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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

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.
 
> > 
> > > > 3. After this patch, res_counter is no longer for general purpose res_counter...
> > > >    It seems to have too many unnecessary accessories for general purpose.  
> > > 
> > > Why not? Soft limits are a feature of any controller. The return of
> > > highest ancestor might be the only policy we impose right now. But as
> > > new controllers start using res_counter, we can clearly add a policy
> > > callback.
> > > 
> > I think you forget that memroy cgroups is an only controller in which the kernel
> > can reduce the usage of resource without any harmful to users.
> > soft-limit is nonsense for general resources, I think.
> > 
> 
> Really? Even for CPUs? soft-limit is a form of shares (please don't
> confuse with cpu.shares). Soft limits is used as a way of implementing
> work conserving controllers.
> 

I don't think cpu needs this. It works under share and no hardlimit.

THanks,
-Kame


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