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SubjectRe: Kswapd in 3.2.0-rc5 is a CPU hog
On Tue, 27 Dec 2011 12:57:31 +0900
Minchan Kim <minchan@kernel.org> wrote:

> On Tue, Dec 27, 2011 at 11:15:43AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Sat, 24 Dec 2011 07:45:03 +1100
> > Dave Chinner <david@fromorbit.com> wrote:
> >
> > > On Fri, Dec 23, 2011 at 03:04:02PM +0400, nowhere wrote:
> > > > В Пт., 23/12/2011 в 21:20 +1100, Dave Chinner пишет:
> > > > > On Fri, Dec 23, 2011 at 01:01:20PM +0400, nowhere wrote:
> > > > > > В Чт., 22/12/2011 в 09:55 +1100, Dave Chinner пишет:
> > > > > > > On Wed, Dec 21, 2011 at 10:52:49AM +0100, Michal Hocko wrote:
> >
> > > > Here is the report of trace-cmd while dd'ing
> > > > https://80.237.6.56/report-dd.xz
> > >
> > > Ok, it's not a shrink_slab() problem - it's just being called ~100uS
> > > by kswapd. The pattern is:
> > >
> > > - reclaim 94 (batches of 32,32,30) pages from iinactive list
> > > of zone 1, node 0, prio 12
> > > - call shrink_slab
> > > - scan all caches
> > > - all shrinkers return 0 saying nothing to shrink
> > > - 40us gap
> > > - reclaim 10-30 pages from inactive list of zone 2, node 0, prio 12
> > > - call shrink_slab
> > > - scan all caches
> > > - all shrinkers return 0 saying nothing to shrink
> > > - 40us gap
> > > - isolate 9 pages from LRU zone ?, node ?, none isolated, none freed
> > > - isolate 22 pages from LRU zone ?, node ?, none isolated, none freed
> > > - call shrink_slab
> > > - scan all caches
> > > - all shrinkers return 0 saying nothing to shrink
> > > 40us gap
> > >
> > > And it just repeats over and over again. After a while, nid=0,zone=1
> > > drops out of the traces, so reclaim only comes in batches of 10-30
> > > pages from zone 2 between each shrink_slab() call.
> > >
> > > The trace starts at 111209.881s, with 944776 pages on the LRUs. It
> > > finishes at 111216.1 with kswapd going to sleep on node 0 with
> > > 930067 pages on the LRU. So 7 seconds to free 15,000 pages (call it
> > > 2,000 pages/s) which is awfully slow....
> > >
> > > vmscan gurus - time for you to step in now...
> > >
> >
> > Can you show /proc/zoneinfo ? I want to know each zone's size.
> >
> > Below is my memo.
> >
> > In trace log, priority = 11 or 12. Then, I think kswapd can reclaim memory
> > to satisfy "sc.nr_reclaimed >= SWAP_CLUSTER_MAX" condition and loops again.
> >
> > Seeing balance_pgdat() and trace log, I guess it does
> >
> > wake up
> >
> > shrink_zone(zone=0(DMA?)) => nothing to reclaim.
> > shrink_slab()
> > shrink_zone(zone=1(DMA32?)) => reclaim 32,32,31 pages
> > shrink_slab()
> > shrink_zone(zone=2(NORMAL?)) => reclaim 13 pages.
> > srhink_slab()
> >
> > sleep or retry.
> >
> > Why shrink_slab() need to be called frequently like this ?
>
> I guess it's caused by small NORMAL zone.

You're right. I confirmed his zoneinfo.

> The scenario I think is as follows,
>
> 1. dd comsumes memory in NORMAL zone
> 2. dd enter direct reclaim and wakeup kswapd
> 3. kswapd reclaims some memory in NORMAL zone until it reclaims high wamrk
> 4. schedule
> 5. dd consumes memory again in NORMAL zone
> 6. kswapd fail to reclaim memory by high watermark due to 5.
> 7. loop again, goto 3.
>
> The point is speed between reclaim VS memory consumption.
> So kswapd cannot reach a point which enough pages are in NORMAL zone.
>
> >
> > BTW. I'm sorry if I miss something ...Why only kswapd reclaims memory
> > while 'dd' operation ? (no direct relcaim by dd.)
> > Is this log record cpu hog after 'dd' ?
>
> If above scenario is right, dd couldn't enter direct reclaim to reclaim memory.
>

I think you're right. IIUC, kswapd's behavior is what we usually see.

Hmm, if I understand correctly,

- dd's speed down is caused by kswapd's cpu consumption.
- kswapd's cpu consumption is enlarged by shrink_slab() (by perf)
- kswapd can't stop because NORMAL zone is small.
- memory reclaim speed is enough because dd can't get enough cpu.

I wonder reducing to call shrink_slab() may be a help but I'm not sure
where lock conention comes from...

Regards,
-Kame








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