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SubjectRe: [PATCH] mm, vmscan: Do not special-case slab reclaim when watermarks are boosted
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On 8/8/19 8:29 PM, Mel Gorman wrote:

...

> Removing the special casing can still indirectly help fragmentation by

I think you mean e.g. 'against fragmentation'?

> avoiding fragmentation-causing events due to slab allocation as pages
> from a slab pageblock will have some slab objects freed. Furthermore,
> with the special casing, reclaim behaviour is unpredictable as kswapd
> sometimes examines slab and sometimes does not in a manner that is tricky
> to tune or analyse.
>
> This patch removes the special casing. The downside is that this is not a
> universal performance win. Some benchmarks that depend on the residency
> of data when rereading metadata may see a regression when slab reclaim
> is restored to its original behaviour. Similarly, some benchmarks that
> only read-once or write-once may perform better when page reclaim is too
> aggressive. The primary upside is that slab shrinker is less surprising
> (arguably more sane but that's a matter of opinion), behaves consistently
> regardless of the fragmentation state of the system and properly obeys
> VM sysctls.
>
> A fsmark benchmark configuration was constructed similar to
> what Dave reported and is codified by the mmtest configuration
> config-io-fsmark-small-file-stream. It was evaluated on a 1-socket machine
> to avoid dealing with NUMA-related issues and the timing of reclaim. The
> storage was an SSD Samsung Evo and a fresh trimmed XFS filesystem was
> used for the test data.
>
> This is not an exact replication of Dave's setup. The configuration
> scales its parameters depending on the memory size of the SUT to behave
> similarly across machines. The parameters mean the first sample reported
> by fs_mark is using 50% of RAM which will barely be throttled and look
> like a big outlier. Dave used fake NUMA to have multiple kswapd instances
> which I didn't replicate. Finally, the number of iterations differ from
> Dave's test as the target disk was not large enough. While not identical,
> it should be representative.
>
> fsmark
> 5.3.0-rc3 5.3.0-rc3
> vanilla shrinker-v1r1
> Min 1-files/sec 4444.80 ( 0.00%) 4765.60 ( 7.22%)
> 1st-qrtle 1-files/sec 5005.10 ( 0.00%) 5091.70 ( 1.73%)
> 2nd-qrtle 1-files/sec 4917.80 ( 0.00%) 4855.60 ( -1.26%)
> 3rd-qrtle 1-files/sec 4667.40 ( 0.00%) 4831.20 ( 3.51%)
> Max-1 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
> Max-5 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
> Max-10 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
> Max-90 1-files/sec 4649.60 ( 0.00%) 4780.70 ( 2.82%)
> Max-95 1-files/sec 4491.00 ( 0.00%) 4768.20 ( 6.17%)
> Max-99 1-files/sec 4491.00 ( 0.00%) 4768.20 ( 6.17%)
> Max 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
> Hmean 1-files/sec 5004.75 ( 0.00%) 5075.96 ( 1.42%)
> Stddev 1-files/sec 1778.70 ( 0.00%) 1369.66 ( 23.00%)
> CoeffVar 1-files/sec 33.70 ( 0.00%) 26.05 ( 22.71%)
> BHmean-99 1-files/sec 5053.72 ( 0.00%) 5101.52 ( 0.95%)
> BHmean-95 1-files/sec 5053.72 ( 0.00%) 5101.52 ( 0.95%)
> BHmean-90 1-files/sec 5107.05 ( 0.00%) 5131.41 ( 0.48%)
> BHmean-75 1-files/sec 5208.45 ( 0.00%) 5206.68 ( -0.03%)
> BHmean-50 1-files/sec 5405.53 ( 0.00%) 5381.62 ( -0.44%)
> BHmean-25 1-files/sec 6179.75 ( 0.00%) 6095.14 ( -1.37%)
>
> 5.3.0-rc3 5.3.0-rc3
> vanillashrinker-v1r1
> Duration User 501.82 497.29
> Duration System 4401.44 4424.08
> Duration Elapsed 8124.76 8358.05
>
> This is showing a slight skew for the max result representing a
> large outlier for the 1st, 2nd and 3rd quartile are similar indicating
> that the bulk of the results show little difference. Note that an
> earlier version of the fsmark configuration showed a regression but
> that included more samples taken while memory was still filling.
>
> Note that the elapsed time is higher. Part of this is that the
> configuration included time to delete all the test files when the test
> completes -- the test automation handles the possibility of testing fsmark
> with multiple thread counts. Without the patch, many of these objects
> would be memory resident which is part of what the patch is addressing.
>
> There are other important observations that justify the patch.
>
> 1. With the vanilla kernel, the number of dirty pages in the system
> is very low for much of the test. With this patch, dirty pages
> is generally kept at 10% which matches vm.dirty_background_ratio
> which is normal expected historical behaviour.
>
> 2. With the vanilla kernel, the ratio of Slab/Pagecache is close to
> 0.95 for much of the test i.e. Slab is being left alone and dominating
> memory consumption. With the patch applied, the ratio varies between
> 0.35 and 0.45 with the bulk of the measured ratios roughly half way
> between those values. This is a different balance to what Dave reported
> but it was at least consistent.
>
> 3. Slabs are scanned throughout the entire test with the patch applied.
> The vanille kernel has periods with no scan activity and then relatively
> massive spikes.
>
> 4. Without the patch, kswapd scan rates are very variable. With the patch,
> the scan rates remain quite stead.
>
> 4. Overall vmstats are closer to normal expectations
>
> 5.3.0-rc3 5.3.0-rc3
> vanilla shrinker-v1r1
> Ops Direct pages scanned 99388.00 328410.00
> Ops Kswapd pages scanned 45382917.00 33451026.00
> Ops Kswapd pages reclaimed 30869570.00 25239655.00
> Ops Direct pages reclaimed 74131.00 5830.00
> Ops Kswapd efficiency % 68.02 75.45
> Ops Kswapd velocity 5585.75 4002.25
> Ops Page reclaim immediate 1179721.00 430927.00
> Ops Slabs scanned 62367361.00 73581394.00
> Ops Direct inode steals 2103.00 1002.00
> Ops Kswapd inode steals 570180.00 5183206.00
>
> o Vanilla kernel is hitting direct reclaim more frequently,
> not very much in absolute terms but the fact the patch
> reduces it is interesting
> o "Page reclaim immediate" in the vanilla kernel indicates
> dirty pages are being encountered at the tail of the LRU.
> This is generally bad and means in this case that the LRU
> is not long enough for dirty pages to be cleaned by the
> background flush in time. This is much reduced by the
> patch.
> o With the patch, kswapd is reclaiming 10 times more slab
> pages than with the vanilla kernel. This is indicative
> of the watermark boosting over-protecting slab
>
> A more complete set of tests were run that were part of the basis
> for introducing boosting and while there are some differences, they
> are well within tolerances.
>
> Bottom line, the special casing kswapd to avoid slab behaviour is
> unpredictable and can lead to abnormal results for normal workloads. This
> patch restores the expected behaviour that slab and page cache is
> balanced consistently for a workload with a steady allocation ratio of
> slab/pagecache pages. It also means that if there are workloads that
> favour the preservation of slab over pagecache that it can be tuned via
> vm.vfs_cache_pressure where as the vanilla kernel effectively ignores
> the parameter when boosting is active.
>
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> Reviewed-by: Dave Chinner <dchinner@redhat.com>
> Cc: stable@vger.kernel.org # v5.0+

Acked-by: Vlastimil Babka <vbabka@suse.cz>

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