linux-kernel - Re: [PATCH 3/5] writeback: dirty rate control

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Date:	Mon, 15 Aug 2011 22:11:38 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: dirty rate control

On Thu, Aug 11, 2011 at 01:10:26AM +0800, Peter Zijlstra wrote:
> On Wed, 2011-08-10 at 22:00 +0800, Wu Fengguang wrote:
> > 
> > > Although I'm not quite sure how he keeps fairness in light of the
> > > sleep time bounding to MAX_PAUSE.
> > 
> > Firstly, MAX_PAUSE will only be applied when the dirty pages rush
> > high (dirty exceeded).  Secondly, the dirty exceeded state is global
> > to all tasks, in which case each task will sleep for MAX_PAUSE equally.
> > So the fairness is still maintained in dirty exceeded state. 
> 
> Its not immediately apparent how dirty_exceeded and MAX_PAUSE interact,
> but having everybody sleep MAX_PAUSE doesn't necessarily mean its fair,
> its only fair if they dirty at the same rate.

Yeah I forget to mention that, but when dirty_exceeded, the tasks will
typically sleep for MAX_PAUSE on every 8 pages, so resulting in the
same dirty rate :)

Thanks,
Fengguang
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