linux-kernel - Re: regression in page writeback

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Date:	Wed, 23 Sep 2009 17:37:53 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Richard Kennedy <richard@....demon.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>
Subject: Re: regression in page writeback

On Wed, Sep 23, 2009 at 05:23:31PM +0800, Peter Zijlstra wrote:
> On Wed, 2009-09-23 at 10:19 +0100, Richard Kennedy wrote:
> > 
> > I am concerned that the background writeout no longer stops when it
> > reaches the background threshold, as balance_dirty_pages requests all
> > dirty pages to be written. No doubt this is good for large linear writes
> > but what about more random write workloads? 
> 
> I've not had time to look over the current code, but write-out not
> stopping on reaching background threshold is a definite bug and needs to
> get fixed.

Yes, 2.6.31 code stops writeback when background threshold is reached.
But new behavior in latest git is to writeback all pages.

The code only checks over_bground_thresh() for kupdate works:

                if (args->for_kupdate && args->nr_pages <= 0 &&
                    !over_bground_thresh())
                        break;

However the background work started by balance_dirty_pages() won't check
over_bground_thresh(). So it will move all dirty pages.

I think it's very weird to check over_bground_thresh() for kupdate
instead of background work. Jens must intended for the latter case.

Thanks,
Fengguang
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