linux-kernel - Re: [RFC] Shared page accounting for memory cgroup

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Date:	Wed, 6 Jan 2010 12:31:50 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [RFC] Shared page accounting for memory cgroup

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-01-06 13:02:58]:

> On Mon, 4 Jan 2010 06:20:31 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-01-04 09:35:28]:
> > 
> > > On Mon, 4 Jan 2010 05:37:52 +0530
> > > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > > 
> > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-01-04 08:51:08]:
> > > > 
> > > > > On Tue, 29 Dec 2009 23:57:43 +0530
> > > > > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > > > > 
> > > > > > Hi, Everyone,
> > > > > > 
> > > > > > I've been working on heuristics for shared page accounting for the
> > > > > > memory cgroup. I've tested the patches by creating multiple cgroups
> > > > > > and running programs that share memory and observed the output.
> > > > > > 
> > > > > > Comments?
> > > > > 
> > > > > Hmm? Why we have to do this in the kernel ?
> > > > >
> > > > 
> > > > For several reasons that I can think of
> > > > 
> > > > 1. With task migration changes coming in, getting consistent data free of races
> > > > is going to be hard.
> > > 
> > > Hmm, Let's see real-worlds's "ps" or "top" command. Even when there are no guarantee
> > > of error range of data, it's still useful.
> > 
> > Yes, my concern is this
> > 
> > 1. I iterate through tasks and calculate RSS
> > 2. I look at memory.usage_in_bytes
> > 
> > If the time in user space between 1 and 2 is large I get very wrong
> > results, specifically if the workload is changing its memory usage
> > drastically.. no?
> > 
> No. If it takes long time, locking fork()/exit() for such long time is the bigger
> issue.
> I recommend you to add memacct subsystem to sum up RSS of all processes's RSS counting
> under a cgroup.  Althoght it may add huge costs in page fault path but implementation
> will be very simple and will not hurt realtime ops.
> There will be no terrible race, I guess.
>

But others hold that lock as well, simple thing like listing tasks and
moving tasks, etc. I expect the usage of shared to be in the same
range.

-- 
	Balbir
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