Re: [RFC][PATCH 0/4] Memory controller soft limit patches (v2)

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

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-02-17 15:36:58]:

> On Tue, 17 Feb 2009 11:09:03 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-02-17 14:10:39]:
> > 
> > > On Tue, 17 Feb 2009 10:11:10 +0530
> > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > 
> > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-02-17 13:03:52]:
> > > > 
> > > > > On Tue, 17 Feb 2009 08:35:26 +0530
> > > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > > > I don't want to add any new big burden to kernel hackers of memory management,
> > > > > they work hard to improve memory reclaim. This patch will change the behavior.
> > > > > 
> > > > 
> > > > I don't think I agree, this approach suggests that before doing global
> > > > reclaim, there are several groups that are using more than their
> > > > share of memory, so it makes sense to reclaim from them first.
> > > > 
> > > 
> > > > 
> > > > > BTW, in typical bad case, several threads on cpus goes into memory recalim at once and
> > > > > all thread will visit this memcg's soft-limit tree at once and soft-limit will
> > > > > not work as desired anyway.
> > > > > You can't avoid this problem at alloc_page() hot-path.
> > > > 
> > > > Even if all threads go into soft-reclaim at once, the tree will become
> > > > empty after a point and we will just return saying there are no more
> > > > memcg's to reclaim from (we remove the memcg from the tree when
> > > > reclaiming), then those threads will go into regular reclaim if there
> > > > is still memory pressure.
> > > 
> > > Yes. the largest-excess group will be removed. So, it seems that it doesn't work
> > > as designed. rbtree is considered as just a hint ? If so, rbtree seems to be
> > > overkill.
> > > 
> > > just a question:
> > > Assume memcg under hierarchy.
> > >    ../group_A/                 usage=1G, soft_limit=900M  hierarchy=1
> > >               01/              usage=200M, soft_limit=100M
> > >               02/              usage=300M, soft_limit=200M
> > >               03/              usage=500M, soft_limit=300M  <==== 200M over.
> > >                  004/          usage=200M, soft_limit=100M
> > >                  005/          usage=300M, soft_limit=200M
> > > 
> > > At memory shortage, group 03's memory will be reclaimed 
> > >   - reclaim memory from 03, 03/004, 03/005
> > > 
> > > When 100M of group 03' memory is reclaimed, group_A 's memory is reclaimd at the
> > > same time, implicitly. Doesn't this break your rb-tree ?
> > > 
> > > I recommend you that soft-limit can be only applied to the node which is top of
> > > hierarchy.
> > 
> > Yes, that can be done, but the reason for putting both was to target
> > the right memcg early.
> > 
> My point is  that sort by rb-tree is broken in above case.
>

OK, I'll explore, experiment and think about adding just the root 

-- 
	Balbir

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