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 09:05:23]:

> On Mon, 16 Feb 2009 16:38:44 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
> > 
> > From: Balbir Singh <balbir@linux.vnet.ibm.com>
> > 
> > Changelog v2...v1
> > 1. Soft limits now support hierarchies
> > 2. Use spinlocks instead of mutexes for synchronization of the RB tree
> > 
> > Here is v2 of the new soft limit implementation. Soft limits is a new feature
> > for the memory resource controller, something similar has existed in the
> > group scheduler in the form of shares. The CPU controllers interpretation
> > of shares is very different though. We'll compare shares and soft limits
> > below.
> > 
> > Soft limits are the most useful feature to have for environments where
> > the administrator wants to overcommit the system, such that only on memory
> > contention do the limits become active. The current soft limits implementation
> > provides a soft_limit_in_bytes interface for the memory controller and not
> > for memory+swap controller. The implementation maintains an RB-Tree of groups
> > that exceed their soft limit and starts reclaiming from the group that
> > exceeds this limit by the maximum amount.
> > 
> > This is an RFC implementation and is not meant for inclusion
> > 
> 
> some thoughts after reading patch.
> 
> 1. As I pointed out, cpuset/mempolicy case is not handled yet.

That should be esy to do with zonelists passed from reclaim path

> 2. I don't like to change usual direct-memory-reclaim path. It will be obstacles
>    for VM-maintaners to improve memory reclaim. memcg's LRU is designed for
>    shrinking memory usage and not for avoiding memory shortage. IOW, it's slow routine
>    for reclaiming memory for memory shortage.

I don't think I agree here. Direct reclaim is the first indication of
shortage and if order 0 pages are short, memcg's above their soft
limit can be targetted first.

> 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.

> 4. please use css_tryget() rather than mem_cgroup_get().

OK, will do

> 5. please remove mem_cgroup from tree at force_empty or rmdir.
>    Just making  memcg->on_tree=false is enough ? I'm in doubt.

force_empty will cause uncharge and we handle it there, but I can add
an explicit call there as well.

> 6. What happens when the-largest-soft-limit-memcg has tons on Anon on swapless
>    system and memory reclaim cannot make enough progress ?

The samething that would happen on regular reclaim, one needs to
decide whether to oom or not from this context for memcg's.

-- 
	Balbir

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