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    SubjectRe: performance "regression" in cfq compared to anticipatory, deadline and noop
    Hi,

    > From: Daniel J Blueman <daniel.blueman@gmail.com>
    > Date: Sun, Aug 24, 2008 09:24:37PM +0100
    >
    > Hi Fabio, Jens,
    >
    ...
    > This was the last test I didn't get around to. Alas, is did help, but
    > didn't give the merging required for full performance:
    >
    > # echo 1 >/proc/sys/vm/drop_caches; dd if=/dev/sda of=/dev/null
    > bs=128k count=2000
    > 262144000 bytes (262 MB) copied, 2.47787 s, 106 MB/s
    >
    > # echo 1 >/proc/sys/vm/drop_caches; hdparm -t /dev/sda
    > Timing buffered disk reads: 308 MB in 3.01 seconds = 102.46 MB/sec
    >
    > It is an improvement over the baseline performance of 2.6.27-rc4:
    >
    > # echo 1 >/proc/sys/vm/drop_caches; dd if=/dev/sda of=/dev/null
    > bs=128k count=2000
    > 262144000 bytes (262 MB) copied, 2.56514 s, 102 MB/s
    >
    > # echo 1 >/proc/sys/vm/drop_caches; hdparm -t /dev/sda
    > Timing buffered disk reads: 294 MB in 3.02 seconds = 97.33 MB/sec
    >
    > Note that platter speed is around 125MB/s (which I get near at smaller
    > read sizes).
    >
    > I feel 128KB read requests are perhaps important, as this is a
    > commonly-used RAID stripe size, and may explain the read-performance
    > drop sometimes we see in hardware vs software RAID benchmarks.
    >
    > How can we generate some ideas or movement on fixing/improving this behaviour?
    >

    Thank you for testing. The blktrace output for this run should be
    interesting, esp. to compare it with a blktrace obtained from anticipatory
    with the same workload - IIRC anticipatory didn't suffer from the problem,
    and anticipatory has a slightly different dispatching mechanism that
    this patch tried to bring into cfq.

    Even if a proper fix may not belong to the elevator itself, I think
    that this couple (this last test + anticipatory) of traces should help
    in better understanding what is still going wrong.

    Thank you in advance.


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