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

[Ying Han <yinghan@google.com>]

On Wed, Aug 31, 2011 at 11:40 PM, Johannes Weiner <jweiner@redhat.com> wrote:
> 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?

Something like :

+enum mem_cgroup_scan_context {
+       SCAN_BY_SYSTEM,
+       SCAN_BY_SYSTEM_UNDER_HIERARCHY,
+       SCAN_BY_LIMIT,
+       SCAN_BY_LIMIT_UNDER_HIERARCHY,
+       NR_SCAN_CONTEXT,
+};

if (global_reclaim(sc))
   context = scan_by_system
else
   context = scan_by_limit

if (target != mem)
   context++;

>
> 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?

you are right about this, there is no much difference on these since
it is counting global reclaim and everyone
is under_hierarchy except root_cgroup. For root cgroup, it is counted
in "_system". (internal)

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)

This value "scanned_pages_by_system" only making senses for root
cgroup, which now could be counted as "# of pages scanned in root lru
under global reclaim".

--Ying

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