From b795854b1fa70f6aee923ae5df74ff7afeaddcaa Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Mon, 7 Oct 2013 11:29:07 +0100 Subject: sched/numa: Set preferred NUMA node based on number of private faults Ideally it would be possible to distinguish between NUMA hinting faults that are private to a task and those that are shared. If treated identically there is a risk that shared pages bounce between nodes depending on the order they are referenced by tasks. Ultimately what is desirable is that task private pages remain local to the task while shared pages are interleaved between sharing tasks running on different nodes to give good average performance. This is further complicated by THP as even applications that partition their data may not be partitioning on a huge page boundary. To start with, this patch assumes that multi-threaded or multi-process applications partition their data and that in general the private accesses are more important for cpu->memory locality in the general case. Also, no new infrastructure is required to treat private pages properly but interleaving for shared pages requires additional infrastructure. To detect private accesses the pid of the last accessing task is required but the storage requirements are a high. This patch borrows heavily from Ingo Molnar's patch "numa, mm, sched: Implement last-CPU+PID hash tracking" to encode some bits from the last accessing task in the page flags as well as the node information. Collisions will occur but it is better than just depending on the node information. Node information is then used to determine if a page needs to migrate. The PID information is used to detect private/shared accesses. The preferred NUMA node is selected based on where the maximum number of approximately private faults were measured. Shared faults are not taken into consideration for a few reasons. First, if there are many tasks sharing the page then they'll all move towards the same node. The node will be compute overloaded and then scheduled away later only to bounce back again. Alternatively the shared tasks would just bounce around nodes because the fault information is effectively noise. Either way accounting for shared faults the same as private faults can result in lower performance overall. The second reason is based on a hypothetical workload that has a small number of very important, heavily accessed private pages but a large shared array. The shared array would dominate the number of faults and be selected as a preferred node even though it's the wrong decision. The third reason is that multiple threads in a process will race each other to fault the shared page making the fault information unreliable. Signed-off-by: Mel Gorman [ Fix complication error when !NUMA_BALANCING. ] Reviewed-by: Rik van Riel Cc: Andrea Arcangeli Cc: Johannes Weiner Cc: Srikar Dronamraju Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/1381141781-10992-30-git-send-email-mgorman@suse.de Signed-off-by: Ingo Molnar --- mm/mmzone.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) (limited to 'mm/mmzone.c') diff --git a/mm/mmzone.c b/mm/mmzone.c index 2ac0afbd68f3..25bb477deb26 100644 --- a/mm/mmzone.c +++ b/mm/mmzone.c @@ -97,20 +97,20 @@ void lruvec_init(struct lruvec *lruvec) INIT_LIST_HEAD(&lruvec->lists[lru]); } -#if defined(CONFIG_NUMA_BALANCING) && !defined(LAST_NID_NOT_IN_PAGE_FLAGS) -int page_nid_xchg_last(struct page *page, int nid) +#if defined(CONFIG_NUMA_BALANCING) && !defined(LAST_NIDPID_NOT_IN_PAGE_FLAGS) +int page_nidpid_xchg_last(struct page *page, int nidpid) { unsigned long old_flags, flags; - int last_nid; + int last_nidpid; do { old_flags = flags = page->flags; - last_nid = page_nid_last(page); + last_nidpid = page_nidpid_last(page); - flags &= ~(LAST_NID_MASK << LAST_NID_PGSHIFT); - flags |= (nid & LAST_NID_MASK) << LAST_NID_PGSHIFT; + flags &= ~(LAST_NIDPID_MASK << LAST_NIDPID_PGSHIFT); + flags |= (nidpid & LAST_NIDPID_MASK) << LAST_NIDPID_PGSHIFT; } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags)); - return last_nid; + return last_nidpid; } #endif -- cgit v1.2.3