Re: [PATCH 0/4] Reduce impact to overall system of SLUB usinghigh-order allocations V2
From: Mel Gorman
Date: Mon May 16 2011 - 04:37:30 EST
On Sat, May 14, 2011 at 10:34:33AM +0200, Colin Ian King wrote:
> On Fri, 2011-05-13 at 15:03 +0100, Mel Gorman wrote:
> > 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 running
> > LARGE COPY AND UNTAR
> > 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
> > appreciated.
> These patches solve the problem for me. I've been soak testing the file
> copy test
> for 3.5 hours with nearly 400 test cycles and observed no lockups at all
> - rock solid. From my observations from the output from vmstat the
> system is behaving sanely.
> Thanks for finding a solution - much appreciated!
Can you tell me if just patches 1 and 4 fix the problem please? It'd be good
to know if this was only a reclaim-related problem. Thanks.
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