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

[Ying Han <yinghan@google.com>]

On Mon, Apr 16, 2012 at 8:15 AM, Michal Hocko <mhocko@suse.cz> wrote:
> 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.

Ok, I will include it on next post.

Thanks !

--Ying

>
>> 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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