Re: [PATCH V3 0/2] memcg softlimit reclaim rework

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 20, 2012 at 08:58:47PM +0200, Michal Hocko wrote:
> On Fri 20-04-12 10:44:14, Ying Han wrote:
> > On Fri, Apr 20, 2012 at 6:17 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > Let me repeat the pros here: no breaking of existing semantics.  No
> > > introduction of unprecedented semantics into the cgroup mess.  No
> > > changing of kernel code necessary (except what we want to tune
> > > anyway).  No computational overhead for you or anyone else.
> > 
> > >
> > > If your only counter argument to this is that you can't be bothered to
> > > slightly adjust your setup, I'm no longer interested in this
> > > discussion.
> > 
> > Before going further, I wanna make sure there is no mis-communication
> > here. As I replied to Michal, I feel that we are mixing up global
> > reclaim and target reclaim policy here.
> 
> I was referring to the global reclaim and my understanding is that
> Johannes did the same when talking about soft reclaim (even though it
> makes some sense to apply the same rules to the hard limit reclaim as
> well - but later to that one...)
> 
> The primary question is whether soft reclaim should be hierarchical or
> not. That is what I've tried to express in other email earlier in this
> thread where I've tried (very briefly) to compare those approaches.
> It currently _is_ hierarchical and your patch changes that so we have to
> be sure that this change in semantic is reasonable. The only workload
> that you seem to consider is when you have a full control over the
> machine while Johannes is considered about containers which might misuse
> your approach to push out working sets of concurrency...
> My concern with hierarchical approach is that it doesn't play well with
> 0 default (which is needed if we want to make soft limit a guarantee,
> right?). I do agree with Johannes about the potential misuse though.  So
> it seems that both approaches have serious issues with configurability.
> Does this summary clarify the issue a bit? Or I am confused as well ;)

Thanks for the nice summary!

A note on the default hierarchical soft limit:

Consider not making the default to be 0, but a special value.  We want
it to mean 'no guarantee' and 'every byte is in excess of the soft
limit', to keep the existing behaviour.  But at the same time, we
wouldn't have to make it inheritable:

    A (soft = default)
      A1 (soft = 10G)
      A2 (soft = 12G)

so in case of global reclaim, A itself would be eligible, but it would
not apply hierarchically to A1 and A2.  They would still only get
reclaimed if their usage would be above their respective soft limits.
Only if you set A's soft limit to 0 or higher it will apply
hierarchically, so that if a parent declares 'no guarantee', no child
is able to override it.

Maybe we can keep -1/~0UL and just treat it a bit differently.

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