linux-kernel - Re: [RFD] Merge task counter into memcg

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Date:	Fri, 13 Apr 2012 10:42:24 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Johannes Weiner <hannes@...xchg.org>,
	Glauber Costa <glommer@...allels.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Walsh <dwalsh@...hat.com>,
	"Daniel P. Berrange" <berrange@...hat.com>,
	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>
Subject: Re: [RFD] Merge task counter into memcg

(2012/04/13 2:41), Tejun Heo wrote:

> Hello, Johannes.

> I'm still split on the issue.
> 
> * #tasks as unit of accounting / limiting is well understood (or at
>   least known).  I think this holds the same to #open files, to a
>   lesser extent.  It means there are and will continue to be people
>   wanting them.  So, they have some value in familiarity - "but... I
>   want to limit the resources consumed by tasks cuz that's what I
>   know!" factor.
> 
> * People could want counting and limiting #tasks or #open files
>   without the overhead of tracking all memory resources.  This stems
>   from the same reason #tasks was used for this sort of things in the
>   first place.  Counting tasks or open files tends to be easier and
>   cheaper than tracking all memory allocations.
> 
> So, there's spectrum of solutions between merging task counter and
> just directing everyone to kmem without distinguishing task resource
> at all, and at the moment voices in my head are succeeding at making
> cases for both directions.  What do you guys think about the above two
> issues?
> 


To be honest, I doubt that task counter is unnecessary...memcg can catch
oom situation well. I often test 'make -j' under memcg.

To the questions
*   It sounds like a 'ulimit' cgroup. How about overwriting
    ulimit values via cgroup ? (sounds joke?) Then, overhead will be small but
    I'm not sure it can be hierarchical and doesn't break userland.

    If people wants to limit the number of tasks, I think interface should provide it
    in the unit of objects. Then, I'm ok to have other subsystem for counting something.
    fork-bomb's memory overhead can be prevent by memcg. What memcg cannot handle
    is ulimit. If forkbomb exhausts all ulimit/tasks, the user cannot login.
    So, having task-limit cgroup subsys for a sandbox will make sense in some situation.

In short, I don't think it's better to have task-counting and fd-counting in memcg.
It's kmem, but it's more than that, I think.
Please provide subsys like ulimit.

Thanks,
-Kame

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