linux-kernel - Re: filesystem benchmarking fun

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Message-Id: <20070516130413.1fd391bf.akpm@linux-foundation.org>
Date:	Wed, 16 May 2007 13:04:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Chuck Ebbert <cebbert@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: filesystem benchmarking fun

On Wed, 16 May 2007 15:53:59 -0400
Chris Mason <chris.mason@...cle.com> wrote:

> > 
> > Should be: it uses first-fit.
> > 
> > >  Looks like ext3 is just walking a list of
> > > bh/jh, maybe we can just sort the silly thing?
> > 
> > The IO scheduler is supposed to do that.
> > 
> > But I don't know what's causing this.
> 
> I had high hopes of blaming cfq, but deadline gives the same results:
> 
> create dir kernel-0 222MB in 5.38 seconds (41.33 MB/s)
> ... [ ~30MB/s here ] ...
> create dir kernel-7 222MB in 8.11 seconds (27.42 MB/s)
> create dir kernel-8 222MB in 18.39 seconds (12.09 MB/s)
> create dir kernel-9 222MB in 6.91 seconds (32.18 MB/s)
> create dir kernel-10 222MB in 24.32 seconds (9.14 MB/s)
> create dir kernel-11 222MB in 12.06 seconds (18.44 MB/s)
> create dir kernel-12 222MB in 10.95 seconds (20.31 MB/s)
> 
> The good news is that if you let it run long enough, the times
> stabilize.  The bad news is:
> 
> create dir kernel-86 222MB in 15.85 seconds (14.03 MB/s)
> create dir kernel-87 222MB in 28.67 seconds (7.76 MB/s)
> create dir kernel-88 222MB in 18.12 seconds (12.27 MB/s)
> create dir kernel-89 222MB in 19.77 seconds (11.25 MB/s)

well hang on.  Doesn't this just mean that the first few runs were writing
into pagecache and the later ones were blocking due to dirty-memory limits?

Or do you have a sync in there?

> echo 2048 > /sys/block/..../nr_requests didn't do it either.
> 
> I guess I'll have systemtap tell me more about the log flushing.
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