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SubjectRe: IO scheduler benchmarking
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>> Why does 2.5.62-mm2 have higher sequential
>> write latency than 2.5.61-mm1?

> And there are various odd interactions in, at least, ext3. You did not
> specify which filesystem was used.

ext2

>> Thr MB/sec CPU% avg lat max latency
>> 2.5.62-mm2-as 8 14.76 52.04% 6.14 4.5
>> 2.5.62-mm2-dline 8 9.91 13.90% 9.41 .8
>> 2.5.62-mm2 8 9.83 15.62% 7.38 408.9

> Fishiness. 2.5.62-mm2 _is_ 2.5.62-mm2-as. Why the 100x difference?

Bad EXTRAVERSION naming on my part. 2.5.62-mm2 _was_ booted with
elevator=cfq.

How it happened:
2.5.61-mm1 tested
2.5.61-mm1-cfq tested and elevator=cfq added to boot flags
2.5.62-mm1 tested (elevator=cfq still in lilo boot boot flags)
Then to test the other two schedulers I changed extraversion and boot
flags.

> That 408 seconds looks suspect.

AFAICT, that's the one request in over 500,000 that took the longest.
The numbers are fairly consistent. How relevant they are is debatable.

> If you want to test write latency, do this:

Your approach is more realistic than tiobench.

> There is a place in VFS where one writing task could accidentally hammer a
> different one. I cannot trigger that, but I'll fix it up in next -mm.

2.5.62-mm3 or 2.5.63-mm1? (-mm3 is running now)

--
Randy Hron
http://home.earthlink.net/~rwhron/kernel/bigbox.html

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