Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 02, 2008 at 09:55:11AM +0200, Jens Axboe wrote:
> On Thu, Oct 02 2008, Andi Kleen wrote:
> > Andrew Morton <akpm@linux-foundation.org> writes:
> > 
> > > On Wed, 1 Oct 2008 20:00:34 -0700 Arjan van de Ven <arjan@infradead.org> wrote:
> > >
> > >> Subject: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
> > >
> > > You proposed this a while back and it didn't happen and I forget
> > > why and the changelog doesn't mention any of that?
> > 
> > XFS tried this some time ago too.
> > 
> > I think the issue was that real user supplied RT applications don't want to 
> > compete with a "pseudo RT" kjournald.
> > 
> > So it would really need a new priority class between RT and normal priority.
> 
> Good point. I think we should mark the IO as sync, and maintain the same
> priority level. Any IO that ends up being waited on is sync by
> definition, we just need to expand the coverage a bit.

That's what XFS has always done - mark the journal I/O as sync.
Still, once you load up the elevator, the sync I/O can still get
delayed for hundreds of milliseconds before dispatch, which was
why I started looking at boosting the priority of the log I/O.
It proved to be much more effective at getting the log I/O
dispatched than the existing "mark it sync" technique....

The RT folk were happy with the idea of journal I/O using the
highest non-RT priority for the journal, but I never got around
to testing that out as I had a bunnch of other stuff to fix at
the time.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com