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

[Nick Piggin]

Jens Axboe wrote:
> 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.

Yep OK, that could be one thing.

> > 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.

This could be another. If Andrew's using IDE, this alone could
make up the entire difference *if* writes are nicely sequential.
I guess they probably aren't, but it could still help.
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