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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