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Date: Thu, 28 Jul 2011 07:52:24 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Block layer roadmap
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To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>, devin122@gmail.com, Stefan Hajnoczi <stefanha@gmail.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>, Jes Sorensen <Jes.Sorensen@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

On 07/28/2011 07:09 AM, Christoph Hellwig wrote:
> On Wed, Jul 27, 2011 at 01:37:31PM +0100, Stefan Hajnoczi wrote:
>> Coroutines in the block layer [Kevin]
>>   * Programming model to simplify block drivers without blocking QEMU threads
>
> Can anyone explain what the whole point of this is?  It really just is
> a bit of syntactic sugar for the current async state machines.  What does
> it buy us over going for real threading?

It is threading--just with a common locking model where a single big 
lock is held to make up for the fact that most of QEMU isn't reentrant.

By restructuring the code to be threaded, we can incrementally remove 
the big lock if we audit for use of non-reentrant functions and 
introduce more granular locking.

The whole ucontext/setjmp thing is just an optimization.  I would hope 
it entirely disappears long term as we promote coroutines to full threads.

Regards,

Anthony Liguori

>
>