tiobench read 50% regression with 2.6.30-rc1

[Zhang, Yanmin]

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@xxxxxxxxxx>
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.

yanmin


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