linux-kernel - Re: [PATCH v3.4-rc5] block: iocontext->nr

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Date:	Tue, 01 May 2012 15:04:49 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>, Tejun Heo <tj@...nel.org>,
	Dave Jones <davej@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Sasha Levin <levinsasha928@...il.com>
Subject: Re: [PATCH v3.4-rc5] block: iocontext->nr_tasks should be initialized to one

Vivek Goyal <vgoyal@...hat.com> writes:

> On Tue, May 01, 2012 at 02:48:41PM -0400, Jeff Moyer wrote:
>> Vivek Goyal <vgoyal@...hat.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?

> 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.

Cheers,
Jeff
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