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

[Michal Hocko <mhocko@suse.cz>]

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;

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

> 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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