linux-kernel - Re: [RFC 2/4] memcg: make it suck faster

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Date:	Wed, 26 Sep 2012 10:03:43 +0100
From:	"Daniel P. Berrange" <berrange@...hat.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, devel@...nvz.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Michal Hocko <mhocko@...e.cz>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC 2/4] memcg: make it suck faster

On Wed, Sep 26, 2012 at 12:53:21PM +0400, Glauber Costa wrote:
> On 09/26/2012 01:02 AM, Andrew Morton wrote:
> >> nomemcg  : memcg compile disabled.
> >> > base     : memcg enabled, patch not applied.
> >> > bypassed : memcg enabled, with patch applied.
> >> > 
> >> >                 base    bypassed
> >> > User          109.12      105.64
> >> > System       1646.84     1597.98
> >> > Elapsed       229.56      215.76
> >> > 
> >> >              nomemcg    bypassed
> >> > User          104.35      105.64
> >> > System       1578.19     1597.98
> >> > Elapsed       212.33      215.76
> >> > 
> >> > So as one can see, the difference between base and nomemcg in terms
> >> > of both system time and elapsed time is quite drastic, and consistent
> >> > with the figures shown by Mel Gorman in the Kernel summit. This is a
> >> > ~ 7 % drop in performance, just by having memcg enabled. memcg functions
> >> > appear heavily in the profiles, even if all tasks lives in the root
> >> > memcg.
> >> > 
> >> > With bypassed kernel, we drop this down to 1.5 %, which starts to fall
> >> > in the acceptable range. More investigation is needed to see if we can
> >> > claim that last percent back, but I believe at last part of it should
> >> > be.
> > Well that's encouraging.  I wonder how many users will actually benefit
> > from this - did I hear that major distros are now using memcg in some
> > system-infrastructure-style code?
> > 
> 
> If they do, they actually be come "users of memcg". This here is aimed
> at non-users of memcg, which given all the whining about it, it seems to
> be plenty.
> 
> Also, I noticed, for instance, that libvirt is now creating memcg
> hierarchies for lxc and qemu as placeholders, before you actually create
> any vm or container.

This is mostly just lazyness on our part. There's no technical reason
why we can't delay creating our intermediate cgroups until we actually
have a VM ready to start, it was just simpler to create them when we
started the main daemon.


Daniel
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