Re: CFQ v2 high cpu load fix(?)

[Nick Piggin]

Jens Axboe wrote:
> On Tue, Oct 12 2004, Nick Piggin wrote:
>
> > Ronny V. Vindenes wrote:
> >
> > > CFQ v2 is much better in a lot of cases, but certain situations trigger
> > > a cpu load so high it starves the rest of the system thus completely
> > > ruining the interactive experience. While casually looking at the
> > > problem, I stumbled upon a patch by Arjan van de Ven sent to lkml on
> > > sept. 1 (Subject: block fixes). Part of it is already included in the
> > > CFQ v2 patches and after applying the rest[1] I'm no longer able to
> > > trigger the problem.
> > >
> > > [1] Patch attached against 2.6.9-rc4-ck1 but applies to rc4-mm1 with
> > > some minor fuzz.
> > >
> > >
> > >
> > > ------------------------------------------------------------------------
> > >
> > > --- patches/linux-2.6.9-rc4-ck1/drivers/block/ll_rw_blk.c	2004-10-12 
> > > 12:25:09.798003278 +0200
> > > +++ linux-2.6.9-rc4-ck1/drivers/block/ll_rw_blk.c	2004-10-12 
> > > 12:25:42.959479479 +0200
> > > @@ -100,7 +100,7 @@
> > > 		nr = q->nr_requests;
> > > 	q->nr_congestion_on = nr;
> > >
> > > -	nr = q->nr_requests - (q->nr_requests / 8) - 1;
> > > +	nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests/16)- 1;
> > > 	if (nr < 1)
> > > 		nr = 1;
> > > 	q->nr_congestion_off = nr;
> >
> >
> > I thought this first hunk looked like a good idea when Arjan sent the
> > patch. Can you check if it alone helps your problem?
>
>
> Yeah agree, it's a good idea to leave a bit of air between congestion on
> and off. Fully explains the cfq v2 excessive sys time for some
> workloads, which is extra nice.

Cool. Can you queue up a patch for when -mm opens again, or shall I?
I can't imagine it should cause any problems but a bit of testing
would be wise.

> > The second hunk should be basically a noop.
>
>
> I don't see what it is trying to achieve, I like the current code
> better.

I think Arjan may have just misread the code a little bit.
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