linux-kernel - Re: [RFC][PATCH] mm: stop balance_dirty

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Date:	Thu, 25 Jun 2009 16:44:50 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Al Boldi <a1426z@...ab.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Kennedy <richard@....demon.co.uk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] mm: stop balance_dirty_pages doing too much work

On Thu, Jun 25 2009, Al Boldi wrote:
> Jens Axboe wrote:
> > On Thu, Jun 25 2009, Al Boldi wrote:
> > > Jens Axboe wrote:
> > > > On Thu, Jun 25 2009, Jens Axboe wrote:
> > > > > On Thu, Jun 25 2009, Peter Zijlstra wrote:
> > > > > > On Wed, 2009-06-24 at 15:27 -0700, Andrew Morton wrote:
> > > > > > > On Wed, 24 Jun 2009 11:38:24 +0100
> > > > > > >
> > > > > > > Richard Kennedy <richard@....demon.co.uk> wrote:
> > > > > > > > When writing to 2 (or more) devices at the same time, stop
> > > > > > > > balance_dirty_pages moving dirty pages to writeback when it has
> > > > > > > > reached the bdi threshold. This prevents balance_dirty_pages
> > > > > > > > overshooting its limits and moving all dirty pages to
> > > > > > > > writeback.
> > > > > > > >
> > > > > > > >
> > > > > > > > Signed-off-by: Richard Kennedy <richard@....demon.co.uk>
> > > > > > > > ---
> > > > > >
> > > > > > Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > > >
> > > > > After doing some integration and update work on the writeback branch,
> > > > > I threw 2.6.31-rc1, 2.6.31-rc1+patch, 2.6.31-rc1+writeback into the
> > > > > test mix. The writeback series include this patch as a prep patch.
> > > > > Results for the mmap write test case:
> > > > >
> > > > > Kernel          Throughput      usr     sys     ctx     util
> > > > > --------------------------------------------------------------
> > > > > vanilla         184MB/sec       19.51%  50.49%  12995   82.88%
> > > > > vanilla         184MB/sec       19.60%  50.77%  12846   83.47%
> > > > > vanilla         182MB/sec       19.25%  51.18%  14692   82.76%
> > > > > vanilla+patch   169MB/sec       18.08%  43.61%   9507   76.38%
> > > > > vanilla+patch   170MB/sec       18.37%  43.46%  10275   76.62%
> > > > > vanilla+patch   165MB/sec       17.59%  42.06%  10165   74.39%
> > > > > writeback       215MB/sec       22.69%  53.23%   4085   92.32%
> > > > > writeback       214MB/sec       24.31%  52.90%   4495   92.40%
> > > > > writeback       208MB/sec       23.14%  52.12%   4067   91.68%
> > > > >
> > > > > To be perfectly clear:
> > > > >
> > > > > vanilla         2.6.31-rc1 stock
> > > > > vanilla+patch   2.6.31-rc1 + bdi_thresh patch
> > > > > writeback       2.6.31-rc1 + bdi_thresh patch + writeback series
> > > > >
> > > > > This is just a single spindle w/ext4, nothing fancy. I'll do a
> > > > > 3-series run with the writeback and this patch backed out, to see if
> > > > > it makes a difference here. I didn't do that initially, since the
> > > > > results were in the range that I expected.
> > > >
> > > > Results for writeback without the bdi_thresh patch
> > > >
> > > > Kernel          Throughput      usr     sys     ctx     util
> > > > --------------------------------------------------------------
> > > > wb-bdi_thresh   211MB/sec       22.71%  53.30%   4050   91.19%
> > > > wb-bdi_thresh   212MB/sec       22.78%  53.55%   4809   91.51%
> > > > wb-bdi_thresh   212MB/sec       22.99%  54.23%   4715   93.10%
> > > >
> > > > Not a lot of difference there, without more than three runs it's hard
> > > > to say what is significant. Could be a small decrease in throughput, if
> > > > the 208MB/sec results from above is an outlier (I think it is,
> > > > ~215MB/sec is usually the most consistent result).
> > >
> > > What's the iowait on these runs?
> >
> > Not sure, I didn't check. Why do you ask?
> 
> iowait gives you an indication of seekactivity.

The test case is random mmap writes to files that have been laid out
sequentially. So it's all seeks. The target drive is an SSD disk though,
so it doesn't matter a whole lot (it's a good SSD).

-- 
Jens Axboe

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