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From: Mel Gorman <mgorman <at> suse.de>
Subject: [PATCH 0/4] Reduce impact to overall system of SLUB using high-order allocations V2
Newsgroups: gmane.comp.file-systems.ext4
Date: Friday 13th May 2011 14:03:20 UTC (over 6 years ago)
Changelog since V1
  o kswapd should sleep if need_resched
  o Remove __GFP_REPEAT from GFP flags when speculatively using high
    orders so direct/compaction exits earlier
  o Remove __GFP_NORETRY for correctness
  o Correct logic in sleeping_prematurely
  o Leave SLUB using the default slub_max_order

There are a few reports of people experiencing hangs when copying
large amounts of data with kswapd using a large amount of CPU which
appear to be due to recent reclaim changes.

SLUB using high orders is the trigger but not the root cause as SLUB
has been using high orders for a while. The following four patches
aim to fix the problems in reclaim while reducing the cost for SLUB
using those high orders.

Patch 1 corrects logic introduced by commit [1741c877: mm:
	kswapd: keep kswapd awake for high-order allocations until
	a percentage of the node is balanced] to allow kswapd to
	go to sleep when balanced for high orders.

Patch 2 prevents kswapd waking up in response to SLUBs speculative
	use of high orders.

Patch 3 further reduces the cost by prevent SLUB entering direct
	compaction or reclaim paths on the grounds that falling
	back to order-0 should be cheaper.

Patch 4 notes that even when kswapd is failing to keep up with
	allocation requests, it should still go to sleep when its
	quota has expired to prevent it spinning.

My own data on this is not great. I haven't really been able to
reproduce the same problem locally.

The test case is simple. "download tar" wgets a large tar file and
stores it locally. "unpack" is expanding it (15 times physical RAM
in this case) and "delete source dirs" is the tarfile being deleted
again. I also experimented with having the tar copied numerous times
and into deeper directories to increase the size but the results were
not particularly interesting so I left it as one tar.

In the background, applications are being launched to time to vaguely
simulate activity on the desktop and to measure how long it takes
applications to start.

Test server, 4 CPU threads, x86_64, 2G of RAM, no PREEMPT, no COMPACTION, X
                      vanilla       fixprematurely  kswapd-nowwake
slub-noexstep  kswapdsleep
download tar           95 ( 0.00%)   94 ( 1.06%)   94 ( 1.06%)   94 (
1.06%)   94 ( 1.06%)
unpack tar            654 ( 0.00%)  649 ( 0.77%)  655 (-0.15%)  589
(11.04%)  598 ( 9.36%)
copy source files       0 ( 0.00%)    0 ( 0.00%)    0 ( 0.00%)    0 (
0.00%)    0 ( 0.00%)
delete source dirs    327 ( 0.00%)  334 (-2.10%)  318 ( 2.83%)  325 (
0.62%)  320 ( 2.19%)
MMTests Statistics: duration
User/Sys Time Running Test (seconds)        1139.7   1142.55   1149.78  
1109.32   1113.26
Total Elapsed Time (seconds)               1341.59   1342.45   1324.90  
1271.02   1247.35

MMTests Statistics: application launch
evolution-wait30     mean     34.92   34.96   34.92   34.92   35.08
gnome-terminal-find  mean      7.96    7.96    8.76    7.80    7.96
iceweasel-table      mean      7.93    7.81    7.73    7.65    7.88

evolution-wait30     stddev    0.96    1.22    1.27    1.20    1.15
gnome-terminal-find  stddev    3.02    3.09    3.51    2.99    3.02
iceweasel-table      stddev    1.05    0.90    1.09    1.11    1.11

Having SLUB avoid expensive steps in reclaim improves performance
by quite a bit with the overall test completing 1.5 minutes
faster. Application launch times were not really affected but it's
not something my test machine was suffering from in the first place
so it's not really conclusive. The kswapd patches also did not appear
to help but again, the test machine wasn't suffering that problem.

These patches are against 2.6.39-rc7. Again, testing would be

 Documentation/vm/slub.txt |    2 +-
 mm/page_alloc.c           |    3 ++-
 mm/slub.c                 |    5 +++--
 3 files changed, 6 insertions(+), 4 deletions(-)

 mm/page_alloc.c |    3 ++-
 mm/slub.c       |    3 ++-
 mm/vmscan.c     |    6 +++++-
 3 files changed, 9 insertions(+), 3 deletions(-)


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