specified:
.RS
.TP
\fI0\ (default)\fP
This provides the traditional (pre-Linux 2.6.13) behavior.
A core dump will not be produced for a process which has
changed credentials (by calling
.BR seteuid (2),
.BR setgid (2),
or similar, or by executing a set-user-ID or set-group-ID program)
or whose binary does not have read permission enabled.
.TP
\fI1\ ("debug")\fP
All processes dump core when possible.
The core dump is owned by the filesystem user ID of the dumping process
and no security is applied.
This is intended for system debugging situations only.
Ptrace is unchecked.
.TP
\fI2\ ("suidsafe")\fP
Any binary which normally would not be dumped (see "0" above)
is dumped readable by root only.
This allows the user to remove the core dump file but not to read it.
For security reasons core dumps in this mode will not overwrite one
another or other files.
This mode is appropriate when administrators are
attempting to debug problems in a normal environment.
.IP
Additionally, since Linux 3.6,
.\" 9520628e8ceb69fa9a4aee6b57f22675d9e1b709
.I /proc/sys/kernel/core_pattern
must either be an absolute pathname
or a pipe command, as detailed in
.BR core (5).
Warnings will be written to the kernel log if
.I core_pattern
does not follow these rules, and no core dump will be produced.
.\" 54b501992dd2a839e94e76aa392c392b55080ce8
.RE
.TP
.I /proc/sys/fs/super-max
This file
controls the maximum number of superblocks, and
thus the maximum number of mounted filesystems the kernel
can have.
You need increase only
.I super-max
if you need to mount more filesystems than the current value in
.I super-max
allows you to.
.TP
.I /proc/sys/fs/super-nr
This file
contains the number of filesystems currently mounted.
.TP
.I /proc/sys/kernel
This directory contains files controlling a range of kernel parameters,
as described below.
.TP
.I /proc/sys/kernel/acct
This file
contains three numbers:
.IR highwater ,
.IR lowwater ,
and
.IR frequency .
If BSD-style process accounting is enabled these values control
its behavior.
If free space on filesystem where the log lives goes below
.I lowwater
percent accounting suspends.
If free space gets above
.I highwater
percent accounting resumes.
.I frequency
determines
how often the kernel checks the amount of free space (value is in
seconds).
Default values are 4, 2 and 30.
That is, suspend accounting if 2% or less space is free; resume it
if 4% or more space is free; consider information about amount of free space
valid for 30 seconds.
.TP
.IR /proc/sys/kernel/cap_last_cap " (since Linux 3.2)"
See
.BR capabilities (7).
.TP
.IR /proc/sys/kernel/cap-bound " (from Linux 2.2 to 2.6.24)"
This file holds the value of the kernel
.I "capability bounding set"
(expressed as a signed decimal number).
This set is ANDed against the capabilities permitted to a process
during
.BR execve (2).
Starting with Linux 2.6.25,
the system-wide capability bounding set disappeared,
and was replaced by a per-thread bounding set; see
.BR capabilities (7).
.TP
.I /proc/sys/kernel/core_pattern
See
.BR core (5).
.TP
.I /proc/sys/kernel/core_uses_pid
See
.BR core (5).
.TP
.I /proc/sys/kernel/ctrl-alt-del
This file
controls the handling of Ctrl-Alt-Del from the keyboard.
When the value in this file is 0, Ctrl-Alt-Del is trapped and
sent to the
.BR init (8)
program to handle a graceful restart.
When the value is greater than zero, Linux's reaction to a Vulcan
Nerve Pinch (tm) will be an immediate reboot, without even
syncing its dirty buffers.
Note: when a program (like dosemu) has the keyboard in "raw"
mode, the ctrl-alt-del is intercepted by the program before it
ever reaches the kernel tty layer, and it's up to the program
to decide what to do with it.
.TP
.IR /proc/sys/kernel/dmesg_restrict " (since Linux 2.6.37)"
The value in this file determines who can see kernel syslog contents.
A value of 0 in this file imposes no restrictions.
If the value is 1, only privileged users can read the kernel syslog.
(See
.BR syslog (2)
for more details.)
Since Linux 3.4,
.\" commit 620f6e8e855d6d447688a5f67a4e176944a084e8
only users with the
.BR CAP_SYS_ADMIN
capability may change the value in this file.
.TP
.IR /proc/sys/kernel/domainname " and " /proc/sys/kernel/hostname
can be used to set the NIS/YP domainname and the
hostname of your box in exactly the same way as the commands
.BR domainname (1)
and
.BR hostname (1),
that is:

.in +4n
.nf
.RB "#" " echo \(aqdarkstar\(aq > /proc/sys/kernel/hostname"
.RB "#" " echo \(aqmydomain\(aq > /proc/sys/kernel/domainname"
.fi
.in

has the same effect as

.in +4n
.nf
.RB "#" " hostname \(aqdarkstar\(aq"
.RB "#" " domainname \(aqmydomain\(aq"
.fi
.in

Note, however, that the classic darkstar.frop.org has the
hostname "darkstar" and DNS (Internet Domain Name Server)
domainname "frop.org", not to be confused with the NIS (Network
Information Service) or YP (Yellow Pages) domainname.
These two
domain names are in general different.
For a detailed discussion
see the
.BR hostname (1)
man page.
.TP
.I /proc/sys/kernel/hotplug
This file
contains the path for the hotplug policy agent.
The default value in this file is
.IR /sbin/hotplug .
.TP
.I /proc/sys/kernel/htab-reclaim
(PowerPC only) If this file is set to a nonzero value,
the PowerPC htab
(see kernel file
.IR Documentation/powerpc/ppc_htab.txt )
is pruned
each time the system hits the idle loop.
.TP
.IR /proc/sys/kernel/kptr_restrict " (since Linux 2.6.38)"
.\" 455cd5ab305c90ffc422dd2e0fb634730942b257
The value in this file determines whether kernel addresses are exposed via
.I /proc
files and other interfaces.
A value of 0 in this file imposes no restrictions.
If the value is 1, kernel pointers printed using the
.I %pK
format specifier will be replaced with zeros unless the user has the
.BR CAP_SYSLOG
capability.
If the value is 2, kernel pointers printed using the
.I %pK
format specifier will be replaced with zeros regardless
of the user's capabilities.
The initial default value for this file was 1,
but the default was changed
.\" commit 411f05f123cbd7f8aa1edcae86970755a6e2a9d9
to 0 in Linux 2.6.39.
Since Linux 3.4,
.\" commit 620f6e8e855d6d447688a5f67a4e176944a084e8
only users with the
.BR CAP_SYS_ADMIN
capability can change the value in this file.
.TP
.I /proc/sys/kernel/l2cr
(PowerPC only) This file
contains a flag that controls the L2 cache of G3 processor
boards.
If 0, the cache is disabled.
Enabled if nonzero.
.TP
.I /proc/sys/kernel/modprobe
This file contains the path for the kernel module loader.
The default value is
.IR /sbin/modprobe .
The file is present only if the kernel is built with the
.B CONFIG_MODULES
.RB ( CONFIG_KMOD
in Linux 2.6.26 and earlier)
option enabled.
It is described by the Linux kernel source file
.I Documentation/kmod.txt
(present only in kernel 2.4 and earlier).
.TP
.IR /proc/sys/kernel/modules_disabled " (since Linux 2.6.31)"
.\" 3d43321b7015387cfebbe26436d0e9d299162ea1
.\" From Documentation/sysctl/kernel.txt
A toggle value indicating if modules are allowed to be loaded
in an otherwise modular kernel.
This toggle defaults to off (0), but can be set true (1).
Once true, modules can be neither loaded nor unloaded,
and the toggle cannot be set back to false.
The file is present only if the kernel is built with the
.B CONFIG_MODULES
option enabled.
.TP
.I /proc/sys/kernel/msgmax
This file defines
a system-wide limit specifying the maximum number of bytes in
a single message written on a System V message queue.
.TP
.IR /proc/sys/kernel/msgmni " (since Linux 2.4)"
This file defines the system-wide limit on the number of
message queue identifiers.
.TP
.I /proc/sys/kernel/msgmnb
This file defines a system-wide parameter used to initialize the
.I msg_qbytes
setting for subsequently created message queues.
The
.I msg_qbytes
setting specifies the maximum number of bytes that may be written to the
message queue.
.TP
.IR /proc/sys/kernel/ostype " and " /proc/sys/kernel/osrelease
These files
give substrings of
.IR /proc/version .
.TP
.IR /proc/sys/kernel/overflowgid " and " /proc/sys/kernel/overflowuid
These files duplicate the files
.I /proc/sys/fs/overflowgid
and
.IR /proc/sys/fs/overflowuid .
.TP
.I /proc/sys/kernel/panic
This file gives read/write access to the kernel variable
.IR panic_timeout .
If this is zero, the kernel will loop on a panic; if nonzero
it indicates that the kernel should autoreboot after this number
of seconds.
When you use the
software watchdog device driver, the recommended setting is 60.
.TP
.IR /proc/sys/kernel/panic_on_oops " (since Linux 2.5.68)"
This file controls the kernel's behavior when an oops
or BUG is encountered.
If this file contains 0, then the system
tries to continue operation.
If it contains 1, then the system
delays a few seconds (to give klogd time to record the oops output)
and then panics.
If the
.I /proc/sys/kernel/panic
file is also nonzero then the machine will be rebooted.
.TP
.IR /proc/sys/kernel/pid_max " (since Linux 2.5.34)"
This file specifies the value at which PIDs wrap around
(i.e., the value in this file is one greater than the maximum PID).
The default value for this file, 32768,
results in the same range of PIDs as on earlier kernels.
On 32-bit platforms, 32768 is the maximum value for
.IR pid_max .
On 64-bit systems,
.I pid_max
can be set to any value up to 2^22
.RB ( PID_MAX_LIMIT ,
approximately 4 million).
.\" Prior to 2.6.10, pid_max could also be raised above 32768 on 32-bit
.\" platforms, but this broke /proc/[pid]
.\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=109513010926152&w=2
.TP
.IR /proc/sys/kernel/powersave-nap " (PowerPC only)"
This file contains a flag.
If set, Linux-PPC will use the "nap" mode of
powersaving,
otherwise the "doze" mode will be used.
.TP
.I /proc/sys/kernel/printk
The four values in this file are
.IR console_loglevel ,
.IR default_message_loglevel ,
.IR minimum_console_level ,
and
.IR default_console_loglevel .
These values influence
.I printk()
behavior when printing or logging error messages.
See
.BR syslog (2)
for more info on the different loglevels.
Messages with a higher priority than
.I console_loglevel
will be printed to the console.
Messages without an explicit priority will be printed with priority
.IR default_message_level .
.I minimum_console_loglevel
is the minimum (highest) value to which
.I console_loglevel
can be set.
.I default_console_loglevel
is the default value for
.IR console_loglevel .
.TP
.IR /proc/sys/kernel/pty " (since Linux 2.6.4)"
This directory contains two files relating to the number of UNIX 98
pseudoterminals (see
.BR pts (4))
on the system.
.TP
.I /proc/sys/kernel/pty/max
This file defines the maximum number of pseudoterminals.
.TP
.I /proc/sys/kernel/pty/nr
This read-only file
indicates how many pseudoterminals are currently in use.
.TP
.I /proc/sys/kernel/random
This directory
contains various parameters controlling the operation of the file
.IR /dev/random .
See
.BR random (4)
for further information.
.TP
.I /proc/sys/kernel/real-root-dev
This file is documented in the Linux kernel source file
.IR Documentation/initrd.txt .
.TP
.IR /proc/sys/kernel/reboot-cmd " (Sparc only) "
This file seems to be a way to give an argument to the SPARC
ROM/Flash boot loader.
Maybe to tell it what to do after
rebooting?
.TP
.I /proc/sys/kernel/rtsig-max
(Only in kernels up to and including 2.6.7; see
.BR setrlimit (2))
This file can be used to tune the maximum number
of POSIX real-time (queued) signals that can be outstanding
in the system.
.TP
.I /proc/sys/kernel/rtsig-nr
(Only in kernels up to and including 2.6.7.)
This file shows the number POSIX real-time signals currently queued.
.TP
.IR /proc/sys/kernel/sched_rr_timeslice_ms " (since Linux 3.9)"
See
.BR sched_rr_get_interval (2).
.TP
.IR /proc/sys/kernel/sem " (since Linux 2.4)"
This file contains 4 numbers defining limits for System V IPC semaphores.
These fields are, in order:
.RS
.IP SEMMSL 8
The maximum semaphores per semaphore set.
.IP SEMMNS 8
A system-wide limit on the number of semaphores in all semaphore sets.
.IP SEMOPM 8
The maximum number of operations that may be specified in a
.BR semop (2)
call.
.IP SEMMNI 8
A system-wide limit on the maximum number of semaphore identifiers.
.RE
.TP
.I /proc/sys/kernel/sg-big-buff
This file
shows the size of the generic SCSI device (sg) buffer.
You can't tune it just yet, but you could change it at
compile time by editing
.I include/scsi/sg.h
and changing
the value of
.BR SG_BIG_BUFF .
However, there shouldn't be any reason to change this value.
.TP
.IR /proc/sys/kernel/shm_rmid_forced " (since Linux 3.1)"
.\" commit b34a6b1da371ed8af1221459a18c67970f7e3d53
.\" See also Documentation/sysctl/kernel.txt
If this file is set to 1, all System V shared memory segments will
be marked for destruction as soon as the number of attached processes
falls to zero;
in other words, it is no longer possible to create shared memory segments
that exist independently of any attached process.
.IP
The effect is as though a
.BR shmctl (2)
.B IPC_RMID
is performed on all existing  segments as well as all segments
created in the future (until this file is reset to 0).
Note that existing segments that are attached to no process will be
immediately destroyed when this file is set to 1.
Setting this option will also destroy segments that were created,
but never attached,
upon termination of the process that created the segment with
.BR shmget (2).
.IP
Setting this file to 1 provides a way of ensuring that
all System V shared memory segments are counted against the
resource usage and resource limits (see the description of
.B RLIMIT_AS
in
.BR getrlimit (2))
of at least one process.
.IP
Because setting this file to 1 produces behavior that is nonstandard
and could also break existing applications,
the default value in this file is 0.
Only set this file to 1 if you have a good understanding
of the semantics of the applications using
System V shared memory on your system.
.TP
.I /proc/sys/kernel/shmall
This file
contains the system-wide limit on the total number of pages of
System V shared memory.
.TP
.I /proc/sys/kernel/shmmax
This file
can be used to query and set the run-time limit
on the maximum (System V IPC) shared memory segment size that can be
created.
Shared memory segments up to 1GB are now supported in the
kernel.
This value defaults to
.BR SHMMAX .
.TP
.IR /proc/sys/kernel/shmmni " (since Linux 2.4)"
This file
specifies the system-wide maximum number of System V shared memory
segments that can be created.
.TP
.I /proc/sys/kernel/sysrq
This file controls the functions allowed to be invoked by the SysRq key.
By default,
the file contains 1 meaning that every possible SysRq request is allowed
(in older kernel versions, SysRq was disabled by default,
and you were required to specifically enable it at run-time,
but this is not the case any more).
Possible values in this file are:

   0 - disable sysrq completely
   1 - enable all functions of sysrq
  >1 - bit mask of allowed sysrq functions, as follows:
          2 - enable control of console logging level
          4 - enable control of keyboard (SAK, unraw)
          8 - enable debugging dumps of processes etc.
         16 - enable sync command
         32 - enable remount read-only
         64 - enable signalling of processes (term, kill, oom-kill)
        128 - allow reboot/poweroff
        256 - allow nicing of all real-time tasks

This file is present only if the
.B CONFIG_MAGIC_SYSRQ
kernel configuration option is enabled.
For further details see the Linux kernel source file
.IR Documentation/sysrq.txt .
.TP
.I /proc/sys/kernel/version
This file contains a string like:

    #5 Wed Feb 25 21:49:24 MET 1998

The "#5" means that
this is the fifth kernel built from this source base and the
date behind it indicates the time the kernel was built.
.TP
.IR /proc/sys/kernel/threads-max " (since Linux 2.3.11)"
This file specifies the system-wide limit on the number of
threads (tasks) that can be created on the system.
.TP
.IR /proc/sys/kernel/zero-paged " (PowerPC only) "
This file
contains a flag.
When enabled (nonzero), Linux-PPC will pre-zero pages in
the idle loop, possibly speeding up get_free_pages.
.TP
.I /proc/sys/net
This directory contains networking stuff.
Explanations for some of the files under this directory can be found in
.BR tcp (7)
and
.BR ip (7).
.TP
.I /proc/sys/net/core/somaxconn
This file defines a ceiling value for the
.I backlog
argument of
.BR listen (2);
see the
.BR listen (2)
manual page for details.
.TP
.I /proc/sys/proc
This directory may be empty.
.TP
.I /proc/sys/sunrpc
This directory supports Sun remote procedure call for network filesystem
(NFS).
On some systems, it is not present.
.TP
.I /proc/sys/vm
This directory contains files for memory management tuning, buffer and
cache management.
.TP
.IR /proc/sys/vm/drop_caches " (since Linux 2.6.16)"
Writing to this file causes the kernel to drop clean caches, dentries, and
inodes from memory, causing that memory to become free.
This can be useful for memory management testing and
performing reproducible filesystem benchmarks.
Because writing to this file causes the benefits of caching to be lost,
it can degrade overall system performance.

To free pagecache, use:

    echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes, use:

    echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes, use:

    echo 3 > /proc/sys/vm/drop_caches

Because writing to this file is a nondestructive operation and dirty objects
are not freeable, the
user should run
.BR sync (8)
first.
.TP
.IR /proc/sys/vm/legacy_va_layout " (since Linux 2.6.9)"
.\" The following is from Documentation/filesystems/proc.txt
If nonzero, this disables the new 32-bit memory-mapping layout;
the kernel will use the legacy (2.4) layout for all processes.
.TP
.IR /proc/sys/vm/memory_failure_early_kill " (since Linux 2.6.32)"
.\" The following is based on the text in Documentation/sysctl/vm.txt
Control how to kill processes when an uncorrected memory error
(typically a 2-bit error in a memory module)
that cannot be handled by the kernel
is detected in the background by hardware.
In some cases (like the page still having a valid copy on disk),
the kernel will handle the failure
transparently without affecting any applications.
But if there is no other up-to-date copy of the data,
it will kill processes to prevent any data corruptions from propagating.

The file has one of the following values:
.RS
.IP 1: 4
Kill all processes that have the corrupted-and-not-reloadable page mapped
as soon as the corruption is detected.
Note this is not supported for a few types of pages, like kernel internally
allocated data or the swap cache, but works for the majority of user pages.
.IP 0: 4
Only unmap the corrupted page from all processes and kill only a process
that tries to access it.
.RE
.IP
The kill is performed using a
.B SIGBUS
signal with
.I si_code
set to
.BR BUS_MCEERR_AO .
Processes can handle this if they want to; see
.BR sigaction (2)
for more details.

This feature is active only on architectures/platforms with advanced machine
check handling and depends on the hardware capabilities.

Applications can override the
.I memory_failure_early_kill
setting individually with the
.BR prctl (2)
.B PR_MCE_KILL
operation.
.IP
Only present if the kernel was configured with
.BR CONFIG_MEMORY_FAILURE .
.TP
.IR /proc/sys/vm/memory_failure_recovery " (since Linux 2.6.32)"
.\" The following is based on the text in Documentation/sysctl/vm.txt
Enable memory failure recovery (when supported by the platform)
.RS
.IP 1: 4
Attempt recovery.
.IP 0: 4
Always panic on a memory failure.
.RE
.IP
Only present if the kernel was configured with
.BR CONFIG_MEMORY_FAILURE .
.TP
.IR /proc/sys/vm/oom_dump_tasks " (since Linux 2.6.25)"
.\" The following is from Documentation/sysctl/vm.txt
Enables a system-wide task dump (excluding kernel threads) to be
produced when the kernel performs an OOM-killing.
The dump includes the following information
for each task (thread, process):
thread ID, real user ID, thread group ID (process ID),
virtual memory size, resident set size,
the CPU that the task is scheduled on,
oom_adj score (see the description of
.IR /proc/[pid]/oom_adj ),
and command name.
This is helpful to determine why the OOM-killer was invoked
and to identify the rogue task that caused it.

If this contains the value zero, this information is suppressed.
On very large systems with thousands of tasks,
it may not be feasible to dump the memory state information for each one.
Such systems should not be forced to incur a performance penalty in
OOM situations when the information may not be desired.

If this is set to nonzero, this information is shown whenever the
OOM-killer actually kills a memory-hogging task.

The default value is 0.
.TP
.IR /proc/sys/vm/oom_kill_allocating_task " (since Linux 2.6.24)"
.\" The following is from Documentation/sysctl/vm.txt
This enables or disables killing the OOM-triggering task in
out-of-memory situations.

If this is set to zero, the OOM-killer will scan through the entire
tasklist and select a task based on heuristics to kill.
This normally selects a rogue memory-hogging task that
frees up a large amount of memory when killed.

If this is set to nonzero, the OOM-killer simply kills the task that
triggered the out-of-memory condition.
This avoids a possibly expensive tasklist scan.

If
.I /proc/sys/vm/panic_on_oom
is nonzero, it takes precedence over whatever value is used in
.IR /proc/sys/vm/oom_kill_allocating_task .

The default value is 0.
.TP
.I /proc/sys/vm/overcommit_memory
This file contains the kernel virtual memory accounting mode.
Values are:
.RS
.IP
0: heuristic overcommit (this is the default)
.br
1: always overcommit, never check
.br
2: always check, never overcommit
.RE
.IP
In mode 0, calls of
.BR mmap (2)
with
.B MAP_NORESERVE
are not checked, and the default check is very weak,
leading to the risk of getting a process "OOM-killed".
Under Linux 2.4 any nonzero value implies mode 1.
In mode 2 (available since Linux 2.6), the total virtual address space
on the system is limited to (SS + RAM*(r/100)),
where SS is the size of the swap space, and RAM
is the size of the physical memory, and r is the contents of the file
.IR /proc/sys/vm/overcommit_ratio .
.TP
.I /proc/sys/vm/overcommit_ratio
See the description of
.IR /proc/sys/vm/overcommit_memory .
.TP
.IR /proc/sys/vm/panic_on_oom " (since Linux 2.6.18)"
.\" The following is adapted from Documentation/sysctl/vm.txt
This enables or disables a kernel panic in
an out-of-memory situation.

If this file is set to the value 0,
the kernel's OOM-killer will kill some rogue process.
Usually, the OOM-killer is able to kill a rogue process and the
system will survive.

If this file is set to the value 1,
then the kernel normally panics when out-of-memory happens.
However, if a process limits allocations to certain nodes
using memory policies
.RB ( mbind  (2)
.BR MPOL_BIND )
or cpusets
.RB ( cpuset (7))
and those nodes reach memory exhaustion status,
one process may be killed by the OOM-killer.
No panic occurs in this case:
because other nodes' memory may be free,
this means the system as a whole may not have reached
an out-of-memory situation yet.

If this file is set to the value 2,
the kernel always panics when an out-of-memory condition occurs.

The default value is 0.
1 and 2 are for failover of clustering.
Select either according to your policy of failover.
.TP
.IR /proc/sys/vm/swappiness
.\" The following is from Documentation/sysctl/vm.txt
The value in this file controls how aggressively the kernel will swap
memory pages.
Higher values increase aggressiveness, lower values
decrease aggressiveness.
The default value is 60.
.TP
.IR /proc/sysrq-trigger " (since Linux 2.4.21)"
Writing a character to this file triggers the same SysRq function as
typing ALT-SysRq-<character> (see the description of
.IR /proc/sys/kernel/sysrq ).
This file is normally writable only by
.IR root .
For further details see the Linux kernel source file
.IR Documentation/sysrq.txt .
.TP
.I /proc/sysvipc
Subdirectory containing the pseudo-files
.IR msg ", "  sem " and "  shm "."
These files list the System V Interprocess Communication (IPC) objects
(respectively: message queues, semaphores, and shared memory)
that currently exist on the system,
providing similar information to that available via
.BR ipcs (1).
These files have headers and are formatted (one IPC object per line)
for easy understanding.
.BR svipc (7)
provides further background on the information shown by these files.
.TP
.I /proc/tty
Subdirectory containing the pseudo-files and subdirectories for
tty drivers and line disciplines.
.TP
.I /proc/uptime
This file contains two numbers: the uptime of the system (seconds),
and the amount of time spent in idle process (seconds).
.TP
.I /proc/version
This string identifies the kernel version that is currently running.
It includes the contents of
.IR /proc/sys/kernel/ostype ,
.I /proc/sys/kernel/osrelease
and
.IR /proc/sys/kernel/version .
For example:
.nf
.in -2
.ft CW
Linux version 1.0.9 (quinlan@phaze) #1 Sat May 14 01:51:54 EDT 1994
.ft
.in +2
.fi
.\" FIXME Document /proc/timer_list
.\" .TP
.\" .IR /proc/timer_list " (since Linux 2.6.21)"
.\" See the 2.6.21 Change log
.\" FIXME Document /proc/timer_stats
.\" .TP
.\" .IR /proc/timer_stats " (since Linux 2.6.21)"
.\" See the 2.6.21 Change log
.TP
.IR /proc/vmstat " (since Linux 2.6)"
This file displays various virtual memory statistics.
.TP
.IR /proc/zoneinfo " (since Linux 2.6.13)"
This file display information about memory zones.
This is useful for analyzing virtual memory behavior.
.\" FIXME more should be said about /proc/zoneinfo
.SH NOTES
Many strings (i.e., the environment and command line) are in
the internal format, with subfields terminated by null bytes (\(aq\\0\(aq),
so you
may find that things are more readable if you use \fIod \-c\fP or \fItr
"\\000" "\\n"\fP to read them.
Alternatively, \fIecho \`cat <file>\`\fP works well.

This manual page is incomplete, possibly inaccurate, and is the kind
of thing that needs to be updated very often.
.\" .SH ACKNOWLEDGEMENTS
.\" The material on /proc/sys/fs and /proc/sys/kernel is closely based on
.\" kernel source documentation files written by Rik van Riel.
.SH SEE ALSO
.BR cat (1),
.BR dmesg (1),
.BR find (1),
.BR free (1),
.BR ps (1),
.BR tr (1),
.BR uptime (1),
.BR chroot (2),
.BR mmap (2),
.BR readlink (2),
.BR syslog (2),
.BR slabinfo (5),
.BR hier (7),
.BR time (7),
.BR arp (8),
.BR hdparm (8),
.BR ifconfig (8),
.BR init (8),
.BR lsmod (8),
.BR lspci (8),
.BR mount (8),
.BR netstat (8),
.BR procinfo (8),
.BR route (8),
.BR sysctl (8)

The Linux kernel source files:
.IR Documentation/filesystems/proc.txt
and
.IR Documentation/sysctl/vm.txt .