linux-kernel - Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock

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Message-ID: <20111227223012.GJ17712@google.com>
Date:	Tue, 27 Dec 2011 14:30:12 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>, avi@...hat.com, nate@...nel.net,
	cl@...ux-foundation.org, oleg@...hat.com, axboe@...nel.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool
 and fix blkcg percpu alloc deadlock

Hello, Andrew.

On Tue, Dec 27, 2011 at 02:21:56PM -0800, Andrew Morton wrote:
> <autorepeat>For those users who don't want the stats, stats shouldn't
> consume any resources at all.

Hmmm.... For common use cases - a few cgroups doing IOs to most likely
single physical device and maybe a couple virtual ones, I don't think
this would show up anywhere both in terms of memory and process
overhead.  While avoding it would be nice, I don't think that should
be the focus of optimization or design decisions.

> And I bet that the majority of the minority who want stats simply want
> to know "how much IO is this cgroup doing", and don't need per-cgroup,
> per-device accounting.
> 
> And it could be that the minority of the minority who want per-device,
> per-cgroup stats only want those for a minority of the time.
> 
> IOW, what happens if we give 'em atomic_add() and be done with it?

I really don't know.  That surely is an enticing idea tho.  Jens,
Vivek, can you guys chime in?  Is gutting out (or drastically
simplifying) cgroup-dev stats an option?  Are there users who are
actually interested in this stuff?

Thanks.

-- 
tejun
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