iozone write 50% regression in kernel 2.6.24-rc1

[Zhang, Yanmin]

Comparing with 2.6.23, iozone sequential write/rewrite (512M) has 50% regression
in kernel 2.6.24-rc1. 2.6.24-rc2 has the same regression.

My machine has 8 processor cores and 8GB memory.

By bisect, I located patch
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=04fbfdc14e5f48463820d6b9807daa5e9c92c51f.


Another behavior: with kernel 2.6.23, if I run iozone for many times after rebooting machine,
the result looks stable. But with 2.6.24-rc1, the first run of iozone got a very small result and
following run has 4Xorig_result.

What I reported is the regression of 2nd/3rd run, because first run has bigger regression.

I also tried to change /proc/sys/vm/dirty_ratio,dirty_backgroud_ratio and didn't get improvement.

-yanmin
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