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

[Johannes Weiner]

On Tue, Aug 30, 2011 at 07:38:39PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 30 Aug 2011 12:17:26 +0200
> Johannes Weiner <jweiner@xxxxxxxxxx> wrote:
> 
> > On Tue, Aug 30, 2011 at 05:56:09PM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Tue, 30 Aug 2011 10:42:45 +0200
> > > Johannes Weiner <jweiner@xxxxxxxxxx> wrote:
>  
> > > > > 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.

It's recorded in C as 'scanned due to parent'.

If you want to track down where pressure comes from, you check the
outer container, B.  If B is scanned due to internal pressure, you
know that C's external pressure comes from B.  If B is scanned due to
external pressure, you know that B's and C's pressure comes from A or
the physical memory limit (the outermost container, so to speak).

The containers are nested.  If C is scanned because of the limit in A,
then this concerns B as well and B must be scanned as well as B, as
C's usage is fully contained in B.

There is not really a direct connection between C and A that is
irrelevant to B, so I see no need to record in C which parent was the
cause of the pressure.  Just that it was /a/ parent and not itself.
Then you can follow the pressure up the hierarchy tree.

Answer to your original question:

	C_scan_due_to_A = C_scan_external - B_scan_internal - A_scan_external

But IMO, having this exact number is not necessary to find the reason
for why C is experiencing memory pressure in the first place, and I
assume that this is the goal.
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