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

    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?
    Thanks.

    --
    Best Regards,
    Peng



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