linux-kernel - Re: regression in page writeback

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Message-ID: <20090925003820.GK2662@think>
Date:	Thu, 24 Sep 2009 20:38:20 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"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>
Subject: Re: regression in page writeback

On Fri, Sep 25, 2009 at 10:11:17AM +1000, Dave Chinner wrote:
> On Thu, Sep 24, 2009 at 11:15:08AM +0800, Wu Fengguang wrote:
> > On Wed, Sep 23, 2009 at 10:00:58PM +0800, Chris Mason wrote:
> > > The only place that actually honors the congestion flag is pdflush.
> > > It's trivial to get pdflush backed up and make it sit down without
> > > making any progress because once the queue congests, pdflush goes away.
> > 
> > Right. I guess that's more or less intentional - to give lowest priority
> > to periodic/background writeback.
> 
> IMO, this is the wrong design. Background writeback should
> have higher CPU/scheduler priority than normal tasks. If there is
> sufficient dirty pages in the system for background writeback to
> be active, it should be running *now* to start as much IO as it can
> without being held up by other, lower priority tasks.

I'd say that an fsync from mutt or vi should be done at a higher prio
than a background streaming writer.

-chris
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