Re: [RFC PATCH 0/8] Reduce filesystem writeback from page reclaim v2

Minchan Kim
2011-07-27T16:18:33Z

On Thu, Jul 21, 2011 at 05:28:42PM +0100, Mel Gorman wrote:
> Warning: Long post with lots of figures. If you normally drink coffee
> and you don't have a cup, get one or you may end up with a case of
> keyboard face.

At last, I get a coffee.

>
> Changelog since v1
> o Drop prio-inode patch. There is now a dependency that the flusher
> threads find these dirty pages quickly.
> o Drop nr_vmscan_throttled counter
> o SetPageReclaim instead of deactivate_page which was wrong
> o Add warning to main filesystems if called from direct reclaim context
> o Add patch to completely disable filesystem writeback from reclaim

It seems to go to the very desirable way.

>
> Testing from the XFS folk revealed that there is still too much
> I/O from the end of the LRU in kswapd. Previously it was considered
> acceptable by VM people for a small number of pages to be written
> back from reclaim with testing generally showing about 0.3% of pages
> reclaimed were written back (higher if memory was low). That writing
> back a small number of pages is ok has been heavily disputed for
> quite some time and Dave Chinner explained it well;
>
> It doesn't have to be a very high number to be a problem. IO
> is orders of magnitude slower than the CPU time it takes to
> flush a page, so the cost of making a bad flush decision is
> very high. And single page writeback from the LRU is almost
> always a bad flush decision.
>
> To complicate matters, filesystems respond very differently to requests
> from reclaim according to Christoph Hellwig;
>
> xfs tries to write it back if the requester is kswapd
> ext4 ignores the request if it's a delayed allocation
> btrfs ignores the request
>
> As a result, each filesystem has different performance characteristics
> when under memory pressure and there are many pages being dirties. In
> some cases, the request is ignored entirely so the VM cannot depend
> on the IO being dispatched.
>
> The objective of this series to to reduce writing of filesystem-backed
> pages from reclaim, play nicely with writeback that is already in
> progress and throttle reclaim appropriately when dirty pages are
> encountered. The assumption is that the flushers will always write
> pages faster than if reclaim issues the IO. The new problem is that
> reclaim has very little control over how long before a page in a
> particular zone or container is cleaned which is discussed later. A
> secondary goal is to avoid the problem whereby direct reclaim splices
> two potentially deep call stacks together.
>
> Patch 1 disables writeback of filesystem pages from direct reclaim
> entirely. Anonymous pages are still written.
>
> Patches 2-4 add warnings to XFS, ext4 and btrfs if called from
> direct reclaim. With patch 1, this "never happens" and
> is intended to catch regressions in this logic in the
> future.
>
> Patch 5 disables writeback of filesystem pages from kswapd unless
> the priority is raised to the point where kswapd is considered
> to be in trouble.
>
> Patch 6 throttles reclaimers if too many dirty pages are being
> encountered and the zones or backing devices are congested.
>
> Patch 7 invalidates dirty pages found at the end of the LRU so they
> are reclaimed quickly after being written back rather than
> waiting for a reclaimer to find them
>
> Patch 8 disables writeback of filesystem pages from kswapd and
> depends entirely on the flusher threads for cleaning pages.
> This is potentially a problem if the flusher threads take a
> long time to wake or are not discovering the pages we need
> cleaned. By placing the patch last, it's more likely that
> bisection can catch if this situation occurs and can be
> easily reverted.

Patch ordering is good, too.

>
> I consider this series to be orthogonal to the writeback work but
> it is worth noting that the writeback work affects the viability of
> patch 8 in particular.
>
> I tested this on ext4 and xfs using fs_mark and a micro benchmark
> that does a streaming write to a large mapping (exercises use-once
> LRU logic) followed by streaming writes to a mix of anonymous and
> file-backed mappings. The command line for fs_mark when botted with
> 512M looked something like
>
> ./fs_mark -d /tmp/fsmark-2676 -D 100 -N 150 -n 150 -L 25 -t 1 -S0 -s 10485760
>
> The number of files was adjusted depending on the amount of available
> memory so that the files created was about 3xRAM. For multiple threads,
> the -d switch is specified multiple times.
>
> 3 kernels are tested.
>
> vanilla 3.0-rc6
> kswapdwb-v2r5 patches 1-7
> nokswapdwb-v2r5 patches 1-8
>
> The test machine is x86-64 with an older generation of AMD processor
> with 4 cores. The underlying storage was 4 disks configured as RAID-0
> as this was the best configuration of storage I had available. Swap
> is on a separate disk. Dirty ratio was tuned to 40% instead of the
> default of 20%.
>
> Testing was run with and without monitors to both verify that the
> patches were operating as expected and that any performance gain was
> real and not due to interference from monitors.

Wow, it seems you would take a long time to finish your experiments.
Thanks for sharing good data.

>
> I've posted the raw reports for each filesystem at
>
> http://www.csn.ul.ie/~mel/postings/reclaim-20110721
>
> Unfortunately, the volume of data is excessive but here is a partial
> summary of what was interesting for XFS.
>
> 512M1P-xfs Files/s mean 32.99 ( 0.00%) 35.16 ( 6.18%) 35.08 ( 5.94%)
> 512M1P-xfs Elapsed Time fsmark 122.54 115.54 115.21
> 512M1P-xfs Elapsed Time mmap-strm 105.09 104.44 106.12
> 512M-xfs Files/s mean 30.50 ( 0.00%) 33.30 ( 8.40%) 34.68 (12.06%)
> 512M-xfs Elapsed Time fsmark 136.14 124.26 120.33
> 512M-xfs Elapsed Time mmap-strm 154.68 145.91 138.83
> 512M-2X-xfs Files/s mean 28.48 ( 0.00%) 32.90 (13.45%) 32.83 (13.26%)
> 512M-2X-xfs Elapsed Time fsmark 145.64 128.67 128.67
> 512M-2X-xfs Elapsed Time mmap-strm 145.92 136.65 137.67
> 512M-4X-xfs Files/s mean 29.06 ( 0.00%) 32.82 (11.46%) 33.32 (12.81%)
> 512M-4X-xfs Elapsed Time fsmark 153.69 136.74 135.11
> 512M-4X-xfs Elapsed Time mmap-strm 159.47 128.64 132.59
> 512M-16X-xfs Files/s mean 48.80 ( 0.00%) 41.80 (-16.77%) 56.61 (13.79%)
> 512M-16X-xfs Elapsed Time fsmark 161.48 144.61 141.19
> 512M-16X-xfs Elapsed Time mmap-strm 167.04 150.62 147.83
>
> The difference between kswapd writing and not writing for fsmark
> in many cases is marginal simply because kswapd was not reaching a
> high enough priority to enter writeback. Memory is mostly consumed
> by filesystem-backed pages so limiting the number of dirty pages
> (dirty_ratio == 40) means that kswapd always makes forward progress
> and avoids the OOM killer.

Looks promising as most of elapsed time is lower than vanilla.

>
> For the streaming-write benchmark, it does make a small difference as
> kswapd is reaching the higher priorities there due to a large number
> of anonymous pages added to the mix. The performance difference is
> marginal though as the number of filesystem pages written is about
> 1/50th of the number of anonymous pages written so it is drowned out.

It does make sense.

>
> I was initially worried about 512M-16X-xfs but it's well within the noise
> looking at the standard deviations from
> http://www.csn.ul.ie/~mel/postings/reclaim-20110721/html-no-monitor/global-dhp-512M-16X__writeback-reclaimdirty-xfs/hydra/comparison.html
>
> Files/s min 25.00 ( 0.00%) 31.10 (19.61%) 32.00 (21.88%)
> Files/s mean 48.80 ( 0.00%) 41.80 (-16.77%) 56.61 (13.79%)
> Files/s stddev 28.65 ( 0.00%) 11.32 (-153.19%) 32.79 (12.62%)
> Files/s max 133.20 ( 0.00%) 81.60 (-63.24%) 154.00 (13.51%)

Yes. it's within the noise so let's not worry about that.

>
> 64 threads writing on a machine with 4 CPUs with 512M RAM has variable
> performance which is hardly surprising.

Fair enough.

>
> The streaming-write benchmarks all completed faster.
>
> The tests were also run with mem=1024M and mem=4608M with the relative
> performance improvement reduced as memory increases reflecting that
> with enough memory there are fewer writes from reclaim as the flusher
> threads have time to clean the page before it reaches the end of
> the LRU.
>
> Here is the same tests except when using ext4
>
> 512M1P-ext4 Files/s mean 37.36 ( 0.00%) 37.10 (-0.71%) 37.66 ( 0.78%)
> 512M1P-ext4 Elapsed Time fsmark 108.93 109.91 108.61
> 512M1P-ext4 Elapsed Time mmap-strm 112.15 108.93 109.10
> 512M-ext4 Files/s mean 30.83 ( 0.00%) 39.80 (22.54%) 32.74 ( 5.83%)
> 512M-ext4 Elapsed Time fsmark 368.07 322.55 328.80
> 512M-ext4 Elapsed Time mmap-strm 131.98 117.01 118.94
> 512M-2X-ext4 Files/s mean 20.27 ( 0.00%) 22.75 (10.88%) 20.80 ( 2.52%)
> 512M-2X-ext4 Elapsed Time fsmark 518.06 493.74 479.21
> 512M-2X-ext4 Elapsed Time mmap-strm 131.32 126.64 117.05
> 512M-4X-ext4 Files/s mean 17.91 ( 0.00%) 12.30 (-45.63%) 16.58 (-8.06%)
> 512M-4X-ext4 Elapsed Time fsmark 633.41 660.70 572.74
> 512M-4X-ext4 Elapsed Time mmap-strm 137.85 127.63 124.07
> 512M-16X-ext4 Files/s mean 55.86 ( 0.00%) 69.90 (20.09%) 42.66 (-30.94%)
> 512M-16X-ext4 Elapsed Time fsmark 543.21 544.43 586.16
> 512M-16X-ext4 Elapsed Time mmap-strm 141.84 146.12 144.01
>
> At first glance, the benefit for ext4 is less clear cut but this
> is due to the standard deviation being very high. Take 512M-4X-ext4
> showing a 45.63% regression for example and we see.
>
> Files/s min 5.40 ( 0.00%) 4.10 (-31.71%) 6.50 (16.92%)
> Files/s mean 17.91 ( 0.00%) 12.30 (-45.63%) 16.58 (-8.06%)
> Files/s stddev 14.34 ( 0.00%) 8.04 (-78.46%) 14.50 ( 1.04%)
> Files/s max 54.30 ( 0.00%) 37.70 (-44.03%) 77.20 (29.66%)
>
> The standard deviation is *massive* meaning that the performance
> loss is well within the noise. The main positive out of this is the

Yes.
ext4 seems to be very sensitive on the situation.

> streaming write benchmarks are generally better.
>
> Where it does benefit is stalls in direct reclaim. Unlike xfs, ext4
> can stall direct reclaim writing back pages. When I look at a separate
> run using ftrace to gather more information, I see;
>
> 512M-ext4 Time stalled direct reclaim fsmark 0.36 0.30 0.31
> 512M-ext4 Time stalled direct reclaim mmap-strm 36.88 7.48 36.24

This data is odd.
[2] and [3] experiment's elapsed time is almost same(117.01, 118.94) but stall time in direct reclaim of
[2] is much fast. Hmm??
Anyway, if we don't write out in kswapd, it seems we can enter direct reclaim path so many time.

> 512M-4X-ext4 Time stalled direct reclaim fsmark 1.06 0.40 0.43
> 512M-4X-ext4 Time stalled direct reclaim mmap-strm 102.68 33.18 23.99
> 512M-16X-ext4 Time stalled direct reclaim fsmark 0.17 0.27 0.30
> 512M-16X-ext4 Time stalled direct reclaim mmap-strm 9.80 2.62 1.28
> 512M-32X-ext4 Time stalled direct reclaim fsmark 0.00 0.00 0.00
> 512M-32X-ext4 Time stalled direct reclaim mmap-strm 2.27 0.51 1.26
>
> Time spent in direct reclaim is reduced implying that bug reports
> complaining about the system becoming jittery when copying large
> files may also be hel.

It would be very good thing.

>
> To show what effect the patches are having, this is a more detailed
> look at one of the tests running with monitoring enabled. It's booted
> with mem=512M and the number of threads running is equal to the number
> of CPU cores. The backing filesystem is XFS.
>
> FS-Mark
> fsmark-3.0.0 3.0.0-rc6 3.0.0-rc6
> rc6-vanilla kswapwb-v2r5 nokswapwb-v2r5
> Files/s min 27.30 ( 0.00%) 31.80 (14.15%) 31.40 (13.06%)
> Files/s mean 30.32 ( 0.00%) 34.34 (11.73%) 34.52 (12.18%)
> Files/s stddev 1.39 ( 0.00%) 1.06 (-31.96%) 1.20 (-16.05%)
> Files/s max 33.60 ( 0.00%) 36.00 ( 6.67%) 36.30 ( 7.44%)
> Overhead min 1393832.00 ( 0.00%) 1793141.00 (-22.27%) 1133240.00 (23.00%)
> Overhead mean 2423808.52 ( 0.00%) 2513297.40 (-3.56%) 1823398.44 (32.93%)
> Overhead stddev 445880.26 ( 0.00%) 392952.66 (13.47%) 420498.38 ( 6.04%)
> Overhead max 3359477.00 ( 0.00%) 3184889.00 ( 5.48%) 3016170.00 (11.38%)
> MMTests Statistics: duration
> User/Sys Time Running Test (seconds) 53.26 52.27 51.88

What is User/Sys?

> Total Elapsed Time (seconds) 137.65 121.95 121.11
>
> Average files per second is increased by a nice percentage that is
> outside the noise. This is also true when I look at the results

Sure.

> without monitoring although the relative performance gain is less.
>
> Time to completion is reduced which is always good ane as it implies
> that IO was consistently higher and this is clearly visible at
>
> http://www.csn.ul.ie/~mel/postings/reclaim-20110721/html-run-monitor/global-dhp-512M__writeback-reclaimdirty-xfs/hydra/blockio-comparison-hydra.png
> http://www.csn.ul.ie/~mel/postings/reclaim-20110721/html-run-monitor/global-dhp-512M__writeback-reclaimdirty-xfs/hydra/blockio-comparison-smooth-hydra.png
>
> kswapd CPU usage is also interesting
>
> http://www.csn.ul.ie/~mel/postings/reclaim-20110721/html-run-monitor/global-dhp-512M__writeback-reclaimdirty-xfs/hydra/kswapdcpu-comparison-smooth-hydra.png
>
> Note how preventing kswapd reclaiming dirty pages pushes up its CPU
> usage as it scans more pages but it does not get excessive due to
> the throttling.

Good to hear.
The concern of this patchset was early OOM kill with too many scanning.
I can throw such concern out from now on.

>
> MMTests Statistics: vmstat
> Page Ins 1481672 1352900 1105364
> Page Outs 38397462 38337199 38366073
> Swap Ins 351918 320883 258868
> Swap Outs 132060 117715 123564
> Direct pages scanned 886587 968087 784109
> Kswapd pages scanned 18931089 18275983 18324613
> Kswapd pages reclaimed 8878200 8768648 8885482
> Direct pages reclaimed 883407 960496 781632
> Kswapd efficiency 46% 47% 48%
> Kswapd velocity 137530.614 149864.559 151305.532
> Direct efficiency 99% 99% 99%
> Direct velocity 6440.879 7938.393 6474.354
> Percentage direct scans 4% 5% 4%
> Page writes by reclaim 170014 117717 123510
> Page reclaim invalidate 0 1221396 1212857
> Page reclaim throttled 0 0 0
> Slabs scanned 23424 23680 23552
> Direct inode steals 0 0 0
> Kswapd inode steals 5560 5500 5584
> Kswapd skipped wait 20 3 5
> Compaction stalls 0 0 0
> Compaction success 0 0 0
> Compaction failures 0 0 0
> Compaction pages moved 0 0 0
> Compaction move failure 0 0 0
>
> These stats are based on information from /proc/vmstat
>
> "Kswapd efficiency" is the percentage of pages reclaimed to pages
> scanned. The higher the percentage is the better because a low
> percentage implies that kswapd is scanning uselessly. As the workload
> dirties memory heavily and is a small machine, the efficiency is low at
> 46% and marginally improves due to a reduced number of pages scanned.
> As memory increases, so does the efficiency as one might expect as
> the flushers have a chance to clean the pages in time.
>
> "Kswapd velocity" is the average number of pages scanned per
> second. The patches increase this as it's no longer getting blocked on
> page writes so it's expected but in general a higher velocity means
> that kswapd is doing more work and consuming more CPU. In this case,
> it is offset by the fact that fewer pages overall are scanned and
> the test completes faster but it explains why CPU usage is higher.

Fair enough.

>
> Page writes by reclaim is what is motivating this series. It goes
> from 170014 pages to 123510 which is a big improvement and we'll see
> later that these writes are for anonymous pages.
>
> "Page reclaim invalided" is very high and implies that a large number
> of dirty pages are reaching the end of the list quickly. Unfortunately,
> this is somewhat unavoidable. Kswapd is scanning pages at a rate
> of roughly 125000 (or 488M) a second on a 512M machine. The best
> possible writing rate of the underlying storage is about 300M/second.
> With the rate of reclaim exceeding the best possible writing speed,
> the system is going to get throttled.

Just out of curiosity.
What is 'Page reclaim throttled'?

>
> FTrace Reclaim Statistics: vmscan
> fsmark-3.0.0 3.0.0-rc6 3.0.0-rc6
> rc6-vanilla kswapwb-v2r5 nokswapwb-v2r5
> Direct reclaims 16173 17605 14313
> Direct reclaim pages scanned 886587 968087 784109
> Direct reclaim pages reclaimed 883407 960496 781632
> Direct reclaim write file async I/O 0 0 0
> Direct reclaim write anon async I/O 0 0 0
> Direct reclaim write file sync I/O 0 0 0
> Direct reclaim write anon sync I/O 0 0 0
> Wake kswapd requests 20699 22048 22893
> Kswapd wakeups 24 20 25
> Kswapd pages scanned 18931089 18275983 18324613
> Kswapd pages reclaimed 8878200 8768648 8885482
> Kswapd reclaim write file async I/O 37966 0 0
> Kswapd reclaim write anon async I/O 132062 117717 123567
> Kswapd reclaim write file sync I/O 0 0 0
> Kswapd reclaim write anon sync I/O 0 0 0
> Time stalled direct reclaim (seconds) 0.08 0.09 0.08
> Time kswapd awake (seconds) 132.11 117.78 115.82
>
> Total pages scanned 19817676 19244070 19108722
> Total pages reclaimed 9761607 9729144 9667114
> %age total pages scanned/reclaimed 49.26% 50.56% 50.59%
> %age total pages scanned/written 0.86% 0.61% 0.65%
> %age file pages scanned/written 0.19% 0.00% 0.00%
> Percentage Time Spent Direct Reclaim 0.15% 0.17% 0.15%
> Percentage Time kswapd Awake 95.98% 96.58% 95.63%
>
> Despite kswapd having higher CPU usage, it spent less time awake which
> is probably a reflection of the test completing faster. File writes

Make sense.

> from kswapd were 0 with the patches applied implying that kswapd was
> not getting to a priority high enough to start writing. The remaining
> writes correlate almost exactly to nr_vmscan_write implying that all
> writes were for anonymous pages.
>
> FTrace Reclaim Statistics: congestion_wait
> Direct number congest waited 0 0 0
> Direct time congest waited 0ms 0ms 0ms
> Direct full congest waited 0 0 0
> Direct number conditional waited 2 17 6
> Direct time conditional waited 0ms 0ms 0ms
> Direct full conditional waited 0 0 0
> KSwapd number congest waited 4 8 10
> KSwapd time congest waited 4ms 20ms 8ms
> KSwapd full congest waited 0 0 0
> KSwapd number conditional waited 0 26036 26283
> KSwapd time conditional waited 0ms 16ms 4ms
> KSwapd full conditional waited 0 0 0

What means congest and conditional?
congest is trace_writeback_congestion_wait and conditional is trace_writeback_wait_iff_congested?

>
> This is based on some of the writeback tracepoints. It's interesting
> to note that while kswapd got throttled about 26000 times with all
> patches applied, it spent negligible time asleep so probably just
> called cond_resched(). This implies that neither the zone nor the
> backing device are rarely truly congested and throttling is necessary
> simply to allow the pages to be written.
>
> MICRO
> MMTests Statistics: duration
> User/Sys Time Running Test (seconds) 32.57 31.18 30.52
> Total Elapsed Time (seconds) 166.29 141.94 148.23
>
> This test is in two stages. The first writes only to a file. The second
> writes to a mix of anonymous and file mappings. Time to completion
> is improved and this is still true with monitoring disabled.

Good.

>
> MMTests Statistics: vmstat
> Page Ins 11018260 10668536 10792204
> Page Outs 16632838 16468468 16449897
> Swap Ins 296167 245878 256038
> Swap Outs 221626 177922 179409
> Direct pages scanned 4129424 5172015 3686598
> Kswapd pages scanned 9152837 9000480 7909180
> Kswapd pages reclaimed 3388122 3284663 3371737
> Direct pages reclaimed 735425 765263 708713
> Kswapd efficiency 37% 36% 42%
> Kswapd velocity 55041.416 63410.455 53357.485
> Direct efficiency 17% 14% 19%
> Direct velocity 24832.666 36438.037 24870.795
> Percentage direct scans 31% 36% 31%
> Page writes by reclaim 347283 180065 179425
> Page writes skipped 0 0 0
> Page reclaim invalidate 0 864018 554666
> Write invalidated 0 0 0
> Page reclaim throttled 0 0 0
> Slabs scanned 14464 13696 13952
> Direct inode steals 470 864 934
> Kswapd inode steals 426 411 317
> Kswapd skipped wait 3255 3381 1437
> Compaction stalls 0 0 2
> Compaction success 0 0 1
> Compaction failures 0 0 1
> Compaction pages moved 0 0 0
> Compaction move failure 0 0 0
>
> Kswapd efficiency is improved slightly. kswapd is operating at roughly
> the same velocity but the number of pages scanned is far lower due
> to the test completing faster.
>
> Direct reclaim efficiency is improved slightly and scanning fewer pages
> (again due to lower time to completion).
>
> Fewer pages are being written from reclaim.
>
> FTrace Reclaim Statistics: vmscan
> micro-3.0.0 3.0.0-rc6 3.0.0-rc6
> rc6-vanilla kswapwb-v2r5 nokswapwb-v2r5
> Direct reclaims 14060 15425 13726
> Direct reclaim pages scanned 3596218 4621037 3613503
> Direct reclaim pages reclaimed 735425 765263 708713
> Direct reclaim write file async I/O 87264 0 0
> Direct reclaim write anon async I/O 10030 9127 15028
> Direct reclaim write file sync I/O 0 0 0
> Direct reclaim write anon sync I/O 0 0 0
> Wake kswapd requests 10424 10346 10786
> Kswapd wakeups 22 22 14
> Kswapd pages scanned 9041353 8889081 7895846
> Kswapd pages reclaimed 3388122 3284663 3371737
> Kswapd reclaim write file async I/O 7277 1710 0
> Kswapd reclaim write anon async I/O 184205 159178 162367
> Kswapd reclaim write file sync I/O 0 0 0
> Kswapd reclaim write anon sync I/O 0 0 0
> Time stalled direct reclaim (seconds) 54.29 5.67 14.29
> Time kswapd awake (seconds) 151.62 129.83 135.98
>
> Total pages scanned 12637571 13510118 11509349
> Total pages reclaimed 4123547 4049926 4080450
> %age total pages scanned/reclaimed 32.63% 29.98% 35.45%
> %age total pages scanned/written 2.29% 1.26% 1.54%
> %age file pages scanned/written 0.75% 0.01% 0.00%
> Percentage Time Spent Direct Reclaim 62.50% 15.39% 31.89%
> Percentage Time kswapd Awake 91.18% 91.47% 91.74%
>
> Time spent in direct reclaim is massively reduced which is surprising

Awesome!

> as this is XFS so it should not have been stalling in the writing
> files anyway. It's possible that the anon writes are completing
> faster so time spent swapping is reduced.
>
> With patches 1-7, kswapd still writes some pages due to it reaching
> higher priorities due to memory pressure but the number of pages it
> writes is significantly reduced and a small percentage of those that
> were written to swap. Patch 8 eliminates it entirely but the benefit is
> not seen in the completion times as the number of writes is so small.

Yes. It seems patch 8's effect is so small in general.
Even it increased direct reclaim time.

>
> FTrace Reclaim Statistics: congestion_wait
> Direct number congest waited 0 0 0
> Direct time congest waited 0ms 0ms 0ms
> Direct full congest waited 0 0 0
> Direct number conditional waited 12345 37713 34841
> Direct time conditional waited 12396ms 132ms 168ms
> Direct full conditional waited 53 0 0
> KSwapd number congest waited 4248 2957 2293
> KSwapd time congest waited 15320ms 10312ms 13416ms
> KSwapd full congest waited 31 1 21
> KSwapd number conditional waited 0 15989 10410
> KSwapd time conditional waited 0ms 0ms 0ms
> KSwapd full conditional waited 0 0 0
>
> Congestion is way down as direct reclaim conditional wait time is
> reduced by about 12 seconds.
>
> Overall, this looks good. Avoiding writes from kswapd improves
> overall performance as expected and eliminating them entirely seems
> to behave well.

I agree with you.

>
> Next I tested on a NUMA configuration of sorts. I don't have a real
> NUMA machine so I booted the same machine with mem=4096M numa=fake=8
> so each node is 512M. Again, the volume of information is high but
> here is a summary of sorts based on a test run with monitors enabled.
>
> 4096M8N-xfs Files/s mean 27.29 ( 0.00%) 27.35 ( 0.20%) 27.91 ( 2.22%)
> 4096M8N-xfs Elapsed Time fsmark 1402.55 1400.77 1382.92
> 4096M8N-xfs Elapsed Time mmap-strm 660.90 596.91 630.05
> 4096M8N-xfs Kswapd efficiency fsmark 72% 71% 13%
> 4096M8N-xfs Kswapd efficiency mmap-strm 39% 40% 31%
> 4096M8N-xfs stalled direct reclaim fsmark 0.00 0.00 0.00
> 4096M8N-xfs stalled direct reclaim mmap-strm 36.37 13.06 56.88
> 4096M8N-4X-xfs Files/s mean 26.80 ( 0.00%) 26.41 (-1.47%) 26.40 (-1.53%)
> 4096M8N-4X-xfs Elapsed Time fsmark 1453.95 1460.62 1470.98
> 4096M8N-4X-xfs Elapsed Time mmap-strm 683.34 663.46 690.01
> 4096M8N-4X-xfs Kswapd efficiency fsmark 68% 67% 8%
> 4096M8N-4X-xfs Kswapd efficiency mmap-strm 35% 34% 6%
> 4096M8N-4X-xfs stalled direct reclaim fsmark 0.00 0.00 0.00
> 4096M8N-4X-xfs stalled direct reclaim mmap-strm 26.45 87.57 46.87
> 4096M8N-2X-xfs Files/s mean 26.22 ( 0.00%) 26.70 ( 1.77%) 27.21 ( 3.62%)
> 4096M8N-2X-xfs Elapsed Time fsmark 1469.28 1439.30 1424.45
> 4096M8N-2X-xfs Elapsed Time mmap-strm 676.77 656.28 655.03
> 4096M8N-2X-xfs Kswapd efficiency fsmark 69% 69% 9%
> 4096M8N-2X-xfs Kswapd efficiency mmap-strm 33% 33% 7%
> 4096M8N-2X-xfs stalled direct reclaim fsmark 0.00 0.00 0.00
> 4096M8N-2X-xfs stalled direct reclaim mmap-strm 52.74 57.96 102.49
> 4096M8N-16X-xfs Files/s mean 25.78 ( 0.00%) 27.81 ( 7.32%) 48.52 (46.87%)
> 4096M8N-16X-xfs Elapsed Time fsmark 1555.95 1554.78 1542.53
> 4096M8N-16X-xfs Elapsed Time mmap-strm 770.01 763.62 844.55
> 4096M8N-16X-xfs Kswapd efficiency fsmark 62% 62% 7%
> 4096M8N-16X-xfs Kswapd efficiency mmap-strm 38% 37% 10%
> 4096M8N-16X-xfs stalled direct reclaim fsmark 0.12 0.01 0.05
> 4096M8N-16X-xfs stalled direct reclaim mmap-strm 1.07 1.09 63.32
>
> The performance differences for fsmark are marginal because the number
> of page written from reclaim is pretty low with this much memory even
> with NUMA enabled. At no point did fsmark enter direct reclaim to
> try and write a page so it's all kswapd. What is important to note is
> the "Kswapd efficiency". Once kswapd cannot write pages at all, its
> efficiency drops rapidly for fsmark as it scans about 5-8 times more
> pages waiting on flusher threads to clean a page from the correct node.
>
> Kswapd not writing pages impairs direct reclaim performance for the
> streaming writer test. Note the times stalled in direct reclaim. In
> all cases, the time stalled in direct reclaim goes way up as both
> direct reclaimers and kswapd get stalled waiting on pages to get
> cleaned from the right node.

Yes. The data is horrible.

>
> Fortunately, kswapd CPU usage does not go to 100% because of the
> throttling. From the 40968M test for example, I see
>
> KSwapd full congest waited 834 739 989
> KSwapd number conditional waited 0 68552 372275
> KSwapd time conditional waited 0ms 16ms 1684ms
> KSwapd full conditional waited 0 0 0
>
> With kswapd avoiding writes, it gets throttled lightly but when it
> writes no pasges at all, it gets throttled very heavily and sleeps.
>
> ext4 tells a slightly different story
>
> 4096M8N-ext4 Files/s mean 28.63 ( 0.00%) 30.58 ( 6.37%) 31.04 ( 7.76%)
> 4096M8N-ext4 Elapsed Time fsmark 1578.51 1551.99 1532.65
> 4096M8N-ext4 Elapsed Time mmap-strm 703.66 655.25 654.86
> 4096M8N-ext4 Kswapd efficiency 62% 69% 68%
> 4096M8N-ext4 Kswapd efficiency 35% 35% 35%
> 4096M8N-ext4 stalled direct reclaim fsmark 0.00 0.00 0.00
> 4096M8N-ext4 stalled direct reclaim mmap-strm 32.64 95.72 152.62
> 4096M8N-2X-ext4 Files/s mean 30.74 ( 0.00%) 28.49 (-7.89%) 28.79 (-6.75%)
> 4096M8N-2X-ext4 Elapsed Time fsmark 1466.62 1583.12 1580.07
> 4096M8N-2X-ext4 Elapsed Time mmap-strm 705.17 705.64 693.01
> 4096M8N-2X-ext4 Kswapd efficiency 68% 68% 67%
> 4096M8N-2X-ext4 Kswapd efficiency 34% 30% 18%
> 4096M8N-2X-ext4 stalled direct reclaim fsmark 0.00 0.00 0.00
> 4096M8N-2X-ext4 stalled direct reclaim mmap-strm 106.82 24.88 27.88
> 4096M8N-4X-ext4 Files/s mean 24.15 ( 0.00%) 23.18 (-4.18%) 23.94 (-0.89%)
> 4096M8N-4X-ext4 Elapsed Time fsmark 1848.41 1971.48 1867.07
> 4096M8N-4X-ext4 Elapsed Time mmap-strm 664.87 673.66 674.46
> 4096M8N-4X-ext4 Kswapd efficiency 62% 65% 65%
> 4096M8N-4X-ext4 Kswapd efficiency 33% 37% 15%
> 4096M8N-4X-ext4 stalled direct reclaim fsmark 0.18 0.03 0.26
> 4096M8N-4X-ext4 stalled direct reclaim mmap-strm 115.71 23.05 61.12
> 4096M8N-16X-ext4 Files/s mean 5.42 ( 0.00%) 5.43 ( 0.15%) 3.83 (-41.44%)
> 4096M8N-16X-ext4 Elapsed Time fsmark 9572.85 9653.66 11245.41
> 4096M8N-16X-ext4 Elapsed Time mmap-strm 752.88 750.38 769.19
> 4096M8N-16X-ext4 Kswapd efficiency 59% 59% 61%
> 4096M8N-16X-ext4 Kswapd efficiency 34% 34% 21%
> 4096M8N-16X-ext4 stalled direct reclaim fsmark 0.26 0.65 0.26
> 4096M8N-16X-ext4 stalled direct reclaim mmap-strm 177.48 125.91 196.92
>
> 4096M8N-16X-ext4 with kswapd writing no pages collapsed in terms of
> performance. Looking at the fsmark logs, in a number of iterations,
> it was barely able to write files at all.
>
> The apparent slowdown for fsmark in 4096M8N-2X-ext4 is well within
> the noise but the reduced time spent in direct reclaim is very welcome.

But 4096M8N-ext4 increased the time and 4096M8N-2X-ext4 is within the noise
as you said. I doubt it's reliability.

>
> Unlike xfs, it's less clear cut if direct reclaim performance is
> impaired but in a few tests, preventing kswapd writing pages did
> increase the time stalled.
>
> Last test is that I've been running this series on my laptop since
> Monday without any problem but it's rarely under serious memory
> pressure. I see nr_vmscan_write is 0 and the number of pages
> invalidated from the end of the LRU is only 10844 after 3 days so
> it's not much of a test.
>
> Overall, having kswapd avoiding writes does improve performance
> which is not a surprise. Dave asked "do we even need IO at all from
> reclaim?". On NUMA machines, the answer is "yes" unless the VM can
> wake the flusher thread to clean a specific node. When kswapd never
> writes, processes can stall for significant periods of time waiting on
> flushers to clean the correct pages. If all writing is to be deferred
> to flushers, it must ensure that many writes on one node would not
> starve requests for cleaning pages on another node.

It's a good answer. :)

>
> I'm currently of the opinion that we should consider merging patches
> 1-7 and discuss what is required before merging. It can be tackled
> later how the flushers can prioritise writing of pages belonging to
> a particular zone before disabling all writes from reclaim. There
> is already some work in this general area with the possibility that
> series such as "writeback: moving expire targets for background/kupdate
> works" could be extended to allow patch 8 to be merged later even if
> the series needs work.

I think you already knew what we need(ie, prioritising the pages in a zone)
In case of NUMA, 1-7 has a problem in ext4 so we have to focus NUMA during remained time.

The alternative of [prioritising the page in a zone] might be Johannes's [mm: per-zone dirty limiting].
It might mitigate NUMA problems.

Overall, I really welcome this approach and would like to merge this in mmotm as soon as possible
for see the side effects in non-NUMA(I will add my reviewed-by soon).
In case of NUMA, we know the problem apparently so I think it could be solved
before it is sent to mainline.

It was a great time to see your data and you makes my coffee delicious. :)
You're a good Barista.
Thanks for your great effort, Mel!

--
Kind regards,
Minchan Kim