Re: [PATCH v3.4-rc5] block: iocontext->nr_tasks should be initialized to one

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, May 01, 2012 at 03:04:49PM -0400, Jeff Moyer wrote:
> Vivek Goyal <vgoyal@redhat.com> writes:
> 
> > On Tue, May 01, 2012 at 02:48:41PM -0400, Jeff Moyer wrote:
> >> Vivek Goyal <vgoyal@redhat.com> writes:
> >> 
> >> > On Tue, May 01, 2012 at 02:31:07PM -0400, Jeff Moyer wrote:
> >> >
> >> > [..]
> >> >> > But as to actual users, I really don't know. I agree it's probably not
> >> >> > that widely used. If google still had that code search, we could get a
> >> >> > better idea :-)
> >> >> 
> >> >> I know of one project: the venerable dump/restore utility uses CLONE_IO.
> >> >
> >> > I thought you wrote cooperating queue logic to fix dump as it was not
> >> > using CLONE_IO and IO from multiple threads was going in separate 
> >> > queues.
> >> 
> >> That's correct.  I believe I sent the patch for dump before the kernel
> >> patch was accepted.  Plus, it can't hurt, right?
> >
> > Ok, so now you have fixed dump to use CLONE_IO.
> >
> > So only other user of coop thing remaining potentially is qemu. I was
> 
> No, that's not the *only* other potential user.  ;-)  I wouldn't be
> surprised if nfsd benefitted from the merging.  I also wouldn't be
> surprised if other 3rd party apps did.  Are you trying to make a case to
> get rid of the queue merging logic?

No, just counting who benefits from coop logic.

> 
> > doing some qemu testing where threads were doing IO to nearby area
> > but no coop merging was taking place. So not sure in practice how well
> > does it work.
> 
> Well, that sounds like it warrants further investigation.
> 
> > Thought, that's irrlevant for this discussion. Thought of mentioning
> > this observation.
> 
> If you can provide a reproducer, I'll be happy to take a look.

I will dig. This was in the context of trying to solve qemu related
IO slowdown with CFQ. Will let you know if I find a concrete reproducer.

Thanks
Vivek