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    SubjectRe: tiobench read 50% regression with 2.6.30-rc1
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    On Thu, 2009-04-09 at 11:57 +0200, Jens Axboe wrote:
    > On Thu, Apr 09 2009, Zhang, Yanmin wrote:
    > > Comparing with 2.6.29's result, tiobench (read) has about 50% regression
    > > with 2.6.30-rc1 on all my machines. Bisect down to below patch.
    > >
    > > b029195dda0129b427c6e579a3bb3ae752da3a93 is first bad commit
    > > commit b029195dda0129b427c6e579a3bb3ae752da3a93
    > > Author: Jens Axboe <jens.axboe@oracle.com>
    > > Date: Tue Apr 7 11:38:31 2009 +0200
    > >
    > > cfq-iosched: don't let idling interfere with plugging
    > >
    > > When CFQ is waiting for a new request from a process, currently it'll
    > > immediately restart queuing when it sees such a request. This doesn't
    > > work very well with streamed IO, since we then end up splitting IO
    > > that would otherwise have been merged nicely. For a simple dd test,
    > > this causes 10x as many requests to be issued as we should have.
    > > Normally this goes unnoticed due to the low overhead of requests
    > > at the device side, but some hardware is very sensitive to request
    > > sizes and there it can cause big slow downs.
    > >
    > >
    > >
    > > Command to start the testing:
    > > #tiotest -k0 -k1 -k3 -f 80 -t 32
    > >
    > > It's a multi-threaded program and starts 32 threads. Every thread does I/O
    > > on its own 80MB file.
    The files should be created before the testing and pls. drop page caches
    by "echo 3 >/proc/sys/vm/drop_caches" before testing.

    >
    > It's not a huge surprise that we regressed there. I'll get this fixed up
    > next week. Can you I talk you into trying to change the 'quantum' sysfs
    > variable for the drive? It's in /sys/block/xxx/queue/iosched where xxx
    > is your drive(s). It's set to 4, if you could try progressively larger
    > settings and retest, that would help get things started.
    I tried 4,8,16,64,128 and didn't find result difference.




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