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

[Al Boldi <a1426z@gawab.com>]

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.

> The 2.4 kernel did this the other way around, and it was really a bad
> idea (no fairness, less optimal scheduling all around due to less
> visibility into what the disk is really doing, several hardware
> properties such as TCQ depth that affect scheduling IOs are truely per
> disk not per partition etc etc)
>
> So I don't think it's likely that per mount is really an option right
> now..

Probably true as it stands right now, but extending io-sched semantics to be 
filesystem aware in the "flattening/partition mapping" phase could improve 
performance a lot.

Thanks!

--
Al


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