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    SubjectRe: Bad SSD performance with recent kernels
    From
    2012/1/30 Wu Fengguang <wfg@linux.intel.com>:
    > On Sun, Jan 29, 2012 at 02:13:51PM +0100, Eric Dumazet wrote:
    >> Le dimanche 29 janvier 2012 à 19:16 +0800, Wu Fengguang a écrit :
    >>
    >>
    >> > Note that as long as buffered read(2) is used, it makes almost no
    >> > difference (well, at least for now) to do "dd bs=128k" or "dd bs=2MB":
    >> > the 128kb readahead size will be used underneath to submit read IO.
    >> >
    >>
    >> Hmm...
    >>
    >> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
    >> 32768+0 enregistrements lus
    >> 32768+0 enregistrements écrits
    >> 4294967296 octets (4,3 GB) copiés, 20,7718 s, 207 MB/s
    >>
    >>
    >> # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
    >> 2048+0 enregistrements lus
    >> 2048+0 enregistrements écrits
    >> 4294967296 octets (4,3 GB) copiés, 27,7824 s, 155 MB/s
    >
    > Interesting. Here are my test results:
    >
    > root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
    > 32768+0 records in
    > 32768+0 records out
    > 4294967296 bytes (4.3 GB) copied, 19.0121 s, 226 MB/s
    > root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
    > 2048+0 records in
    > 2048+0 records out
    > 4294967296 bytes (4.3 GB) copied, 19.0214 s, 226 MB/s
    >
    > Maybe the /dev/sda performance bug on your machine is sensitive to timing?
    I got similar result:
    128k: 224M/s
    1M: 182M/s

    1M block size is slow, I guess it's CPU related.

    And as for the big regression with newer kernel than 2.6.38, please
    check if idle=poll
    helps. CPU idle dramatically impacts disk performance and even latest
    cpuidle governor
    doesn't help for some CPUs.

    Thanks,
    Shaohua
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