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    SubjectRe: [PATCH] cfq-iosched: reduce write depth only if sync was delayed
    Date
    Corrado Zoccolo <czoccolo@gmail.com> writes:

    > Hi Jeff,
    > On Wed, Dec 9, 2009 at 7:09 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
    >> Corrado Zoccolo <czoccolo@gmail.com> writes:
    >>
    >>> The numbers look good. Now, there is no penalty in having low_latency
    >>> set for sequential writes, and just a small penalty for random ones.
    >>> The fact that random reads are faster with low_latency set is interesting.
    >>> Is the test is running with your patched tiobench (so that the number
    >>> of random operations is comparable with sequential ones)?
    >>
    >> No, I forgot all about that.  The number of random operations defaults
    >> to 4000, which is pretty low.  I'll re-run the tests with a number
    >> comparable to the sequential runs.  Sorry about that.
    >>
    > N.P.
    > if you have time, can you also re-run the test changing:
    > iosched/fifo_expire_async to 8 ?
    > I hope that reducing the expire_async, will make cfq quicker at switching
    > between the different threads, allowing more parallelism for seq
    > writers on your hw.
    > If this is the case, I think I can try to estimate the
    > fifo_expire_async in the autotuning patch.

    Sorry this took so long. I've been rather busy of late.

    Cheers,
    Jeff

    low_latency=1, fifo_expire_async=8

    Unit information
    ================
    File size = megabytes
    Blk Size = bytes
    Rate = megabytes per second
    CPU% = percentage of CPU used during the test
    Latency = milliseconds
    Lat% = percent of requests that took longer than X seconds
    CPU Eff = Rate divided by CPU% - throughput per cpu load

    Sequential Reads
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 88.39 89.74% 16.388 2032.62 0.00000 0.00000 98
    2.6.32 8192 65536 16 90.77 185.3% 32.213 2175.99 0.00000 0.00000 49

    Random Reads
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 19.62 25.74% 71.827 3397.26 0.00000 0.00000 76
    2.6.32 8192 65536 16 23.82 55.01% 103.361 4075.53 0.00000 0.00000 43

    Sequential Writes
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 108.28 1007.% 12.984 5643.55 0.00076 0.00000 11
    2.6.32 8192 65536 16 112.40 2014.% 25.430 8592.98 0.00839 0.00000 6

    Random Writes
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 63.94 337.7% 22.885 6047.22 0.00076 0.00000 19
    2.6.32 8192 65536 16 61.94 662.5% 46.997 12759.69 0.15411 0.00000 9


    low_latency=0, fifo_expire_async=8

    Unit information
    ================
    File size = megabytes
    Blk Size = bytes
    Rate = megabytes per second
    CPU% = percentage of CPU used during the test
    Latency = milliseconds
    Lat% = percent of requests that took longer than X seconds
    CPU Eff = Rate divided by CPU% - throughput per cpu load

    Sequential Reads
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 89.13 88.30% 15.872 3101.39 0.00000 0.00000 101
    2.6.32 8192 65536 16 86.78 161.7% 30.794 4909.02 0.00000 0.00000 54

    Random Reads
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 20.21 26.64% 69.863 4285.42 0.00000 0.00000 76
    2.6.32 8192 65536 16 20.10 52.75% 139.761 5986.94 0.00076 0.00000 38

    Sequential Writes
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 108.74 1020.% 13.070 5331.78 0.00076 0.00000 11
    2.6.32 8192 65536 16 112.18 2020.% 25.559 7903.16 0.00992 0.00000 6

    Random Writes
    File Blk Num Avg Maximum Lat% Lat% CPU
    Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
    ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
    2.6.32 8192 65536 8 64.53 337.8% 22.671 5388.77 0.00000 0.00000 19
    2.6.32 8192 65536 16 61.75 668.9% 47.265 13271.37 0.12894 0.00000 9
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