linux-kernel - Re: filesystem benchmarking fun

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Date:	Wed, 16 May 2007 15:53:59 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Chuck Ebbert <cebbert@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: filesystem benchmarking fun

On Wed, May 16, 2007 at 12:33:42PM -0700, Andrew Morton wrote:
> On Wed, 16 May 2007 15:13:39 -0400
> Chris Mason <chris.mason@...cle.com> wrote:
> 
> > > 
> > > If that's still working then the problem will _probably_ be directory
> > > writeout.  Possibly inodes, but they should be well-laid-out.
> > > 
> > > Were you using dir_index?  That might be screwing things up.
> > 
> > Yes, dir_index.  A quick test of mkfs.ext3 -O ^dir_index seems to still
> > have the problem.  Even though the inodes are well laid out, is the
> > order they get written sane?
> 
> 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)

echo 2048 > /sys/block/..../nr_requests didn't do it either.

I guess I'll have systemtap tell me more about the log flushing.

-chris

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