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Message-Id: <1365710278-6807-1-git-send-email-mgorman@suse.de>
Date: Thu, 11 Apr 2013 20:57:48 +0100
From: Mel Gorman <mgorman@...e.de>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jiri Slaby <jslaby@...e.cz>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
Rik van Riel <riel@...hat.com>,
Zlatko Calusic <zcalusic@...sync.net>,
Johannes Weiner <hannes@...xchg.org>,
dormando <dormando@...ia.net>, Michal Hocko <mhocko@...e.cz>,
Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: [PATCH 0/10] Reduce system disruption due to kswapd V3
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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