Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Michal Hocko <mhocko@suse.cz>]

On Thu 12-04-12 04:22:33, Johannes Weiner wrote:
> On Wed, Apr 11, 2012 at 08:45:33PM -0400, Rik van Riel wrote:
> > On 04/11/2012 06:00 PM, Ying Han wrote:
> > >1. If soft_limit are all set to MAX, it wastes first three periority iterations
> > >without scanning anything.
> > >
> > >2. By default every memcg is eligibal for softlimit reclaim, and we can also
> > >set the value to MAX for special memcg which is immune to soft limit reclaim.
> > >
> > >This idea is based on discussion with Michal and Johannes from LSF.
> > 
> > Combined with patch 2/5, would this not result in always
> > returning "reclaim from this memcg" for groups without a
> > configured softlimit, while groups with a configured
> > softlimit only get reclaimed from when they are over
> > their limit?
> > 
> > Is that the desired behaviour when a system has some
> > cgroups with a configured softlimit, and some without?
> 
> Yes, in general I think this new behaviour is welcome.
> 
> In the past, soft limits were only used to give excess memory a lower
> priority and there was no particular meaning associated with "being
> below your soft limit".  This change makes it so that soft limits are
> actually a minimum guarantee, too, so you wouldn't get reclaimed if
> you behaved (if possible):
> 
> 		A-unconfigured		B-below-softlimit
> old:		reclaim			reclaim
> new:		reclaim			no reclaim (if possible)
> 
> The much less obvious change here, however, is that we no longer put
> extra pressure on groups above their limit compared to unconfigured
> groups:
> 
> 		A-unconfigured		B-above-softlimit
> old:		reclaim			reclaim twice
> new:		reclaim			reclaim

Agreed and I guess that the above should be a part of the changelog.
This is changing previous behavior and we should rather be explicit
about that.

> I still think that it's a reasonable use case to put a soft limit on a
> workload to "nice" it memory-wise, without looking at the machine as a
> whole and configuring EVERY cgroup based on global knowledge and
> static partitioning of the machine.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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