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Date: Thu, 28 Jul 2011 13:54:03 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] Block layer roadmap
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To: Kevin Wolf <kwolf@redhat.com>
Cc: devin122@gmail.com, Jes Sorensen <Jes.Sorensen@redhat.com>, Jagane Sundar <jagane@sundar.org>, Dor Laor <dlaor@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, Feiran Zheng <famcool@gmail.com>, Frediano Ziglio <freddy77@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Christoph Hellwig <hch@lst.de>

On Thu, Jul 28, 2011 at 1:35 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 28.07.2011 14:09, schrieb Christoph Hellwig:
>> On Wed, Jul 27, 2011 at 01:37:31PM +0100, Stefan Hajnoczi wrote:
>>> Coroutines in the block layer [Kevin]
>>> =A0* Programming model to simplify block drivers without blocking QEMU =
threads
>>
>> Can anyone explain what the whole point of this is? =A0It really just is
>> a bit of syntactic sugar for the current async state machines. =A0What d=
oes
>> it buy us over going for real threading?
>
> The only current block driver that really does everything in an async
> state machine is qed. It's definitely not nice code, and having to
> convert all of the other block drivers to this would be a lot of work.

Thanks Kevin :).  I do agree with the clumsiness of async callback
programming - a lot of code is spent bundling and unbundling variables
to pass between functions, not to mention that the control flow is
much harder to follow.

> So if it only means that we're making things async that would block the
> VCPU until now, isn't that a great improvement already?
>
> The advantage compared to threading is that it allows an easy and
> incremental transition.

Also remember that we already have a threads-based implementation of
coroutines.  Later on we can do fine-grained locking and switch to
threads directly instead of coroutines, if need be.

Stefan