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Gmane
From: Mel Gorman <mgorman <at> suse.de>
Subject: [PATCH 0/10] Reduce system disruption due to kswapd V3
Newsgroups: gmane.linux.kernel
Date: Thursday 11th April 2013 19:57:48 UTC (over 3 years ago)
Big change is again related to proportional reclaim.

Changelog since V2
o Preserve ratio properly for proportional scanning		(kamezawa)

Changelog since V1
o Rename ZONE_DIRTY to ZONE_TAIL_LRU_DIRTY			(andi)
o Reformat comment in shrink_page_list				(andi)
o Clarify some comments						(dhillf)
o Rework how the proportional scanning is preserved
o Add PageReclaim check before kswapd starts writeback
o Reset sc.nr_reclaimed on every full zone scan

Kswapd and page reclaim behaviour has been screwy in one way or the other
for a long time. Very broadly speaking it worked in the far past because
machines were limited in memory so it did not have that many pages to scan
and it stalled congestion_wait() frequently to prevent it going completely
nuts. In recent times it has behaved very unsatisfactorily with some of
the problems compounded by the removal of stall logic and the introduction
of transparent hugepage support with high-order reclaims.

There are many variations of bugs that are rooted in this area. One example
is reports of a large copy operations or backup causing the machine to
grind to a halt or applications pushed to swap. Sometimes in low memory
situations a large percentage of memory suddenly gets reclaimed. In other
cases an application starts and kswapd hits 100% CPU usage for prolonged
periods of time and so on. There is now talk of introducing features like
an extra free kbytes tunable to work around aspects of the problem instead
of trying to deal with it. It's compounded by the problem that it can be
very workload and machine specific.

This series aims at addressing some of the worst of these problems without
attempting to fundmentally alter how page reclaim works.

Patches 1-2 limits the number of pages kswapd reclaims while still obeying
	the anon/file proportion of the LRUs it should be scanning.

Patches 3-4 control how and when kswapd raises its scanning priority and
	deletes the scanning restart logic which is tricky to follow.

Patch 5 notes that it is too easy for kswapd to reach priority 0 when
	scanning and then reclaim the world. Down with that sort of thing.

Patch 6 notes that kswapd starts writeback based on scanning priority which
	is not necessarily related to dirty pages. It will have kswapd
	writeback pages if a number of unqueued dirty pages have been
	recently encountered at the tail of the LRU.

Patch 7 notes that sometimes kswapd should stall waiting on IO to complete
	to reduce LRU churn and the likelihood that it'll reclaim young
	clean pages or push applications to swap. It will cause kswapd
	to block on IO if it detects that pages being reclaimed under
	writeback are recycling through the LRU before the IO completes.

Patch 8 shrinks slab just once per priority scanned or if a zone is
otherwise
	unreclaimable to avoid hammering slab when kswapd has to skip a
	large number of pages.

Patches 9-10 are cosmetic but balance_pgdat() might be easier to follow.

This was tested using memcached+memcachetest while some background IO
was in progress as implemented by the parallel IO tests implement in MM
Tests. memcachetest benchmarks how many operations/second memcached can
service and it is run multiple times. It starts with no background IO and
then re-runs the test with larger amounts of IO in the background to
roughly
simulate a large copy in progress.  The expectation is that the IO should
have little or no impact on memcachetest which is running entirely in
memory.

                                         3.9.0-rc6                  
3.9.0-rc6
                                           vanilla           
lessdisrupt-v3r6
Ops memcachetest-0M             10868.00 (  0.00%)          10932.00 ( 
0.59%)
Ops memcachetest-749M           10976.00 (  0.00%)          10986.00 ( 
0.09%)
Ops memcachetest-2498M           3406.00 (  0.00%)          10871.00
(219.17%)
Ops memcachetest-4246M           2402.00 (  0.00%)          10936.00
(355.29%)
Ops io-duration-0M                  0.00 (  0.00%)              0.00 ( 
0.00%)
Ops io-duration-749M               15.00 (  0.00%)              9.00 (
40.00%)
Ops io-duration-2498M             107.00 (  0.00%)             27.00 (
74.77%)
Ops io-duration-4246M             193.00 (  0.00%)             47.00 (
75.65%)
Ops swaptotal-0M                    0.00 (  0.00%)              0.00 ( 
0.00%)
Ops swaptotal-749M             155965.00 (  0.00%)             25.00 (
99.98%)
Ops swaptotal-2498M            335917.00 (  0.00%)            287.00 (
99.91%)
Ops swaptotal-4246M            463021.00 (  0.00%)              0.00 ( 
0.00%)
Ops swapin-0M                       0.00 (  0.00%)              0.00 ( 
0.00%)
Ops swapin-749M                     0.00 (  0.00%)              0.00 ( 
0.00%)
Ops swapin-2498M               139128.00 (  0.00%)              0.00 ( 
0.00%)
Ops swapin-4246M               156276.00 (  0.00%)              0.00 ( 
0.00%)
Ops minorfaults-0M            1677257.00 (  0.00%)        1642376.00 ( 
2.08%)
Ops minorfaults-749M          1819566.00 (  0.00%)        1572243.00 (
13.59%)
Ops minorfaults-2498M         1842140.00 (  0.00%)        1652508.00 (
10.29%)
Ops minorfaults-4246M         1796116.00 (  0.00%)        1651464.00 ( 
8.05%)
Ops majorfaults-0M                  6.00 (  0.00%)              6.00 ( 
0.00%)
Ops majorfaults-749M               55.00 (  0.00%)             49.00 (
10.91%)
Ops majorfaults-2498M           20936.00 (  0.00%)            110.00 (
99.47%)
Ops majorfaults-4246M           22487.00 (  0.00%)            185.00 (
99.18%)

Note how the vanilla kernels performance collapses when there is enough IO
taking place in the background. This drop in performance is part of users
complain of when they start backups. Note how the swapin and major fault
figures indicate that processes were being pushed to swap prematurely. With
the series applied, there is no noticable performance drop and while there
is still some swap activity, it's tiny.

                             3.9.0-rc6   3.9.0-rc6
                               vanillalessdisrupt-v3r6
Page Ins                       1281068       89224
Page Outs                     15697620    11478616
Swap Ins                        295654           0
Swap Outs                       659499         312
Direct pages scanned                 0       78668
Kswapd pages scanned           7166977     4416457
Kswapd pages reclaimed         1185518     1051751
Direct pages reclaimed               0       72993
Kswapd efficiency                  16%         23%
Kswapd velocity               5558.640    3420.614
Direct efficiency                 100%         92%
Direct velocity                  0.000      60.930
Percentage direct scans             0%          1%
Page writes by reclaim         2044715     2922251
Page writes file               1385216     2921939
Page writes anon                659499         312
Page reclaim immediate            4040         218
Page rescued immediate               0           0
Slabs scanned                    35456       26624
Direct inode steals                  0           0
Kswapd inode steals              19898        1420
Kswapd skipped wait                  0           0
THP fault alloc                     11          51
THP collapse alloc                 574         609
THP splits                           9           6
THP fault fallback                   0           0
THP collapse fail                    0           0
Compaction stalls                    0           0
Compaction success                   0           0
Compaction failures                  0           0
Page migrate success                 0           0
Page migrate failure                 0           0
Compaction pages isolated            0           0
Compaction migrate scanned           0           0
Compaction free scanned              0           0
Compaction cost                      0           0
NUMA PTE updates                     0           0
NUMA hint faults                     0           0
NUMA hint local faults               0           0
NUMA pages migrated                  0           0
AutoNUMA cost                        0           0

Note that kswapd efficiency is slightly improved. Unfortunately, also note
that there is a small amount of direct reclaim due to kswapd no longer
reclaiming
the world. Using ftrace it would appear that the direct reclaim stalls are
mostly
harmless with the vast bulk of the stalls incurred by dd

      2 gzip-3111
      5 memcachetest-12607
     26 tclsh-3109
     67 tee-3110
     89 flush-8:0-286
   2055 dd-12795

There is a risk that kswapd not reclaiming the world may mean that it
stays awake balancing zones, does not stall on the appropriate events
and continually scans pages it cannot reclaim consuming CPU. This will
be visible as continued high CPU usage but in my own tests I only saw a
single spike lasting less than a second and I did not observe any problems
related to reclaim while running the series on my desktop.

 include/linux/mmzone.h |  17 ++
 mm/vmscan.c            | 461
++++++++++++++++++++++++++++++-------------------
 2 files changed, 305 insertions(+), 173 deletions(-)

-- 
1.8.1.4

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CD: 3ms