2.6.30-rc deadline scheduler performance regression for iozone over NFS

[Jeff Moyer <jmoyer@redhat.com>]

Hi,

I've been working on CFQ improvements for interleaved I/Os between
processes, and noticed a regression in performance when using the
deadline I/O scheduler.  The test uses a server configured with a cciss
array and 1Gb/s ethernet.

The iozone command line was:
  iozone -s 2000000 -r 64 -f /mnt/test/testfile -i 1 -w

The numbers in the nfsd's row represent the number of nfsd "threads".
These numbers (in MB/s) represent the average of 5 runs.

               v2.6.29

nfsd's  |   1    |  2   |   4   |   8
--------+---------------+-------+------
deadline| 43207 | 67436 | 96289 | 107590

              2.6.30-rc1

nfsd's  |   1   |   2   |   4   |   8
--------+---------------+-------+------
deadline| 43732 | 68059 | 76659 | 83231

    2.6.30-rc3.block-for-linus

nfsd's  |   1   |   2   |   4   |   8
--------+---------------+-------+------
deadline| 46102 | 71151 | 83120 | 82330


Notice the drop for 4 and 8 threads.  It may be worth noting that the
default number of NFSD threads is 8.

Cheers,
Jeff