linux-kernel - Re: [PATCH 21/32] fs/aio: aio_wq isn't used in memory reclaim path

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Message-ID: <x494o9nqrk0.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 05 Jan 2011 10:49:19 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org
Subject: Re: [PATCH 21/32] fs/aio: aio_wq isn't used in memory reclaim path

Benjamin LaHaise <bcrl@...ck.org> writes:

> On Wed, Jan 05, 2011 at 09:50:57AM -0500, Jeff Moyer wrote:
>> > Yeap.  Do you agree that the concurrency limit is necessary?  If not,
>> > we can just put everything onto system_wq.
>> 
>> I'm not sure whether it's strictly necessary (there may very well be a
>> need for this in the usb gadgetfs code), but keeping it the same at
>> least seems safe.
>
> Limiting concurrency on aio requests is exactly the opposite of what the 
> usb gadgetfs requires.

I'll have to dig on what their requirements are.  After briefly looking
at mailing list archives, it appears they use the aio workqueue to queue
work after a completed I/O.  I think Zach actually had posted a patch to
change them over to using their own workqueue for that.  At any rate, it
may well be that they don't have a concurrency requirement (in fact, it
would be surprising if they did).  However, I wasn't going to propose
changing the way things were done w/o someone chiming in and saying they
needed it.

> It's similarly bad for filesystem aio when there's a mix of small and
> large requests in flight.

Well, the aio workqueue isn't actually used by the filesystem aio paths
at all (except for the fput_work, and that's being moved to the system
workqueue).

Cheers,
Jeff
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