linux-kernel - Re: [RFC PATCH] v2 mm: balance_dirty_pages. reduce calls to global_page

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Date:	Mon, 7 Sep 2009 12:42:31 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"chris.mason" <chris.mason@...cle.com>,
	linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH] v2 mm: balance_dirty_pages.  reduce calls to
	global_page_state to reduce cache references

On Mon, Sep 07 2009, Richard Kennedy wrote:
> On Sun, 2009-09-06 at 20:42 +0200, Jens Axboe wrote:
> > On Fri, Sep 04 2009, Richard Kennedy wrote:
> > > Reducing the number of times balance_dirty_pages calls global_page_state
> > > reduces the cache references and so improves write performance on a
> > > variety of workloads.
> > > 
> > > 'perf stats' of simple fio write tests shows the reduction in cache
> > > access.
> > > Where the test is fio 'write,mmap,600Mb,pre_read' on AMD AthlonX2 with
> > > 3Gb memory (dirty_threshold approx 600 Mb)
> > > running each test 10 times, dropping the fasted & slowest values then
> > > taking 
> > > the average & standard deviation
> > > 
> > > 		average (s.d.) in millions (10^6)
> > > 2.6.31-rc8	648.6 (14.6)
> > > +patch		620.1 (16.5)
> > 
> > This patch looks good to me, I have workloads too here where up to 10%
> > of the time is spent in balance_dirty_pages() because of this. I'll give
> > this patch a go on the box and test in question tomorrow, but it looks
> > promising.
> > 
> 
> Thanks Jens, 
> 
> It will be interesting to see how it works on different hardware &
> workload. How many cores are you going to run it on?
> wow 10% in balance_dirty_pages! Is that on a large server? or do you
> think its peculiar to your workload?

~10% is rounded, it was 8.9x%. Pretty bad! It's a 64-way, and there's
nothing special about the workload (just buffered IO with 4k blocks).
It's just going rather fast.

-- 
Jens Axboe

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