linux-kernel - Re: regression in page writeback

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Message-Id: <1253606965.8439.281.camel@twins>
Date:	Tue, 22 Sep 2009 10:09:25 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"Li, Shaohua" <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"richard@....demon.co.uk" <richard@....demon.co.uk>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: regression in page writeback

On Tue, 2009-09-22 at 16:05 +0800, Wu Fengguang wrote:
> 
> I'm not sure how this patch stopped the "overshooting" behavior.
> Maybe it managed to not start the background pdflush, or the started
> pdflush thread exited because it found writeback is in progress by
> someone else?
> 
> -               if (bdi_nr_reclaimable) {
> +               if (bdi_nr_reclaimable > bdi_thresh) {

The idea is that we shouldn't move more pages from dirty -> writeback
when there's not actually that much dirty left.

Now, I'm not sure about the > bdi_thresh part, I've suggested to maybe
use bdi_thresh/2 a few times, but it generally didn't seem to make much
of a difference.



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