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

Michal Hocko
2011-06-09T15:00:33Z

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