Re: 2.6.20-rc2+: CFQ halving disk throughput.

Jens Axboe
2007-01-02T08:44:51Z

On Mon, Jan 01 2007, Andrew Morton wrote:
> On Mon, 01 Jan 2007 23:46:58 +0100
> Rene Herman <rene.herman@gmail.com> wrote:
>
> > > Everything seems fine in the dmesg. Performance degradation is
> > > probably some other issue in -rc kernel. I'm suspecting recently
> > > fixed block layer bug. If it's still the same in the next -rc,
> > > please report.
> >
> > In fact, it's CFQ. The PATA thing was a red herring. 2.6.20-rc2 and 3
> > give me ~ 24 MB/s from "hdparm t /dev/hda" while 2.6.20-rc1 and below
> > give me ~ 50 MB/s.
> >
> > Jens: this is due to "[PATCH] cfq-iosched: tighten allow merge
> > criteria", 719d34027e1a186e46a3952e8a24bf91ecc33837:
> >
> > http://www2.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=719d34027e1a186e46a3952e8a24bf91ecc33837
> >
> > If I revert that one, I have my 50 M/s back. config and dmesg attached
> > in case they're useful.
>
> The patch would appear to need this fix:
>
> --- a/block/cfq-iosched.c~a
> +++ a/block/cfq-iosched.c
> @@ -592,7 +592,7 @@ static int cfq_allow_merge(request_queue
> if (cfqq == RQ_CFQQ(rq))
> return 1;
>
> - return 1;
> + return 0;
> }
>
> static inline void
> _
>
> But that might not fix things...

Yeah it is, but I don't think it'll fix it (if anything, it'll be more
conservative).

--
Jens Axboe