Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v5)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> [2009-03-13 16:17:25]:

> > > hm
> > > I read past discussion. so, I think we discuss many aspect at once.
> > > So, my current thinking is below, 
> > > 
> > > (1) if the group don't have any soft limit shrinking page, 
> > >     mem_cgroup_soft_limit_reclaim() spent time unnecessary.
> > >     -> right.
> > 
> > If the soft limit RB tree is empty, we don't spend any time at all.
> > Are you referring to something else? Am I missing something? The tree
> > will be empty if no group is over the soft limit.
> 
> maybe, I am missing anything.
> May I ask your following paragraph meaning?
> 
> 
> > I experimented a *lot* with zone reclaim and found it to be not so
> > effective. Here is why
> > 
> > 1. We have no control over priority or how much to scan, that is
> > controlled by balance_pgdat(). If we find that we are unable to scan
> > anything, we continue scanning with the scan > 0 check, but we scan
> > the same pages and the same number, because shrink_zone does scan >>
> > priority.
> 
> I thought this sentense mean soft-limit-shrinking spent a lot of time.
> if not, could you please tell me what makes so slower?
> 

Let me summarize below


> and, you wrote:
> 
> > 
> > Yes, I sent that reason out as comments to Kame's patches. kswapd or
> > balance_pgdat controls the zones, priority and in effect how many
> > pages we scan while doing reclaim. I did lots of experiments and found
> > that if soft limit reclaim occurred from kswapd, soft_limit_reclaim
> > would almost always fail and shrink_zone() would succeed, since it
> > looks at the whole zone and is always able to find some pages at all
> > priority levels. It also does not allow for targetted reclaim based on
> > how much we exceed the soft limit by. 
> 
> but, if "soft_limit_reclaim fail and shrink_zone() succeed" don't cause
> any performance degression, I don't find why kswapd is wrong.
> 
> I guess you and kamezawa-san know it detail. but my understanding don't reach it.
> Could you please tell me what so slowness.

Here is what I saw in my experiments

1. Kame's scan logic, selects shrink_zone for the mem cgroup, but the
   pages scanned and reclaimed from depend on priority and watermarks
   of the zone and *not* at all on the soft limit parameters.
2. Because soft limit reclaim fails to reclaim anythoing (due to 1),
   shrink_zone which is called, does reclaiming indepedent of any
   knowledge of soft limits, which does not work as expected.



-- 
	Balbir

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