Re: kswapd0: excessive CPU usage

[Mel Gorman <mgorman@suse.de>]

On Thu, Nov 08, 2012 at 10:22:05PM -0600, Seth Jennings wrote:
> On 11/02/2012 02:45 PM, Jiri Slaby wrote:
> > On 11/02/2012 11:53 AM, Jiri Slaby wrote:
> >> On 11/02/2012 11:44 AM, Zdenek Kabelac wrote:
> >>>>> Yes, applying this instead of the revert fixes the issue as well.
> >>>
> >>> I've applied this patch on 3.7.0-rc3 kernel - and I still see excessive
> >>> CPU usage - mainly  after  suspend/resume
> >>>
> >>> Here is just simple  kswapd backtrace from running kernel:
> >>
> >> Yup, this is what we were seeing with the former patch only too. Try to
> >> apply the other one too:
> >> https://patchwork.kernel.org/patch/1673231/
> >>
> >> For me I would say, it is fixed by the two patches now. I won't be able
> >> to report later, since I'm leaving to a conference tomorrow.
> > 
> > Damn it. It recurred right now, with both patches applied. After I
> > started a java program which consumed some more memory. Though there are
> > still 2 gigs free, kswap is spinning:
> > [<ffffffff810b00da>] __cond_resched+0x2a/0x40
> > [<ffffffff811318a0>] shrink_slab+0x1c0/0x2d0
> > [<ffffffff8113478d>] kswapd+0x66d/0xb60
> > [<ffffffff810a25d0>] kthread+0xc0/0xd0
> > [<ffffffff816aa29c>] ret_from_fork+0x7c/0xb0
> > [<ffffffffffffffff>] 0xffffffffffffffff
> 
> I'm also hitting this issue in v3.7-rc4.  It appears that the last
> release not effected by this issue was v3.3.  Bisecting the changes
> included for v3.4-rc1 showed that this commit introduced the issue:
> 
> fe2c2a106663130a5ab45cb0e3414b52df2fff0c is the first bad commit
> commit fe2c2a106663130a5ab45cb0e3414b52df2fff0c
> Author: Rik van Riel <riel@redhat.com>
> Date:   Wed Mar 21 16:33:51 2012 -0700
> 
>     vmscan: reclaim at order 0 when compaction is enabled
> ...
> 
> This is plausible since the issue seems to be in the kswapd + compaction
> realm.  I've yet to figure out exactly what about this commit results in
> kswapd spinning.
> 
> I would be interested if someone can confirm this finding.
> 

I cannot confirm the actual finding as I don't see the same sort of
problems. However, this does make sense and was more or less expected.
Reclaiming at order-0 would have forced compaction to be used more instead
of lumpy reclaim (less CPU usage but greater system distruption that is
harder to measure). Shortly after, lumpy reclaim was removed entirely so
now larger amounts of CPU time is spent compacting memory that previously
would have been reclaimed.

-- 
Mel Gorman
SUSE Labs

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