Re: Time sliced CFQ io scheduler

[Jens Axboe <axboe@suse.de>]

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