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

[Jens Axboe <axboe@suse.de>]

On Fri, May 21 2004, Nick Piggin wrote:
> 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?

2.4 will give up merging faster than 2.6, elevator_linus will stop
looking for a merge point if the sequence drops to zero. 2.6 will always
merge. So that could explain the fewer writes.

> 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 think that's pretty similar. Andrew didn't say what device he was
testing on, but 2.4 ide defaults to max 64k where 2.6 defaults to 128k.

> 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.

Andrew, you did numbers for deadline previously as well, but no rq
statistics there? As for nr_requests that's true, would be worth a shot
to bump available requests in 2.6.

-- 
Jens Axboe