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


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