linux-kernel - Re: [PATCH 01/13] writeback: IO-less balance_dirty

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Date:	Thu, 18 Nov 2010 14:40:06 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Theodore Ts'o <tytso@....edu>, 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>, tglx <tglx@...utronix.de>
Subject: Re: [PATCH 01/13] writeback: IO-less balance_dirty_pages()

On Thu, 2010-11-18 at 21:26 +0800, Wu Fengguang wrote:
> On Thu, Nov 18, 2010 at 09:04:34PM +0800, Peter Zijlstra wrote:
> > On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
> > > - avoid useless (eg. zero pause time) balance_dirty_pages() calls
> > > - avoid too small pause time (less than  10ms, which burns CPU power)
> > > - avoid too large pause time (more than 100ms, which hurts responsiveness)
> > > - avoid big fluctuations of pause times 
> > 
> > If you feel like playing with sub-jiffies timeouts (a way to avoid that
> > HZ=>100 assumption), the below (totally untested) patch might be of
> > help..
> 
> Assuming there are HZ=10 users.
> 
> - when choosing such a coarse granularity, do they really care about
>   responsiveness? :)

No of course not, they usually care about booting their system,.. I've
been told booting Linux on a 10Mhz FPGA is 'fun' :-)

> - will the use of hrtimer add a little code size and/or runtime
>   overheads, and hence hurt the majority HZ=100 users?

Yes it will add code and runtime overhead, but it would allow you to
have 1ms timeouts even on a HZ=100 system, as opposed to a 10ms minimum.

Anyway, I'm not saying you should do it, I just wondered if we had the
API, saw we didn't and thought it might be nice to offer it if desired.


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