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

[Johannes Weiner <hannes@cmpxchg.org>]

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

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.

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