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


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