Re: [PATCH V8 1/2] mm: memcg softlimit reclaim rework

[Glauber Costa <glommer@parallels.com>]

On 08/20/2012 10:30 PM, Ying Han wrote:
> Not exactly. Here reclaiming from root is mainly for "reclaiming from
> root's exclusive lru", which links the page includes:
> 1. processes running under root
> 2. reparented pages from rmdir memcg under root
> 3. bypassed pages
> 
> Setting root cgroup's softlimit = 0 has the implication of putting
> those pages to likely to reclaim, which works fine. The question is
> that if no other memcg is above its softlimit, would it be a problem
> to adding a bit extra pressure to root which always is eligible for
> softlimit reclaim ( usage is always greater than softlimit).
> 
> As an example, it works fine in our environment since we don't
> explicitly put any process under root. Most of  the pages linked in
> root lru would be reparented pages which should be reclaimed prior to
> others.

Keep in mind that not all environments will be specialized to the point
of having root memcg empty. This basically treats root memcg as a trash
bin, and can be very detrimental to use cases where actual memory is
present in there.

It would maybe be better to have all this garbage to go to a separate
place, like a shadow garbage memcg, which is invisible to the
filesystem, and is always the first to be reclaimed from, in any
circumstance.


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