Re: Poor performance with 2.5.52, load and process in D state

["Paolo Ciarrocchi" <ciarrocchi@linuxmail.org>]

From: Andrew Morton <akpm@digeo.com>
> Paolo Ciarrocchi wrote:
> > 
> > From: Andrew Morton <akpm@digeo.com>
> > > Paolo Ciarrocchi wrote:
> > > >
> > > > Hi all,
> > > > I booted 2.5.52 with the following parmater:
> > > > apm=off mem=32M (not sure about the amount, anyway I can reproduce
> > > > the problem for sure with 32M and 40M)
> > > >
> > > > Then I tried the osdb (www.osdb.org) benchmark with
> > > > 40M of data.
> > > >
> > > > $./bin/osdb-pg --nomulti
> > > >
> > > > the result is that aftwer a few second running top I see the postmaster
> > > > process in D state and a lot if iowait.
> > >
> > > What exactly _is_ the issue?  The machine is achieving 25% CPU utilisation
> > > in user code, 6-9% in system code.  It is doing a lot of I/O, and is
> > > getting work done.
> > 
> > Ok, I'm back with the results of the osdb test against 2.4.19 and 2.5.52
> > Both the kernel booted with apm=off mem=40M
> > osdb ran with 40M of data.
> > To summarize the results:
> > 2.4.19 "Single User Test"       806.78 seconds  (0:13:26.78)
> > 2.5.52 "Single User Test"       3771.85 seconds (1:02:51.85)
> > 
> 
> I could reproduce this.

And this is good ;-)

> What's happening is that when the test starts up it does a lot of writing
> which causes 2.4 to do a bunch of swapout.  So for the rest of the test
> 2.4 has an additional 8MB of cache available.
> 
> The problem of write activity causing swapout was fixed in 2.5.  It
> does not swap out at all in this test.  But this time, we want it to.
> 
> End result: 2.4 has ~20 megabytes of cache for the test and 2.5 has ~12
> megabytes.   The working pagecache set is around 16 MB, so we're right on
> the edge - it makes 2.5 run 10x slower.  You can get most of this back by
> boosting /proc/sys/vm/swappiness.  I think the default of 60 is too unswappy
> really.  I run my machines at 80.

Thank you for the clear explanation, 
if you want I can run the test with different values of /proc/sys/vm/swappines
and post the results, let me know it it is a good idea or just a waste of time.

 
> Tuning swappiness doesn't get all the performance back.  2.5's memory
> footprint is generally larger - we still need to work that down.

Yes, it seems that 2.5 doesn/t fit very well on box with low memory. 

> If this was a real database server I'd expect that memory would end
> up getting swapped out anyway.  But it doesn't happen in this test,
> which is actually quite light in its I/O demands.

Indeed! I thought that booting the box with mem=40M was enought to
force the machine swapping. Is it this test good to "simulate" the 
workload of a _real_ database ?
 
> With mem=128m, 2.5 is 10% faster than 2.4.  Some of this is due to
> the enhancements to copy_*_user() for poorly-aligned copies on Intel
> CPUs.

Oh yes, I see it as well.

Thanks,
             Paolo
 

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