Re: how to make memory.memsw.failcnt is nonzero

[Michal Hocko <mhocko@suse.cz>]

On Mon 30-01-12 10:34:49, Peng Haitao wrote:
> 
> Michal Hocko said the following on 2012-1-4 0:04:
> > On Wed 28-12-11 17:23:04, Peng Haitao wrote:
> >>
> >> memory.memsw.failcnt shows the number of memory+Swap hits limits.
> >> So I think when memory+swap usage is equal to limit, memsw.failcnt should be nonzero.
> >>
> >> I test as follows:
> >>
> >> # uname -a
> >> Linux K-test 3.2.0-rc7-17-g371de6e #2 SMP Wed Dec 28 12:02:52 CST 2011 x86_64 x86_64 x86_64 GNU/Linux
> >> # mkdir /cgroup/memory/group
> >> # cd /cgroup/memory/group/
> >> # echo 10M > memory.limit_in_bytes
> >> # echo 10M > memory.memsw.limit_in_bytes
> >> # echo $$ > tasks
> >> # dd if=/dev/zero of=/tmp/temp_file count=20 bs=1M
> >> Killed
> >> # cat memory.memsw.failcnt
> >> 0
> >> # grep "failcnt" /var/log/messages | tail -2
> >> Dec 28 17:05:52 K-test kernel: memory: usage 10240kB, limit 10240kB, failcnt 21
> >> Dec 28 17:05:52 K-test kernel: memory+swap: usage 10240kB, limit 10240kB, failcnt 0
> >>
> >> memory+swap usage is equal to limit, but memsw.failcnt is zero.
> >>
> > Please note that memsw.limit_in_bytes is triggered only if we have
> > consumed some swap space already (and the feature is primarily intended
> > to stop extensive swap usage in fact).
> > It goes like this: If we trigger hard limit (memory.limit_in_bytes) then
> > we start the direct reclaim (with swap available). If we trigger memsw
> > limit then we try to reclaim without swap available. We will OOM if we
> > cannot reclaim enough to satisfy the respective limit.
> > 
> > The other part of the answer is, yes there is something wrong going
> > on her because we definitely shouldn't OOM. The workload is a single
> > threaded and we have a plenty of page cache that could be reclaimed
> > easily. On the other hand we end up with:
> > # echo $$ > tasks 
> > /dev/memctl/a# echo 10M > memory.limit_in_bytes 
> > /dev/memctl/a# echo 10M > memory.memsw.limit_in_bytes 
> > /dev/memctl/a# dd if=/dev/zero of=/tmp/temp_file count=20 bs=1M
> > Killed
> > /dev/memctl/a# cat memory.stat 
> > cache 9265152
> > [...]
> > 
> > So there is almost 10M of page cache that we can simply reclaim. If we
> > use 40M limit then we are OK. So this looks like the small limit somehow
> > tricks our math in the reclaim path and we think there is nothing to
> > reclaim.
> > I will look into this.
> 
> Have any conclusion for this?

I am sorry, but I didn't get to this. The last two months were really
busy and I am leaving for a long vacation next week. It's still on my
todo list...

> Thanks.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic