Re: [patch] Revert "memcg: add memory.vmscan_stat"

Johannes Weiner
2011-09-01T06:41:25Z

On Wed, Aug 31, 2011 at 11:05:51PM -0700, Ying Han wrote:
> On Tue, Aug 30, 2011 at 1:42 AM, Johannes Weiner <jweiner@redhat.com> wrote:
> > You want to look at A and see whether its limit was responsible for
> > reclaim scans in any children.  IMO, that is asking the question
> > backwards.  Instead, there is a cgroup under reclaim and one wants to
> > find out the cause for that.  Not the other way round.
> >
> > In my original proposal I suggested differentiating reclaim caused by
> > internal pressure (due to own limit) and reclaim caused by
> > external/hierarchical pressure (due to limits from parents).
> >
> > If you want to find out why C is under reclaim, look at its reclaim
> > statistics.  If the _limit numbers are high, C's limit is the problem.
> > If the _hierarchical numbers are high, the problem is B, A, or
> > physical memory, so you check B for _limit and _hierarchical as well,
> > then move on to A.
> >
> > Implementing this would be as easy as passing not only the memcg to
> > scan (victim) to the reclaim code, but also the memcg /causing/ the
> > reclaim (root_mem):
> >
> >        root_mem == victim -> account to victim as _limit
> >        root_mem != victim -> account to victim as _hierarchical
> >
> > This would make things much simpler and more natural, both the code
> > and the way of tracking down a problem, IMO.
>
> This is pretty much the stats I am currently using for debugging the
> reclaim patches. For example:
>
> scanned_pages_by_system 0
> scanned_pages_by_system_under_hierarchy 50989
>
> scanned_pages_by_limit 0
> scanned_pages_by_limit_under_hierarchy 0
>
> "_system" is count under global reclaim, and "_limit" is count under
> per-memcg reclaim.
> "_under_hiearchy" is set if memcg is not the one triggering pressure.

I don't get this distinction between _system and _limit. How is it
orthogonal to _limit vs. _hierarchy, i.e. internal vs. external?

If the system scans memcgs then no limit is at fault. It's just
external pressure.

For example, what is the distinction between scanned_pages_by_system
and scanned_pages_by_system_under_hierarchy? The reason for
scanned_pages_by_system would be, per your definition, neither due to
the limit (_by_system -> global reclaim) nor not due to the limit
(!_under_hierarchy -> memcg is the one triggering pressure)