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    SubjectRe: 2.6.30-rc deadline scheduler performance regression for iozone over NFS
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    On Thu, May 14, 2009 at 02:26:09PM -0400, Trond Myklebust wrote:
    > On Thu, 2009-05-14 at 13:55 -0400, J. Bruce Fields wrote:
    > > On Wed, May 13, 2009 at 07:45:38PM -0400, Trond Myklebust wrote:
    > > > 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
    > >
    > > I agree, the calculation doesn't look right. But where do you get the
    > > 2/3 number from?
    >
    > That's the sk_stream_wspace() vs. sk_stream_min_wspace() comparison.

    Oh, I see, so looking at their implementations,

    sk_stream_wspace(sk) < sk_stream_min_wspace(sk)

    is equivalent to sk_wmem_queued/2 < sk_->sndbuf - sk_wmem_queued, or
    sk_wmem_queued < 2/3 sndbuf, got it. I didn't understand that the point
    of this patch was just to do that calculation around--now I see.--b.


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