linux-kernel - Re: [PATCH 00/13] IO-less dirty throttling

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Date:	Thu, 18 Nov 2010 11:19:05 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@....de>, Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/13] IO-less dirty throttling

On Thu, Nov 18, 2010 at 10:50:39AM +0800, Wu Fengguang wrote:
> On Thu, Nov 18, 2010 at 09:59:00AM +0800, Andrew Morton wrote:
> > On Thu, 18 Nov 2010 12:40:51 +1100 Dave Chinner <david@...morbit.com> wrote:
> > 
> > > 
> > > There's no point
> > > waking a dirtier if all they can do is write a single page before
> > > they are throttled again - IO is most efficient when done in larger
> > > batches...
> > 
> > That assumes the process was about to do another write.  That's
> > reasonable on average, but a bit sad for interactive/rtprio tasks.  At
> > some stage those scheduler things should be brought into the equation.
> 
> The interactive/rtprio tasks are given 1/4 bonus in
> global_dirty_limits(). So when there are lots of heavy dirtiers,
> the interactive/rtprio tasks will get soft throttled at
> (6~8)*bdi_bandwidth. We can increase that to (12~16)*bdi_bandwidth
> or whatever.

Even better :) It seems that this break in balance_dirty_pages() will
make them throttle free, unless they themselves generate dirty data
faster than the disk can write:

        if (nr_dirty <= (background_thresh + dirty_thresh) / 2)
                break;

Thanks,
Fengguang

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