Re: [RFC][PATCH] mm: stop balance_dirty_pages doing too much work

[Jens Axboe]

On Fri, Jun 26 2009, Richard Kennedy wrote:
> On Thu, 2009-06-25 at 11:10 +0200, 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > > > ---
> > > 
> > > Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > 
> > 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.
> 
> Intriguing numbers. It would tell us a lot if we could find out why
> vanilla + patch is slower than vanilla. I'll run some tests using mmap
> and see if I can find anything.
> What block size are you using ?

It's using 4kb block size.

> I see that the last test of each group is the slowest. I wonder if this
> is showing a slowdown over time or just noise? Any chance you could run
> more tests in each group?

The runs are actually inverted, so the last entry is the first run. It's
a bit confusing. So the first run is usually the odd one out, after that
they are stable.

-- 
Jens Axboe

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