Re: memcg: softlimit on internal nodes

[Michal Hocko <mhocko@suse.cz>]

On Mon 22-04-13 08:57:03, Tejun Heo wrote:
> On Mon, Apr 22, 2013 at 05:19:08PM +0200, Michal Hocko wrote:
> > We can try to be clever during the outside pressure and prefer
> > reclaiming over soft limit groups first. Which we used to do and will
> > do after rework as well. As a side effect of that a properly designed
> > hierachy with opt-in soft limited groups can actually accomplish some
> > isolation is a nice side effect but no _guarantee_.
> 
> Okay, so it *is* a soft limit.  Good.  If so, a subtree going over the
> limit of course forces reclaim on its children even though their
> individual configs aren't over limit.  It's exactly the same as
> hardlimit.  There doesn't need to be any difference and there's
> nothing questionable or interesting about it.
> 
> Also, then, a cgroup which has been configured explicitly shouldn't be
> disadvantaged compared to a cgroup with a limit configured.  ie. the
> current behavior of giving maximum to the knob on creation is the
> correct one. 

Although the default limit is correct it is impractical for use
because it doesn't allow for "I behave do not reclaim me if you can"
cases. And we can implement such a behavior really easily with backward
compatibility and new interfaces (aka reuse the soft limit for that).

I am approaching this from a simple perspective. Reclaim from everybody
who doesn't care about the soft limit (it hasn't been set for that
group) or who is above the soft limit. If that is sufficient to meet the
reclaim target then there is no reason to touch groups that _do_ care
about soft limit and they are under. Although this doesn't give you
any guarantee it can give a certain prioritization for groups in the
overcommit situations and that is what soft limit was intended for from
the very beginning.

> The knob should create *extra* pressure.  It shouldn't
> lessen the pressure.  When populated weith other cgroups with limits
> configured, it would change the relative pressure felt by each but in
> general it's a limiting mechanism not an isolation one.  I think the
> bulk of confusion is coming from this, so please make that abundantly
> clear.
> 
> And, if people want a mechanism for isolation / lessening of pressure,
> which looks like a valid use case to me, add another knob for that
> which is prioritized under both hard and soft limits.  That is the
> only sensible way to do it.

No, please no yet another knob. We have too many of them already. And
even those that are here for a long time can be confusing as one can
see.

> Alright, no complaint anymore.  Thanks.
> 
> -- 
> tejun
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-- 
Michal Hocko
SUSE Labs

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