On Tue, 30 Aug 2011 12:17:26 +0200
Johannes Weiner [off-list ref] wrote:

On Tue, Aug 30, 2011 at 05:56:09PM +0900, KAMEZAWA Hiroyuki wrote:

quoted

On Tue, 30 Aug 2011 10:42:45 +0200
Johannes Weiner [off-list ref] wrote:

quoted

quoted

quoted

Assume 3 cgroups in a hierarchy.

	A
       /
      B
     /
    C

C's scan contains 3 causes.
	C's scan caused by limit of A.
	C's scan caused by limit of B.
	C's scan caused by limit of C.

If we make hierarchy sum at read, we think
	B's scan_stat = B's scan_stat + C's scan_stat
But in precice, this is

	B's scan_stat = B's scan_stat caused by B +
			B's scan_stat caused by A +
			C's scan_stat caused by C +
			C's scan_stat caused by B +
			C's scan_stat caused by A.

In orignal version.
	B's scan_stat = B's scan_stat caused by B +
			C's scan_stat caused by B +

After this patch,
	B's scan_stat = B's scan_stat caused by B +
			B's scan_stat caused by A +
			C's scan_stat caused by C +
			C's scan_stat caused by B +
			C's scan_stat caused by A.

Hmm...removing hierarchy part completely seems fine to me.

I see.

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.

hmm. I have no strong opinion.

I do :-)

BTW,  how to calculate C's lru scan caused by A finally ?

            A
           /
          B
         /
        C

At scanning LRU of C because of A's limit, where stats are recorded ?

If we record it in C, we lose where the memory pressure comes from.
If we record it in A, we lose where scan happens.
I'm sorry I'm a little confused.

Thanks,
-Kame





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