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

Nick Piggin
2004-05-22T01:01:07Z

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.