linux-kernel - Re: 2.6.30-rc deadline scheduler performance regression for iozone over NFS

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Date:	Wed, 13 May 2009 15:06:55 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Olga Kornievskaia <aglo@...i.umich.edu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Jim Rees <rees@...ch.edu>, linux-nfs@...r.kernel.org
Subject: Re: 2.6.30-rc deadline scheduler performance regression for iozone  over NFS

Olga Kornievskaia <aglo@...i.umich.edu> writes:

> On Wed, May 13, 2009 at 12:32 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
>> On Wed, 13 May 2009 12:20:57 -0400 Olga Kornievskaia <aglo@...i.umich.edu> wrote:
>>
>>> I believe what you are seeing is how well TCP autotuning performs.
>>> What old NFS code was doing is disabling autotuning and instead using
>>> #nfsd thread to scale TCP recv window. You are providing an example of
>>> where setting TCP buffer sizes outperforms TCP autotuning. While this
>>> is a valid example, there is also an alternative example of where old
>>> NFS design hurts performance.
>>
>> <scratches head>
>>
>> Jeff's computer got slower.  Can we fix that?
>
> We realize that decrease performance is a problem and understand that
> reverting the patch might be the appropriate course of action!

I wasn't suggesting that we just revert the patch.  I was just looking
for some guidance on diagnosing and hopefully fixing the regression.

> But we are curious why this is happening. Jeff if it's not too much trouble
> could you generate tcpdumps for both cases. We are curious what are
> the max window sizes in both cases? Also could you give us your tcp and
> network sysctl values for the testing environment (both client and server
> values) that you can get with "sysctl -a | grep tcp" and also
> " | grep net.core".

http://people.redhat.com/jmoyer/iozone-regression.tar

I'm happy to continue to help track this down.  If you want to reproduce
this in your own environment, though, you can probably do it with a
ramdisk served up via nfs with the nfs client and server on the same
gig-e network.

> Poor performance using TCP autotuning can be demonstrated outside
> of NFS but using Iperf. It can be shown that iperf will work better if "-w"
> flag is used. When this flag is set, Iperf calls setsockopt() call which in
> the kernel turns off autotuning.
>
> As for fixing this it would be great if we could get some help from the
> TCP kernel folks?

Then we'll need to add netdev to the CC, but probably from a message
that has more background on the problem (we've even trimmed the
offending commit and performance numbers from the email at this point).

Cheers,
Jeff
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