Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging

[Corrado Zoccolo <czoccolo@gmail.com>]

On Wed, Dec 30, 2009 at 7:45 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> On Wed, Dec 30 2009, Corrado Zoccolo wrote:
>> Non rotational devices' performances are not affected by
>> distance of requests, so there is no point in having overhead
>> to merge queues of nearby requests.
>
> If the distance is zero, it may still make a big difference (at least
> for writes). This check would be better as "ncq and doesn't suck", ala
>
>        blk_queue_nonrot(q) && tagged
>
> like we do elsewhere.

For reads, though, even flash cards and netbook ssds are completely
unaffected. I have done few experiments on my available disks:
* http://dl.dropbox.com/u/3525644/service_time.png (I used the
program: http://dl.dropbox.com/u/3525644/stride.c to get the graphs).

For distance 0, I think request merging will be more effective than
queue merging, moreover I think the multi-thread trick to have large
I/O depth is used for reads, not writes (where simply issuing buffered
writes already achieves a similar effect), so I think it is safe to
disable it for all non-rotational devices.

Thanks,
Corrado
>
> --
> Jens Axboe
>
>