Re: Time sliced CFQ io scheduler

From: Nick Piggin
Date: Wed Dec 08 2004 - 02:16:53 EST

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.

What were the main things I had to do... hmm, I think the main one was
to not start on a new batch until all requests from a previous batch
are reported to have completed. So eg. you get all reads completing
before you start issuing any more writes. The write->read side of things
isn't so clear cut with your "smart" write caches on the IO systems, but
no doubt that helps a bit.

Of course, after you do all that your database performance has well and
truly gone down the shitter. It is also hampered by the more fundamental
issue that read anticipating can block up the pipe for IO that is cached
on the controller/disks and would get satisfied immediately.

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