Re: [Qemu-devel] converting the block layer from coroutines to threads

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/24/2012 08:22 PM, Anthony Liguori wrote:
> Virtio really wants each virtqueue to be processed in a separate
> thread.  On a multicore system, there's considerable improvement doing
> this.  I think that's where we ought to start.

Well, that's where we ought to *get*.  Stefan's work is awesome but with
the current feature set it would be hard to justify it upstream.

To get it upstream we need to generalize it and make it work well with
the block layer.  And vice versa make the block layer work well with
threads, which is what I care about here.

> We really just need the block layer to be re-entrant, we don't
> actually need qcow2 or anything else that uses coroutines to use full
> threads.

Once you can issue I/O from two threads at the same-time (such as
streaming in the iothread and guest I/O in the virtqueue thread),
everything already needs to be thread-safe.  It is a pretty short step
from there to thread pools for everything.

> Or at least, as far as I know, we don't have any performance data to
> suggest that we do.

No, it's not about speed, though of course it only works if there is no
performance dip.  It is just an enabling step.

That said, my weekend officially begins now. :)

Paolo