linux-kernel - Re: [PATCH] memcg: update documentation to describe usage_in

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Message-Id: <20110328193108.07965b4a.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 28 Mar 2011 19:31:08 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: update documentation to describe usage_in_bytes

On Mon, 28 Mar 2011 11:48:20 +0200
Michal Hocko <mhocko@...e.cz> wrote:

> On Mon 28-03-11 18:11:27, KAMEZAWA Hiroyuki wrote:
> > On Mon, 28 Mar 2011 09:43:42 +0200
> > Michal Hocko <mhocko@...e.cz> wrote:
> > 
> > > On Mon 28-03-11 13:25:50, Daisuke Nishimura wrote:
> > > > From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> [...]
> > > > +5.5 usage_in_bytes
> > > > +
> > > > +As described in 2.1, memory cgroup uses res_counter for tracking and limiting
> > > > +the memory usage. memory.usage_in_bytes shows the current res_counter usage for
> > > > +memory, and DOESN'T show a actual usage of RSS and Cache. It is usually bigger
> > > > +than the actual usage for a performance improvement reason. 
> > > 
> > > Isn't an explicit mention about caching charges better?
> > > 
> > 
> > It's difficult to distinguish which is spec. and which is implemnation details...
> 
> Sure. At least commit log should contain the implementation details IMO,
> though.
> 
> > 
> > My one here ;)
> > ==
> > 5.5 usage_in_bytes
> > 
> > For efficiency, as other kernel components, memory cgroup uses some optimization to
> > avoid unnecessary cacheline false sharing. usage_in_bytes is affected by the
> > method and doesn't show 'exact' value of usage, it's an fuzz value for efficient
> > access. (Of course, when necessary, it's synchronized.)
> > In usual, the value (RSS+CACHE) in memory.stat shows more exact value. IOW,
> 
> - In usual, the value (RSS+CACHE) in memory.stat shows more exact value. IOW,
> + (RSS+CACHE) value from memory.stat shows more exact value and should be used
> + by userspace. IOW,
> 
> ?
> 

seems good. Nishimura-san, could you update ?

Thanks,
-Kame

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