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authorMel Gorman <mgorman@suse.de>2012-10-08 16:32:33 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-10-09 16:22:49 +0900
commit2a1402aa044b55c2d30ab0ed9405693ef06fb07c (patch)
treed286a88cc0882663143ec67d54cd3b1247ef2829 /mm/mmu_notifier.c
parent661c4cb9b829110cb68c18ea05a56be39f75a4d2 (diff)
mm: compaction: acquire the zone->lru_lock as late as possible
Richard Davies and Shaohua Li have both reported lock contention problems in compaction on the zone and LRU locks as well as significant amounts of time being spent in compaction. This series aims to reduce lock contention and scanning rates to reduce that CPU usage. Richard reported at https://lkml.org/lkml/2012/9/21/91 that this series made a big different to a problem he reported in August: http://marc.info/?l=kvm&m=134511507015614&w=2 Patch 1 defers acquiring the zone->lru_lock as long as possible. Patch 2 defers acquiring the zone->lock as lock as possible. Patch 3 reverts Rik's "skip-free" patches as the core concept gets reimplemented later and the remaining patches are easier to understand if this is reverted first. Patch 4 adds a pageblock-skip bit to the pageblock flags to cache what pageblocks should be skipped by the migrate and free scanners. This drastically reduces the amount of scanning compaction has to do. Patch 5 reimplements something similar to Rik's idea except it uses the pageblock-skip information to decide where the scanners should restart from and does not need to wrap around. I tested this on 3.6-rc6 + linux-next/akpm. Kernels tested were akpm-20120920 3.6-rc6 + linux-next/akpm as of Septeber 20th, 2012 lesslock Patches 1-6 revert Patches 1-7 cachefail Patches 1-8 skipuseless Patches 1-9 Stress high-order allocation tests looked ok. Success rates are more or less the same with the full series applied but there is an expectation that there is less opportunity to race with other allocation requests if there is less scanning. The time to complete the tests did not vary that much and are uninteresting as were the vmstat statistics so I will not present them here. Using ftrace I recorded how much scanning was done by compaction and got this 3.6.0-rc6 3.6.0-rc6 3.6.0-rc6 3.6.0-rc6 3.6.0-rc6 akpm-20120920 lockless revert-v2r2 cachefail skipuseless Total free scanned 360753976 515414028 565479007 17103281 18916589 Total free isolated 2852429 3597369 4048601 670493 727840 Total free efficiency 0.0079% 0.0070% 0.0072% 0.0392% 0.0385% Total migrate scanned 247728664 822729112 1004645830 17946827 14118903 Total migrate isolated 2555324 3245937 3437501 616359 658616 Total migrate efficiency 0.0103% 0.0039% 0.0034% 0.0343% 0.0466% The efficiency is worthless because of the nature of the test and the number of failures. The really interesting point as far as this patch series is concerned is the number of pages scanned. Note that reverting Rik's patches massively increases the number of pages scanned indicating that those patches really did make a difference to CPU usage. However, caching what pageblocks should be skipped has a much higher impact. With patches 1-8 applied, free page and migrate page scanning are both reduced by 95% in comparison to the akpm kernel. If the basic concept of Rik's patches are implemened on top then scanning then the free scanner barely changed but migrate scanning was further reduced. That said, tests on 3.6-rc5 indicated that the last patch had greater impact than what was measured here so it is a bit variable. One way or the other, this series has a large impact on the amount of scanning compaction does when there is a storm of THP allocations. This patch: Compaction's migrate scanner acquires the zone->lru_lock when scanning a range of pages looking for LRU pages to acquire. It does this even if there are no LRU pages in the range. If multiple processes are compacting then this can cause severe locking contention. To make matters worse commit b2eef8c0 ("mm: compaction: minimise the time IRQs are disabled while isolating pages for migration") releases the lru_lock every SWAP_CLUSTER_MAX pages that are scanned. This patch makes two changes to how the migrate scanner acquires the LRU lock. First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages if the lock is contended. This reduces the number of times it unnecessarily disables and re-enables IRQs. The second is that it defers acquiring the LRU lock for as long as possible. If there are no LRU pages or the only LRU pages are transhuge then the LRU lock will not be acquired at all which reduces contention on zone->lru_lock. [minchan@kernel.org: augment comment] [akpm@linux-foundation.org: tweak comment text] Signed-off-by: Mel Gorman <mgorman@suse.de> Acked-by: Rik van Riel <riel@redhat.com> Cc: Richard Davies <richard@arachsys.com> Cc: Shaohua Li <shli@kernel.org> Cc: Avi Kivity <avi@redhat.com> Acked-by: Rafael Aquini <aquini@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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