Re: Time sliced CFQ io scheduler

Jens Axboe
2004-12-08T06:56:34Z

On Tue, Dec 07 2004, Andrew Morton wrote:
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> > The desktop is ok with "as" simply because it's
> > normally optimal to stop writes completely
>
> AS doesn't "stop writes completely". With the current settings it
> apportions about 1/3 of the disk's bandwidth to writes.
>
> This thing Jens has found is for direct-io writes only. It's a bug.

Indeed. It's a special case one, but nasty for that case.

> The other problem with AS is that it basically doesn't work at all with a
> TCQ depth greater than four or so, and lots of people blindly look at
> untuned SCSI benchmark results without realising that. If a distro is

That's pretty easy to fix. I added something like that to cfq, and it's
not a lot of lines of code (grep for rq_in_driver and cfq_max_depth).

> always selecting CFQ then they've probably gone and deoptimised all their
> IDE users.

Andrew, AS has other issues, it's not a case of AS always being faster
at everything.

> AS needs another iteration of development to fix these things. Right now
> it's probably the case that we need CFQ or deadline for servers and AS for
> desktops. That's awkward.

Currently I think the time sliced cfq is the best all around. There's
still a few kinks to be shaken out, but generally I think the concept is
sounder than AS.

--
Jens Axboe