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diff --git a/Documentation/vm/page_migration b/Documentation/vm/page_migration new file mode 100644 index 000000000000..0dd4ef30c361 --- /dev/null +++ b/Documentation/vm/page_migration @@ -0,0 +1,175 @@ +Page migration +-------------- + +Page migration allows the moving of the physical location of pages between +nodes in a numa system while the process is running. This means that the +virtual addresses that the process sees do not change. However, the +system rearranges the physical location of those pages. + +The main intend of page migration is to reduce the latency of memory access +by moving pages near to the processor where the process accessing that memory +is running. + +Page migration allows a process to manually relocate the node on which its +pages are located through the MF_MOVE and MF_MOVE_ALL options while setting +a new memory policy via mbind(). The pages of process can also be relocated +from another process using the sys_migrate_pages() function call. The +migrate_pages function call takes two sets of nodes and moves pages of a +process that are located on the from nodes to the destination nodes. +Page migration functions are provided by the numactl package by Andi Kleen +(a version later than 0.9.3 is required. Get it from +ftp://ftp.suse.com/pub/people/ak). numactl provided libnuma which +provides an interface similar to other numa functionality for page migration. +cat /proc/<pid>/numa_maps allows an easy review of where the pages of +a process are located. See also the numa_maps manpage in the numactl package. + +Manual migration is useful if for example the scheduler has relocated +a process to a processor on a distant node. A batch scheduler or an +administrator may detect the situation and move the pages of the process +nearer to the new processor. At some point in the future we may have +some mechanism in the scheduler that will automatically move the pages. + +Larger installations usually partition the system using cpusets into +sections of nodes. Paul Jackson has equipped cpusets with the ability to +move pages when a task is moved to another cpuset (See ../cpusets.txt). +Cpusets allows the automation of process locality. If a task is moved to +a new cpuset then also all its pages are moved with it so that the +performance of the process does not sink dramatically. Also the pages +of processes in a cpuset are moved if the allowed memory nodes of a +cpuset are changed. + +Page migration allows the preservation of the relative location of pages +within a group of nodes for all migration techniques which will preserve a +particular memory allocation pattern generated even after migrating a +process. This is necessary in order to preserve the memory latencies. +Processes will run with similar performance after migration. + +Page migration occurs in several steps. First a high level +description for those trying to use migrate_pages() from the kernel +(for userspace usage see the Andi Kleen's numactl package mentioned above) +and then a low level description of how the low level details work. + +A. In kernel use of migrate_pages() +----------------------------------- + +1. Remove pages from the LRU. + + Lists of pages to be migrated are generated by scanning over + pages and moving them into lists. This is done by + calling isolate_lru_page(). + Calling isolate_lru_page increases the references to the page + so that it cannot vanish while the page migration occurs. + It also prevents the swapper or other scans to encounter + the page. + +2. Generate a list of newly allocates page. These pages will contain the + contents of the pages from the first list after page migration is + complete. + +3. The migrate_pages() function is called which attempts + to do the migration. It returns the moved pages in the + list specified as the third parameter and the failed + migrations in the fourth parameter. The first parameter + will contain the pages that could still be retried. + +4. The leftover pages of various types are returned + to the LRU using putback_to_lru_pages() or otherwise + disposed of. The pages will still have the refcount as + increased by isolate_lru_pages() if putback_to_lru_pages() is not + used! The kernel may want to handle the various cases of failures in + different ways. + +B. How migrate_pages() works +---------------------------- + +migrate_pages() does several passes over its list of pages. A page is moved +if all references to a page are removable at the time. The page has +already been removed from the LRU via isolate_lru_page() and the refcount +is increased so that the page cannot be freed while page migration occurs. + +Steps: + +1. Lock the page to be migrated + +2. Insure that writeback is complete. + +3. Make sure that the page has assigned swap cache entry if + it is an anonyous page. The swap cache reference is necessary + to preserve the information contain in the page table maps while + page migration occurs. + +4. Prep the new page that we want to move to. It is locked + and set to not being uptodate so that all accesses to the new + page immediately lock while the move is in progress. + +5. All the page table references to the page are either dropped (file + backed pages) or converted to swap references (anonymous pages). + This should decrease the reference count. + +6. The radix tree lock is taken. This will cause all processes trying + to reestablish a pte to block on the radix tree spinlock. + +7. The refcount of the page is examined and we back out if references remain + otherwise we know that we are the only one referencing this page. + +8. The radix tree is checked and if it does not contain the pointer to this + page then we back out because someone else modified the mapping first. + +9. The mapping is checked. If the mapping is gone then a truncate action may + be in progress and we back out. + +10. The new page is prepped with some settings from the old page so that + accesses to the new page will be discovered to have the correct settings. + +11. The radix tree is changed to point to the new page. + +12. The reference count of the old page is dropped because the radix tree + reference is gone. + +13. The radix tree lock is dropped. With that lookups become possible again + and other processes will move from spinning on the tree lock to sleeping on + the locked new page. + +14. The page contents are copied to the new page. + +15. The remaining page flags are copied to the new page. + +16. The old page flags are cleared to indicate that the page does + not use any information anymore. + +17. Queued up writeback on the new page is triggered. + +18. If swap pte's were generated for the page then replace them with real + ptes. This will reenable access for processes not blocked by the page lock. + +19. The page locks are dropped from the old and new page. + Processes waiting on the page lock can continue. + +20. The new page is moved to the LRU and can be scanned by the swapper + etc again. + +TODO list +--------- + +- Page migration requires the use of swap handles to preserve the + information of the anonymous page table entries. This means that swap + space is reserved but never used. The maximum number of swap handles used + is determined by CHUNK_SIZE (see mm/mempolicy.c) per ongoing migration. + Reservation of pages could be avoided by having a special type of swap + handle that does not require swap space and that would only track the page + references. Something like that was proposed by Marcelo Tosatti in the + past (search for migration cache on lkml or linux-mm@kvack.org). + +- Page migration unmaps ptes for file backed pages and requires page + faults to reestablish these ptes. This could be optimized by somehow + recording the references before migration and then reestablish them later. + However, there are several locking challenges that have to be overcome + before this is possible. + +- Page migration generates read ptes for anonymous pages. Dirty page + faults are required to make the pages writable again. It may be possible + to generate a pte marked dirty if it is known that the page is dirty and + that this process has the only reference to that page. + +Christoph Lameter, March 8, 2006. + |