Re: Time sliced CFQ io scheduler

Jens Axboe
2004-12-08T13:49:21Z

On Wed, Dec 08 2004, Nick Piggin wrote:
> On Wed, 2004-12-08 at 07:58 +0100, Jens Axboe wrote:
> > On Wed, Dec 08 2004, Nick Piggin wrote:
> > > On Tue, 2004-12-07 at 18:25 -0800, Andrew Morton wrote:
>
> > > I think we could detect when a disk asks for more than, say, 4
> > > concurrent requests, and in that case turn off read anticipation
> > > and all the anti-starvation for TCQ by default (with the option
> > > to force it back on).
> >
> > CFQ only allows a certain depth a the hardware level, you can control
> > that. I don't think you should drop the AS behaviour in that case, you
> > should look at when the last request comes in and what type it is.
> >
> > With time sliced cfq I'm seeing some silly SCSI disk behaviour as well,
> > it gets harder to get good read bandwidth as the disk is trying pretty
> > hard to starve me. Maybe killing write back caching would help, I'll
> > have to try.
> >
>
> I "fixed" this in AS. It gets (or got, last time we checked, many months
> ago) pretty good read latency even with a big write and a very large
> tag depth.

This problem was also caused by the dispatch sort bug. So you were
right, it was 'some little bug' in the code :)

--
Jens Axboe