On 08/23/2012 02:27 AM, Ying Han wrote:

On Tue, Aug 21, 2012 at 2:29 AM, Glauber Costa <glommer@parallels.com
<mailto:glommer@parallels.com>> wrote:

    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.


We can certainly do something like that, and actually we have the
*special* cgroup setup today in google's environment. It is mainly
targeting for pages that are allocated not on behalf of applications,
but more of 
system maintainess overhead. One example would be kernel thread memory
charging.

In this case, it might make sense to put those reparented pages to
a separate cgroup. However I do wonder with the following questions:

1.  it might only make sense to do that if something else running under
root. As we know, root is kind of special in memcg where there is no
limit on it. So I wonder what would be the real life use case to put
something under root?

It is a common desktop environment cgroups usage to separate some
special processes only in memcg, and bind their memory usage. This is
even mentioned in the docs as an example case, if I recall correctly.

Systemd, for instance, places all services in cgroups. All the rest of
the system, applications user launches, etc, would live in the root memcg.

2.  even the reparented pages are mixed together with pages from process
running under root, the LRU mechanism should still take effect of
evicting cold pages first. if the reparent pages are the left-over pages
from the removed cgroups, I would assume they are the candidate to
reclaim first.

I think this is a big assumption. To begin with, pages *just* reparented
will still be hot. They can be a lot.

I am curious that in your environment, do you have things running root? 

Our environment is also pretty easy, in the sense that all processes
that matters lives in a cgroup, 1 per-container. But say the admin wants
to install a management deamon, for instance, it won't necessarily be
accounted.

In any case, our use case is not my top concern here. Rather, it is the
general usage for people other than us.

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