Re: [rfc patch 4/6] memcg: reclaim statistics

[Ying Han <yinghan@google.com>]

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On Mon, May 16, 2011 at 4:10 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Thu, May 12, 2011 at 12:33:50PM -0700, Ying Han wrote:
> > On Thu, May 12, 2011 at 7:53 AM, Johannes Weiner <hannes@cmpxchg.org>
> wrote:
> >
> > > TODO: write proper changelog.  Here is an excerpt from
> > > http://lkml.kernel.org/r/20110428123652.GM12437@cmpxchg.org:
> > >
> > > : 1. Limit-triggered direct reclaim
> > > :
> > > : The memory cgroup hits its limit and the task does direct reclaim
> from
> > > : its own memcg.  We probably want statistics for this separately from
> > > : background reclaim to see how successful background reclaim is, the
> > > : same reason we have this separation in the global vmstat as well.
> > > :
> > > :       pgscan_direct_limit
> > > :       pgfree_direct_limit
> > >
> >
> > Can we use "pgsteal_" instead? Not big fan of the naming but want to make
> > them consistent to other stats.
>
> Actually, I thought what KAME-san said made sense.  'Stealing' is a
> good fit for reclaim due to outside pressure.  But if the memcg is
> target-reclaimed from the inside because it hit the limit, is
> 'stealing' the appropriate term?
>
> > > : 2. Limit-triggered background reclaim
> > > :
> > > : This is the watermark-based asynchroneous reclaim that is currently
> in
> > > : discussion.  It's triggered by the memcg breaching its watermark,
> > > : which is relative to its hard-limit.  I named it kswapd because I
> > > : still think kswapd should do this job, but it is all open for
> > > : discussion, obviously.  Treat it as meaning 'background' or
> > > : 'asynchroneous'.
> > > :
> > > :       pgscan_kswapd_limit
> > > :       pgfree_kswapd_limit
> > >
> >
> > Kame might have this stats on the per-memcg bg reclaim patch. Just
> mention
> > here since it will make later merge
> > a bit harder
>
> I'll have a look, thanks for the heads up.
>
> > > : 3. Hierarchy-triggered direct reclaim
> > > :
> > > : A condition outside the memcg leads to a task directly reclaiming
> from
> > > : this memcg.  This could be global memory pressure for example, but
> > > : also a parent cgroup hitting its limit.  It's probably helpful to
> > > : assume global memory pressure meaning that the root cgroup hit its
> > > : limit, conceptually.  We don't have that yet, but this could be the
> > > : direct softlimit reclaim Ying mentioned above.
> > > :
> > > :       pgscan_direct_hierarchy
> > > :       pgsteal_direct_hierarchy
> > >
> >
> >  The stats for soft_limit reclaim from global ttfp have been merged in
> mmotm
> > i believe as the following:
> >
> > "soft_direct_steal"
> > "soft_direct_scan"
> >
> > I wonder we might want to separate that out from the other case where the
> > reclaim is from the parent triggers its limit.
>
> The way I implemented soft limits in 6/6 is to increase pressure on
> exceeding children whenever hierarchical reclaim is taking place.
>
> This changes soft limit from
>
>        Global memory pressure: reclaim from exceeding memcg(s) first
>
> to
>
>        Memory pressure on a memcg: reclaim from all its children,
>        with increased pressure on those exceeding their soft limit
>        (where global memory pressure means root_mem_cgroup and all
>        existing memcgs are considered its children)
>
> which makes the soft limit much more generic and more powerful, as it
> allows the admin to prioritize reclaim throughout the hierarchy, not
> only for global memory pressure.  Consider one memcg with two
> subgroups.  You can now prioritize reclaim to prefer one subgroup over
> another through soft limiting.
>
> This is one reason why I think that the approach of maintaining a
> global list of memcgs that exceed their soft limits is an inferior
> approach; it does not take the hierarchy into account at all.
>


This scheme would not provide a natural way of counting pages that
> were reclaimed because of the soft limit, and thus I still oppose the
> merging of soft limit counters.
>
> The proposal we discussed during LSF ( implemented in the patch " memcg:
revisit soft_limit reclaim on contention") takes consideration
of hierarchical reclaim. The memcg is linked in the list if it exceeds the
soft_limit, and the soft_limit reclaim per-memcg is calling
mem_cgroup_hierarchical_reclaim().

The current "soft_steal" and "soft_scan" is counting pages being steal/scan
 inside mem_cgroup_hierarchical_reclaim() w check_soft checking, which then
counts pages being reclaimed because of soft_limit and also counting the
hierarchical reclaim.

Sorry If i missed something before reading through your whole patch set.

--Ying

       Hannes
>

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