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    SubjectRe: [patch 4/8] memcg: rework soft limit reclaim
    On Thu 02-06-11 22:25:29, Ying Han wrote:
    > On Thu, Jun 2, 2011 at 2:55 PM, Ying Han <yinghan@google.com> wrote:
    > > On Tue, May 31, 2011 at 11:25 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
    > >> Currently, soft limit reclaim is entered from kswapd, where it selects
    [...]
    > >> diff --git a/mm/vmscan.c b/mm/vmscan.c
    > >> index c7d4b44..0163840 100644
    > >> --- a/mm/vmscan.c
    > >> +++ b/mm/vmscan.c
    > >> @@ -1988,9 +1988,13 @@ static void shrink_zone(int priority, struct zone *zone,
    > >>                unsigned long reclaimed = sc->nr_reclaimed;
    > >>                unsigned long scanned = sc->nr_scanned;
    > >>                unsigned long nr_reclaimed;
    > >> +               int epriority = priority;
    > >> +
    > >> +               if (mem_cgroup_soft_limit_exceeded(root, mem))
    > >> +                       epriority -= 1;
    > >
    > > Here we grant the ability to shrink from all the memcgs, but only
    > > higher the priority for those exceed the soft_limit. That is a design
    > > change
    > > for the "soft_limit" which giving a hint to which memcgs to reclaim
    > > from first under global memory pressure.
    >
    >
    > Basically, we shouldn't reclaim from a memcg under its soft_limit
    > unless we have trouble reclaim pages from others.

    Agreed.

    > Something like the following makes better sense:
    >
    > diff --git a/mm/vmscan.c b/mm/vmscan.c
    > index bdc2fd3..b82ba8c 100644
    > --- a/mm/vmscan.c
    > +++ b/mm/vmscan.c
    > @@ -1989,6 +1989,8 @@ restart:
    > throttle_vm_writeout(sc->gfp_mask);
    > }
    >
    > +#define MEMCG_SOFTLIMIT_RECLAIM_PRIORITY 2
    > +
    > static void shrink_zone(int priority, struct zone *zone,
    > struct scan_control *sc)
    > {
    > @@ -2001,13 +2003,13 @@ static void shrink_zone(int priority, struct zone *zone,
    > unsigned long reclaimed = sc->nr_reclaimed;
    > unsigned long scanned = sc->nr_scanned;
    > unsigned long nr_reclaimed;
    > - int epriority = priority;
    >
    > - if (mem_cgroup_soft_limit_exceeded(root, mem))
    > - epriority -= 1;
    > + if (!mem_cgroup_soft_limit_exceeded(root, mem) &&
    > + priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
    > + continue;

    yes, this makes sense but I am not sure about the right(tm) value of the
    MEMCG_SOFTLIMIT_RECLAIM_PRIORITY. 2 sounds too low. You would do quite a
    lot of loops
    (DEFAULT_PRIORITY-MEMCG_SOFTLIMIT_RECLAIM_PRIORITY) * zones * memcg_count
    without any progress (assuming that all of them are under soft limit
    which doesn't sound like a totally artificial configuration) until you
    allow reclaiming from groups that are under soft limit. Then, when you
    finally get to reclaiming, you scan rather aggressively.

    Maybe something like 3/4 of DEFAULT_PRIORITY? You would get 3 times
    over all (unbalanced) zones and all cgroups that are above the limit
    (scanning max{1/4096+1/2048+1/1024, 3*SWAP_CLUSTER_MAX} of the LRUs for
    each cgroup) which could be enough to collect the low hanging fruit.
    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic
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