linux-kernel - Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up time

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Date:	Thu, 14 Apr 2011 22:08:17 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up
 time

> > I'm only testing on a desktop with 2 drives. I use a simple test to
> > write 2gb to sda then 2gb to sdb while recording the threshold values.
> > On 2.6.39-rc3, after the 2nd write starts it take approx 90 seconds for
> > sda's threshold value to drop from its maximum to minimum and sdb's to
> > rise from min to max. So this seems much too slow for normal desktop
> > workloads. 
> 
> Yes.
> 
> > I haven't tested with this patch on 2.6.39-rc3 yet, but I'm just about
> > to set that up. 
> 
> It will sure help, but the problem is now the low-memory NAS servers..
> 
> Fortunately my patchset could make the dirty pages ramp up much more
> fast than the ramp up speed of the per-bdi threshold, and is also less
> sensitive to the fluctuations of per-bdi thresholds in JBOD setup.

Look at the attached graph. You cannot notice an obvious "rampup"
stage in the number of dirty pages (red line) at all :)

Thanks,
Fengguang

Download attachment "balance_dirty_pages-pages.png" of type "image/png" (81675 bytes)