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

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Message-ID: <x49ws8j686r.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 14 May 2009 11:00:28 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	Olga Kornievskaia <aglo@...i.umich.edu>,
	"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

Trond Myklebust <trond.myklebust@....uio.no> writes:

> On Thu, 2009-05-14 at 09:34 -0400, Jeff Moyer wrote:
>> Trond Myklebust <trond.myklebust@....uio.no> writes:
>> 
>> > On Wed, 2009-05-13 at 15:29 -0400, Jeff Moyer wrote:
>> >> Hi, netdev folks.  The summary here is:
>> >> 
>> >> A patch added in the 2.6.30 development cycle caused a performance
>> >> regression in my NFS iozone testing.  The patch in question is the
>> >> following:
>> >> 
>> >> commit 47a14ef1af48c696b214ac168f056ddc79793d0e
>> >> Author: Olga Kornievskaia <aglo@...i.umich.edu>
>> >> Date:   Tue Oct 21 14:13:47 2008 -0400
>> >> 
>> >>     svcrpc: take advantage of tcp autotuning
>> >>  
>> >> which is also quoted below.  Using 8 nfsd threads, a single client doing
>> >> 2GB of streaming read I/O goes from 107590 KB/s under 2.6.29 to 65558
>> >> KB/s under 2.6.30-rc4.  I also see more run to run variation under
>> >> 2.6.30-rc4 using the deadline I/O scheduler on the server.  That
>> >> variation disappears (as does the performance regression) when reverting
>> >> the above commit.
>> >
>> > It looks to me as if we've got a bug in the svc_tcp_has_wspace() helper
>> > function. I can see no reason why we should stop processing new incoming
>> > RPC requests just because the send buffer happens to be 2/3 full. If we
>> > see that we have space for another reply, then we should just go for it.
>> > OTOH, we do want to ensure that the SOCK_NOSPACE flag remains set, so
>> > that the TCP layer knows that we're congested, and that we'd like it to
>> > increase the send window size, please.
>> >
>> > Could you therefore please see if the following (untested) patch helps?
>> 
>> I'm seeing slightly better results with the patch:
>> 
>> 71548
>> 75987
>> 71557
>> 87432
>> 83538
>> 
>> But that's still not up to the speeds we saw under 2.6.29.  The packet
>> capture for one run can be found here:
>>   http://people.redhat.com/jmoyer/trond.pcap.bz2
>> 
>> Cheers,
>> Jeff
>
> Yes. Something is very wrong there...
>
> See for instance frame 1195, where the client finishes sending a whole
> series of READ requests, and we go into a flurry of ACKs passing
> backwards and forwards, but no data. It looks as if the NFS server isn't
> processing anything, probably because the window size falls afoul of the
> svc_tcp_has_wspace()...
>
> Does something like this help?

Sorry for the previous, stupid question.  I applied the patch in
addition the last one and here are the results:

70327
71561
68760
69199
65324

A packet capture for this run is available here:
  http://people.redhat.com/jmoyer/trond2.pcap.bz2

Any more ideas?  ;)

-Jeff
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