linux-kernel - Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty

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Date:	Mon, 22 Sep 2008 16:41:33 -0700
From:	"Michael Rubin" <mrubin@...gle.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	"Andrea Righi" <righi.andrea@...il.com>, balbir@...ux.vnet.ibm.com,
	menage@...gle.com, kamezawa.hiroyu@...fujitsu.com,
	dave@...ux.vnet.ibm.com, chlunde@...g.uio.no, dpshah@...gle.com,
	eric.rannaud@...il.com, fernando@....ntt.co.jp, agk@...rceware.org,
	m.innocenti@...eca.it, s-uchida@...jp.nec.com, ryov@...inux.co.jp,
	matt@...ehost.com, dradford@...ehost.com,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio

On Fri, Sep 12, 2008 at 1:18 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> One thing to think about please: Michael Rubin is hitting problems with
> the existing /proc/sys/vm/dirty-ratio.  Its present granularity of 1%
> is just too coarse for really large machines, and as
> memory-size/disk-speed ratios continue to increase, this will just get
> worse.

Re-sending since I top-posted before. Never again. Also adding more
thoughts on a byte based interface.

Currently the problem we are hitting is that we cannot specify pdflush
to have background limits less than 1% of memory. I am currently
finishing up a patch right now that adds a dirty_ratio_millis
interface.  I hope to submit the patch to LKML by the end of the week.

The idea is that we don't want to break backwards compatibility and we
also don't want to have two conflicting knobs in the sysctl or
/proc/sys/vm/ space. I thought adding a new knob for those who want to
specify finer grained functionality was a compromise. So the patch has
a vm_dirty_ratio and a vm_dirty_ratio_millis interface. The first to
specify 0-100% and the second to specify .0 to .999%.

So to represent 0.125% of RAM we set
vm_dirty_ratio = 0
vm_dirty_ratio_millis = 125

The same for the background_ratio.

I would also prefer using a bytes interface but I am not sure how to
offer that without  either removing the legacy interface of the ratios
or by offering a concurrent interface that might be confusing such as
when users are looking at the old one and not aware of a new one.

Any feedback?

mrubin
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