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

Ying Han
2011-06-16T00:33:38Z

On Wed, Jun 15, 2011 at 3:57 PM, Ying Han <yinghan@google.com> wrote:
> 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?

Also

+bool mem_cgroup_soft_limit_exceeded(struct mem_cgroup *mem)
+{
+ return res_counter_soft_limit_excess(&mem->res);
+}

--Ying
>
> 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
>>
>