Re: [RFC PATCH] v2 mm: balance_dirty_pages. reduce calls to global_page_state to reduce cache references

[Richard Kennedy <richard@rsk.demon.co.uk>]

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?

regards
Richard