Re: Kswapd in 3.2.0-rc5 is a CPU hog

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 27 Dec 2011 06:50:08 +0400
"Nikolay S." <nowhere@hakkenden.ath.cx> wrote:

> В Вт., 27/12/2011 в 11:15 +0900, KAMEZAWA Hiroyuki пишет:
> > 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.
> 

Thanks, 
Qeustion:
 1. does this system has no swap ?
 2. What version of kernel which you didn't see the kswapd issue ?
 3. Is this real host ? or virtualized ?


> $ cat /proc/zoneinfo 
...
Node 0, zone    DMA32
  pages free     19620
        min      14715
        low      18393
        high     22072
        scanned  0
        spanned  1044480
        present  896960
    nr_free_pages 19620
    nr_inactive_anon 43203
    nr_active_anon 206577
    nr_inactive_file 412249
    nr_active_file 126151

Then, DMA32(zone=1) files are enough large (> 32 << 12)
Hmm. assuming all frees are used for file(of dd)


(412249 + 126151 + 19620) >> 12 = 136

So, 32, 32, 30 scan seems to work as desgined.

> Node 0, zone   Normal
>   pages free     2854
>         min      2116
>         low      2645
>         high     3174
>         scanned  0
>         spanned  131072
>         present  129024
>     nr_free_pages 2854
>     nr_inactive_anon 20682
>     nr_active_anon 10262
>     nr_inactive_file 47083
>     nr_active_file 11292

Hmm, NORMAL is much smaller than DMA32. (only 500MB.)

Then, at priority=12,

  13 << 12 = 53248

13 pages per a scan seems to work as designed.
To me,  it seems kswapd does usual work...reclaim small memory until free
gets enough. And it seems 'dd' allocates its memory from ZONE_DMA32 because
of gfp_t fallbacks.


Memo.

1. why shrink_slab() should be called per zone, which is not zone aware.
   Isn't it enough to call it per priority ?

2. what spinlock contention that perf showed ?
   And if shrink_slab() doesn't consume cpu as trace shows, why perf 
   says shrink_slab() is heavy..

3. because 8/9 of memory is in DMA32, calling shrink_slab() frequently
   at scanning NORMAL seems to be time wasting.
 

Thanks,
-Kame





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