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    SubjectRe: [PATCH] mm: memcg: fix over reclaiming mem cgroup
    On Tue 24-01-12 09:18:21, Balbir Singh wrote:
    > On Tue, Jan 24, 2012 at 12:44 AM, Ying Han <yinghan@google.com> wrote:
    > > On Mon, Jan 23, 2012 at 5:02 AM, Michal Hocko <mhocko@suse.cz> wrote:
    > >> On Sat 21-01-12 22:49:23, Hillf Danton wrote:
    > >>> In soft limit reclaim, overreclaim occurs when pages are reclaimed from mem
    > >>> group that is under its soft limit, or when more pages are reclaimd than the
    > >>> exceeding amount, then performance of reclaimee goes down accordingly.
    > >>
    > >> First of all soft reclaim is more a help for the global memory pressure
    > >> balancing rather than any guarantee about how much we reclaim for the
    > >> group.
    > >> We need to do more changes in order to make it a guarantee.
    > >> For example you implementation will cause severe problems when all
    > >> cgroups are soft unlimited (default conf.) or when nobody is above the
    > >> limit but the total consumption triggers the global reclaim. Therefore
    > >> nobody is in excess and you would skip all groups and only bang on the
    > >> root memcg.
    > >>
    >
    > True, ideally soft reclaim should not turn on and allow global reclaim
    > to occur in the scenario mentioned.
    >
    > >> Ying Han has a patch which basically skips all cgroups which are under
    > >> its limit until we reach a certain reclaim priority but even for this we
    > >> need some additional changes - e.g. reverse the current default setting
    > >> of the soft limit.
    > >>
    >
    > I'd be wary of that approach, because it might be harder to explain
    > the working of soft limits,

    This is an attempt to turn the soft reclaim into a "guarantee". Changing
    the default value from unlimited to 0 basically says that everybody will
    be considered under memory pressure unless the soft limit setting says
    otherwise.
    This btw. has been the case with the double (global and per-cgroup) LRUs
    as well. It was just hidden.

    > I'll look at the discussion thread mentioned earlier for the benefits
    > of that approach.
    >
    > >> Anyway, I like the nr_to_reclaim reduction idea because we have to do
    > >> this in some way because the global reclaim starts with ULONG
    > >> nr_to_scan.
    > >
    > > Agree with Michal where there are quite a lot changes we need to get
    > > in for soft limit before any further optimization.
    > >
    > > Hillf, please refer to the patch from Johannes
    > > https://lkml.org/lkml/2012/1/13/99 which got quite a lot recent
    > > discussions. I am expecting to get that in before further soft limit
    > > changes.
    >
    > Balbir
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    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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