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Mel Gorman authored
The zonelist cache (zlc) was introduced to skip over zones that were recently known to be full. This avoided expensive operations such as the cpuset checks, watermark calculations and zone_reclaim. The situation today is different and the complexity of zlc is harder to justify. 1) The cpuset checks are no-ops unless a cpuset is active and in general are a lot cheaper. 2) zone_reclaim is now disabled by default and I suspect that was a large source of the cost that zlc wanted to avoid. When it is enabled, it's known to be a major source of stalling when nodes fill up and it's unwise to hit every other user with the overhead. 3) Watermark checks are expensive to calculate for high-order allocation requests. Later patches in this series will reduce the cost of the watermark checking. 4) The most important issue is that in the current implementation it is possible for a failed THP allocation to mark a zone full for order-0 allocations and cause a fallback to remote nodes. The last issue could be addressed with additional complexity but as the benefit of zlc is questionable, it is better to remove it. If stalls due to zone_reclaim are ever reported then an alternative would be to introduce deferring logic based on a timeout inside zone_reclaim itself and leave the page allocator fast paths alone. The impact on page-allocator microbenchmarks is negligible as they don't hit the paths where the zlc comes into play. Most page-reclaim related workloads showed no noticeable difference as a result of the removal. The impact was noticeable in a workload called "stutter". One part uses a lot of anonymous memory, a second measures mmap latency and a third copies a large file. In an ideal world the latency application would not notice the mmap latency. On a 2-node machine the results of this patch are stutter 4.3.0-rc1 4.3.0-rc1 baseline nozlc-v4 Min mmap 20.9243 ( 0.00%) 20.7716 ( 0.73%) 1st-qrtle mmap 22.0612 ( 0.00%) 22.0680 ( -0.03%) 2nd-qrtle mmap 22.3291 ( 0.00%) 22.3809 ( -0.23%) 3rd-qrtle mmap 25.2244 ( 0.00%) 25.2396 ( -0.06%) Max-90% mmap 48.0995 ( 0.00%) 28.3713 ( 41.02%) Max-93% mmap 52.5557 ( 0.00%) 36.0170 ( 31.47%) Max-95% mmap 55.8173 ( 0.00%) 47.3163 ( 15.23%) Max-99% mmap 67.3781 ( 0.00%) 70.1140 ( -4.06%) Max mmap 24447.6375 ( 0.00%) 12915.1356 ( 47.17%) Mean mmap 33.7883 ( 0.00%) 27.7944 ( 17.74%) Best99%Mean mmap 27.7825 ( 0.00%) 25.2767 ( 9.02%) Best95%Mean mmap 26.3912 ( 0.00%) 23.7994 ( 9.82%) Best90%Mean mmap 24.9886 ( 0.00%) 23.2251 ( 7.06%) Best50%Mean mmap 22.0157 ( 0.00%) 22.0261 ( -0.05%) Best10%Mean mmap 21.6705 ( 0.00%) 21.6083 ( 0.29%) Best5%Mean mmap 21.5581 ( 0.00%) 21.4611 ( 0.45%) Best1%Mean mmap 21.3079 ( 0.00%) 21.1631 ( 0.68%) Note that the maximum stall latency went from 24 seconds to 12 which is still bad but an improvement. The milage varies considerably 2-node machine on an earlier test went from 494 seconds to 47 seconds and a 4-node machine that tested an earlier version of this patch went from a worst case stall time of 6 seconds to 67ms. The nature of the benchmark is inherently unpredictable as it is hammering the system and the milage will vary between machines. There is a secondary impact with potentially more direct reclaim because zones are now being considered instead of being skipped by zlc. In this particular test run it did not occur so will not be described. However, in at least one test the following was observed 1. Direct reclaim rates were higher. This was likely due to direct reclaim being entered instead of the zlc disabling a zone and busy looping. Busy looping may have the effect of allowing kswapd to make more progress and in some cases may be better overall. If this is found then the correct action is to put direct reclaimers to sleep on a waitqueue and allow kswapd make forward progress. Busy looping on the zlc is even worse than when the allocator used to blindly call congestion_wait(). 2. There was higher swap activity as direct reclaim was active. 3. Direct reclaim efficiency was lower. This is related to 1 as more scanning activity also encountered more pages that could not be immediately reclaimed In that case, the direct page scan and reclaim rates are noticeable but it is not considered a problem for a few reasons 1. The test is primarily concerned with latency. The mmap attempts are also faulted which means there are THP allocation requests. The ZLC could cause zones to be disabled causing the process to busy loop instead of reclaiming. This looks like elevated direct reclaim activity but it's the correct action to take based on what processes requested. 2. The test hammers reclaim and compaction heavily. The number of successful THP faults is highly variable but affects the reclaim stats. It's not a realistic or reasonable measure of page reclaim activity. 3. No other page-reclaim intensive workload that was tested showed a problem. 4. If a workload is identified that benefitted from the busy looping then it should be fixed by having direct reclaimers sleep on a wait queue until woken by kswapd instead of busy looping. We had this class of problem before when congestion_waits() with a fixed timeout was a brain damaged decision but happened to benefit some workloads. If a workload is identified that relied on the zlc to busy loop then it should be fixed correctly and have a direct reclaimer sleep on a waitqueue until woken by kswapd. Signed-off-by:
Mel Gorman <mgorman@techsingularity.net> Acked-by:
David Rientjes <rientjes@google.com> Acked-by:
Christoph Lameter <cl@linux.com> Acked-by:
Vlastimil Babka <vbabka@suse.cz> Acked-by:
Michal Hocko <mhocko@suse.com> Acked-by:
Johannes Weiner <hannes@cmpxchg.org> Cc: Vitaly Wool <vitalywool@gmail.com> Cc: Rik van Riel <riel@redhat.com> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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