Re: Random file I/O regressions in 2.6 [patch+results]

[Nick Piggin]

Andrew Morton wrote:

> Open questions are:
>
> a) Why is 2.6 write coalescing so superior to 2.4?
>
> b) Why is 2.6 issuing more read requests, for less data?
>
> c) Why is Alexey seeing dissimilar results?


Interesting. I am not too familiar with 2.4's IO scheduler,
but 2.6's have pretty comprehensive merging systems. Could
that be helping, Jens? Or is 2.4 pretty equivalent?

What about things like maximum request size for 2.4 vs 2.6
for example? This is another thing that can have an impact,
especially for writes.

I'll take a guess at b, and say it could be as-iosched.c.
Another thing might be that 2.6 has smaller nr_requests than
2.4, although you are unlikely to hid the read side limit
with only 16 threads if they are doing sync IO.

As for question c, has Alexey confirmed that it is indeed
2.6-bk which has problems?
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