On Thu, Mar 31, 2016 at 10:50:32AM +0200, Vlastimil Babka wrote: > The goal here is to reduce latency (and increase success) of direct async > compaction by making it focus more on the goal of creating a high-order page, > at some expense of thoroughness. > > This is based on an older attempt [1] which I didn't finish as it seemed that > it increased longer-term fragmentation. Now it seems it doesn't, and we have > kcompactd for that goal. The main patch (3) makes migration scanner skip whole > order-aligned blocks as soon as isolation fails in them, as it takes just one > unmigrated page to prevent a high-order buddy page from fully merging. > > Patch 4 then attempts to reduce the excessive freepage scanning (such as > reported in [2]) by allocating migration targets directly from freelists. Here > we just need to be sure that the free pages are not from the same block as the > migrated pages. This is also limited to direct async compaction and is not > meant to replace the more thorough free scanner for other scenarios. I don't like that another algorithm is introduced for async compaction. As you know, we already suffer from corner case that async compaction have (such as compaction deferring doesn't work if we only do async compaction). It makes further analysis/improvement harder. Generally, more difference on async compaction would cause more problem later. In suggested approach, possible risky places I think is finish condition and deferring logic. Scanner meet position would be greatly affected by system load. If there are no processes and async compaction isn't aborted, freepage scanner will be at the end of the zone and we can scan migratable page until we reach there. But, in the other case that the system has some load, async compaction would be aborted easily and freepage scanner will be at the some of point of the zone and async compaction's scanning power can be limited a lot. And, with different algorithm, it doesn't make sense to share same deferring logic. Async compaction can succeed even if sync compaction continually fails. I hope that we don't make async/sync compaction more diverse. I'd be more happy if we can apply such a change to both async/sync direct compaction. > > [1] https://lkml.org/lkml/2014/7/16/988 > [2] http://www.spinics.net/lists/linux-mm/msg97475.html > > Testing was done using stress-highalloc from mmtests, configured for order-4 > GFP_KERNEL allocations: > > 4.6-rc1 4.6-rc1 4.6-rc1 > patch2 patch3 patch4 > Success 1 Min 24.00 ( 0.00%) 27.00 (-12.50%) 43.00 (-79.17%) > Success 1 Mean 30.20 ( 0.00%) 31.60 ( -4.64%) 51.60 (-70.86%) > Success 1 Max 37.00 ( 0.00%) 35.00 ( 5.41%) 73.00 (-97.30%) > Success 2 Min 42.00 ( 0.00%) 32.00 ( 23.81%) 73.00 (-73.81%) > Success 2 Mean 44.00 ( 0.00%) 44.80 ( -1.82%) 78.00 (-77.27%) > Success 2 Max 48.00 ( 0.00%) 52.00 ( -8.33%) 81.00 (-68.75%) > Success 3 Min 91.00 ( 0.00%) 92.00 ( -1.10%) 88.00 ( 3.30%) > Success 3 Mean 92.20 ( 0.00%) 92.80 ( -0.65%) 91.00 ( 1.30%) > Success 3 Max 94.00 ( 0.00%) 93.00 ( 1.06%) 94.00 ( 0.00%) > > While the eager skipping of unsuitable blocks from patch 3 didn't affect > success rates, direct freepage allocation did improve them. Direct freepage allocation changes compaction algorithm a lot. It removes limitation that we cannot get freepages from behind the migration scanner so we can get freepage easily. It would be achieved by other compaction algorithm changes (such as your pivot change or my compaction algorithm change or this patchset). For the long term, this limitation should be removed for sync compaction (at least direct sync compaction), too. What's the reason that you don't apply this algorithm to other cases? Is there any change in fragmentation? Thanks. > > 4.6-rc1 4.6-rc1 4.6-rc1 > patch2 patch3 patch4 > User 2587.42 2566.53 2413.57 > System 482.89 471.20 461.71 > Elapsed 1395.68 1382.00 1392.87 > > Times are not so useful metric for this benchmark as main portion is the > interfering kernel builds, but results do hint at reduced system times. > > 4.6-rc1 4.6-rc1 4.6-rc1 > patch2 patch3 patch4 > Direct pages scanned 163614 159608 123385 > Kswapd pages scanned 2070139 2078790 2081385 > Kswapd pages reclaimed 2061707 2069757 2073723 > Direct pages reclaimed 163354 159505 122304 > > Reduced direct reclaim was unintended, but could be explained by more > successful first attempt at (async) direct compaction, which is attempted > before the first reclaim attempt in __alloc_pages_slowpath(). > > Compaction stalls 33052 39853 55091 > Compaction success 12121 19773 37875 > Compaction failures 20931 20079 17216 > > Compaction is indeed more successful, and thus less likely to get deferred, > so there are also more direct compaction stalls. > > Page migrate success 3781876 3326819 2790838 > Page migrate failure 45817 41774 38113 > Compaction pages isolated 7868232 6941457 5025092 > Compaction migrate scanned 168160492 127269354 87087993 > Compaction migrate prescanned 0 0 0 > Compaction free scanned 2522142582 2326342620 743205879 > Compaction free direct alloc 0 0 920792 > Compaction free dir. all. miss 0 0 5865 > Compaction cost 5252 4476 3602 > > Patch 2 reduces migration scanned pages by 25% thanks to the eager skipping. > Patch 3 reduces free scanned pages by 70%. The portion of direct allocation > misses to all direct allocations is less than 1% which should be acceptable. > Interestingly, patch 3 also reduces migration scanned pages by another 30% on > top of patch 2. The reason is not clear, but we can rejoice nevertheless. s/Patch 2/Patch 3 s/Patch 3/Patch 4 > Vlastimil Babka (4): > mm, compaction: wrap calculating first and last pfn of pageblock > mm, compaction: reduce spurious pcplist drains > mm, compaction: skip blocks where isolation fails in async direct > compaction > mm, compaction: direct freepage allocation for async direct compaction > > include/linux/vm_event_item.h | 1 + > mm/compaction.c | 189 ++++++++++++++++++++++++++++++++++-------- > mm/internal.h | 5 ++ > mm/page_alloc.c | 27 ++++++ > mm/vmstat.c | 2 + > 5 files changed, 191 insertions(+), 33 deletions(-) > > -- > 2.7.3 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. 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