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

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Date:	Tue, 07 Oct 2008 16:34:22 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Michael Rubin <mrubin@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Righi <righi.andrea@...il.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

Michael Rubin wrote:
> 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.
> 

Just provide a vm_dirty_ration_in_bytes interface and keep it in sync with
vm_dirty_ratio (they are just two representations of the same internal value)
and for higher resolution propose that users use the bytes interface.



-- 
	Balbir
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