Re: [patch 4/8] memcg: rework soft limit reclaim

Ying Han
2011-06-15T22:58:05Z

On Fri, Jun 10, 2011 at 12:36 AM, Michal Hocko <mhocko@suse.cz> wrote:
> On Thu 09-06-11 17:00:26, Michal Hocko wrote:
>> 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.
>
> There is also another problem. I have just realized that this code path
> is shared with the cgroup direct reclaim. We shouldn't care about soft
> limit in such a situation. It would be just a wasting of cycles. So we
> have to:
>
> if (current_is_kswapd() &&
>        !mem_cgroup_soft_limit_exceeded(root, mem) &&
>        priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
>        continue;

Agreed.

>
> Maybe the condition would have to be more complex for per-cgroup
> background reclaim, though.

That would be the same logic for per-memcg direct reclaim. In general,
we don't consider soft_limit
unless the global memory pressure. So the condition could be something like:

> if ( global_reclaim(sc) &&
>        !mem_cgroup_soft_limit_exceeded(root, mem) &&
>        priority > MEMCG_SOFTLIMIT_RECLAIM_PRIORITY)
>        continue;

make sense?

Thanks

--Ying
>
>> 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
>