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

[Jeff Moyer <jmoyer@redhat.com>]

Trond Myklebust <trond.myklebust@fys.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@citi.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