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Subject[PATCH 0/5] Prevent kswapd dumping excessive amounts of memory in response to high-order allocations V3
Date
There was a minor bug in V2 that led to this release.  I'm hopeful it'll
stop kswapd going mad on Simon's machine and might also alleviate some of
the "too much free memory" problem.

Changelog since V2
o Add clarifying comments
o Properly check that the zone is balanced for order-0
o Treat zone->all_unreclaimable properly

Changelog since V1
o Take classzone into account
o Ensure that kswapd always balances at order-09
o Reset classzone and order after reading
o Require a percentage of a node be balanced for high-order allocations,
not just any zone as ZONE_DMA could be balanced when the node in general
is a mess

Simon Kirby reported the following problem

We're seeing cases on a number of servers where cache never fully
grows to use all available memory. Sometimes we see servers with 4
GB of memory that never seem to have less than 1.5 GB free, even with
a constantly-active VM. In some cases, these servers also swap out
while this happens, even though they are constantly reading the working
set into memory. We have been seeing this happening for a long time;
I don't think it's anything recent, and it still happens on 2.6.36.

After some debugging work by Simon, Dave Hansen and others, the prevaling
theory became that kswapd is reclaiming order-3 pages requested by SLUB
too aggressive about it.

There are two apparent problems here. On the target machine, there is a small
Normal zone in comparison to DMA32. As kswapd tries to balance all zones, it
would continually try reclaiming for Normal even though DMA32 was balanced
enough for callers. The second problem is that sleeping_prematurely() does
not use the same logic as balance_pgdat() when deciding whether to sleep
or not. This keeps kswapd artifically awake.

A number of tests were run and the figures from previous postings will look
very different for a few reasons. One, the old figures were forcing my network
card to use GFP_ATOMIC in attempt to replicate Simon's problem. Second, I
previous specified slub_min_order=3 again in an attempt to reproduce Simon's
problem. In this posting, I'm depending on Simon to say whether his problem is
fixed or not and these figures are to show the impact to the ordinary cases.
Finally, the "vmscan" figures are taken from /proc/vmstat instead of the
tracepoints. There is less information but recording is less disruptive.

The first test of relevance was postmark with a process running in the
background reading a large amount of anonymous memory in blocks. The
objective was to vaguely simulate what was happening on Simon's machine
and it's memory intensive enough to have kswapd awake.

POSTMARK
traceonly kanyzone
Transactions per second: 156.00 ( 0.00%) 153.00 (-1.96%)
Data megabytes read per second: 21.51 ( 0.00%) 21.52 ( 0.05%)
Data megabytes written per second: 29.28 ( 0.00%) 29.11 (-0.58%)
Files created alone per second: 250.00 ( 0.00%) 416.00 (39.90%)
Files create/transact per second: 79.00 ( 0.00%) 76.00 (-3.95%)
Files deleted alone per second: 520.00 ( 0.00%) 420.00 (-23.81%)
Files delete/transact per second: 79.00 ( 0.00%) 76.00 (-3.95%)

MMTests Statistics: duration
User/Sys Time Running Test (seconds) 16.58 17.4
Total Elapsed Time (seconds) 218.48 222.47

VMstat Reclaim Statistics: vmscan
Direct reclaims 0 4
Direct reclaim pages scanned 0 203
Direct reclaim pages reclaimed 0 184
Kswapd pages scanned 326631 322018
Kswapd pages reclaimed 312632 309784
Kswapd low wmark quickly 1 4
Kswapd high wmark quickly 122 475
Kswapd skip congestion_wait 1 0
Pages activated 700040 705317
Pages deactivated 212113 203922
Pages written 9875 6363

Total pages scanned 326631 322221
Total pages reclaimed 312632 309968
%age total pages scanned/reclaimed 95.71% 96.20%
%age total pages scanned/written 3.02% 1.97%
proc vmstat: Faults
Major Faults 300 254
Minor Faults 645183 660284
Page ins 493588 486704
Page outs 4960088 4986704
Swap ins 1230 661
Swap outs 9869 6355

Performance is mildly affected because kswapd is no longer doing as much
work and the background memory consumer process is getting in the way. Note
that kswapd scanned and reclaimed fewer pages as it's less aggressive and
overall fewer pages were scanned and reclaimed. Swap in/out is particularly
reduced again reflecting kswapd throwing out fewer pages.

The slight performance impact is unfortunate here but it looks like a direct
result of kswapd being less aggressive. As the bug report is about too many
pages being freed by kswapd, it may have to be accepted for now.

The second test is a streaming IO benchmark that was previously used by
Johannes to show regressions in page reclaim.

MICRO
traceonly kanyzone
User/Sys Time Running Test (seconds) 29.29 28.87
Total Elapsed Time (seconds) 492.18 488.79

VMstat Reclaim Statistics: vmscan
Direct reclaims 2128 1460
Direct reclaim pages scanned 2284822 1496067
Direct reclaim pages reclaimed 148919 110937
Kswapd pages scanned 15450014 16202876
Kswapd pages reclaimed 8503697 8537897
Kswapd low wmark quickly 3100 3397
Kswapd high wmark quickly 1860 7243
Kswapd skip congestion_wait 708 801
Pages activated 9635 9573
Pages deactivated 1432 1271
Pages written 223 1130

Total pages scanned 17734836 17698943
Total pages reclaimed 8652616 8648834
%age total pages scanned/reclaimed 48.79% 48.87%
%age total pages scanned/written 0.00% 0.01%

proc vmstat: Faults
Major Faults 165 221
Minor Faults 9655785 9656506
Page ins 3880 7228
Page outs 37692940 37480076
Swap ins 0 69
Swap outs 19 15

Again fewer pages are scanned and reclaimed as expected and this time the test
completed faster. Note that kswapd is hitting its watermarks faster (low and
high wmark quickly) which I expect is due to kswapd reclaiming fewer pages.

I also ran fs-mark, iozone and sysbench but there is nothing interesting
to report in the figures. Performance is not significantly changed and the
reclaim statistics look reasonable.

include/linux/mmzone.h | 3 +-
mm/page_alloc.c | 8 ++-
mm/vmscan.c | 132 +++++++++++++++++++++++++++++++++++++++++-------
3 files changed, 120 insertions(+), 23 deletions(-)


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