Re: CFQ will be the new default IO scheduler - why?

[Al Boldi <a1426z@gawab.com>]

Arjan van de Ven wrote:
> Al Boldi wrote:
> > Arjan van de Ven wrote:
> >>>> Should there be a default scheduler per filesystem?  As some
> >>>> filesystems may perform better/worse with one over another?
> >>>
> >>> It's currently perDevice, and should probably be extended to perMount.
> >>
> >> Hi,
> >
> > Hi!
> >
> >> per mount is going to be "not funny". I assume the situation you are
> >> aiming for is the "3 partitions on a disk, each wants its own
> >> elevator". The way the kernel currently works is that IO requests the
> >> filesystem does are first flattened into an IO for the entire device
> >> (eg the partition mapping is done) and THEN the IO scheduler gets
> >> involved to schedule the IO on a per disk basis.
> >
> > IC.  That probably explains why concurrent io-procs have such a hard
> > time getting through to the disk.  They probably just hang in the
> > flatting phase, waiting for something to take care of their requests.
>
> flattening is just an addition in the cpu, that's just really boring and
> shouldn't be visible anywhere performance wise

Try this on 2.6 and 2.4 respectively:
# cat /dev/hda > /dev/null
< switch to another vt >
< login >
< start timing >
< wait for shell >
< stop timing >
< wait for dcache to be gobbled by cat and repeat login as necessary >

On my system 2.4.31 (2sec) is at least twice as fast as 2.6.17 (4-10sec) 
depending on io-scheduler, with noop/deadline performing best, albeit a lot 
of noise (scrubbing the disk), and anti/cfq performing worst, albeit quieter 
(just hanging around).

Thanks!

--
Al

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