Re: [PATCH 0/4] Reduce impact to overall system of SLUB usinghigh-order allocations V2

[Mel Gorman]

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.

-- 
Mel Gorman
SUSE Labs
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