linux-kernel - Re: [PATCH 00/23] per device dirty throttling -v10

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Date:	Tue, 11 Sep 2007 22:31:32 -0400
From:	"John Stoffel" <john@...ffel.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, akpm@...ux-foundation.org, neilb@...e.de,
	dgc@....com, tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com,
	richard@....demon.co.uk, torvalds@...ux-foundation.org
Subject: Re: [PATCH 00/23] per device dirty throttling -v10

Peter> Per device dirty throttling patches These patches aim to
Peter> improve balance_dirty_pages() and directly address three
Peter> issues:

Peter>   1) inter device starvation
Peter>   2) stacked device deadlocks
Peter>   3) inter process starvation

Peter> 1 and 2 are a direct result from removing the global dirty
Peter> limit and using per device dirty limits. By giving each device
Peter> its own dirty limit is will no longer starve another device,
Peter> and the cyclic dependancy on the dirty limit is broken.

Ye haa!  This should be a big improvement.  

Peter> In order to efficiently distribute the dirty limit across the
Peter> independant devices a floating proportion is used, this will
Peter> allocate a share of the total limit proportional to the
Peter> device's recent activity.

I'm not sure I like or agree with this.  Shouldn't we be limiting
based on the device's capability to sustain traffic?  So if I have a
RAID device which can read/write a total of 100Mb/sec, while at the
same time I've got a CF device which can do 5Mb/sec, shouldn't we be
more strongly limiting the CF device, even if it is the only device
being written to?  

Of course, I haven't read the patches yet, nor am I qualified to
comment on them in any meanginful way I think.  Hopefully I'm just
missing something key here in the explanation.

Peter> 3 is done by also scaling the dirty limit proportional to the
Peter> current task's recent dirty rate.

Do you mean task or device here?  I'm just wondering how well this
works with a bunch of devices with wildly varying speeds.  

John
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