Re: [Qemu-devel] virtio-blk multiqueue support in qemu.

[Michael S. Tsirkin]

On Thu, Jul 09, 2015 at 02:22:48PM +0100, Stefan Hajnoczi wrote:
> On Wed, Jul 08, 2015 at 05:37:12PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Jul 02, 2015 at 09:21:21AM +0800, Fam Zheng wrote:
> > > On Wed, 07/01 22:45, Naredula Janardhana Reddy wrote:
> > > > HI,
> > > >      I am trying to improve the throughput of  virtio-blk for my kernel 
> > > > (
> > > > https://github.com/naredula-jana/Jiny-Kernel  )  to make use of the
> > > > multi-core using multi-queue. But I found the latest qemu  does not 
> > > > support
> > > > multiqueue for virtio-blk, The feature flag does not return multi-queue
> > > > support. Do we have any patches to the qemu for enabling multi-queue for
> > > > virtio-blk similar to that of virtio-net.  Can someone help some 
> > > > pointers
> > > > related to this..
> > > 
> > > There was a series last year adding support for multi-queue but wan't 
> > > merged to
> > > the code base:
> > > 
> > > http://comments.gmane.org/gmane.comp.emulators.qemu/289126
> > > 
> > > Fam
> > 
> > Looks like it was mostly about the missing GPL license on some files.
> > Or did I miss some bigger issues?
> 
> I reviewed the series again.  The most important part:
> 
> It was not a true multiqueue implementation - all the virtqueues are
> handled from a single thread in QEMU.  The reason for this is that
> QEMU's block layer isn't threadsafe so a lot of work is necessary to
> support true multiqueue.
> 
> Also, the performance numbers quoted cannot be attributed to multiqueue
> due to the number of additional changes the series made.  In the
> meantime, some of these additional changes have been merged as separate
> series.
> 
> On the positive side, we do need something similar to the last 3 patches
> to expose multiqueue to the guest.  But it will require much more work
> in QEMU to achieve true multiqueue.
> 
> Stefan

Might this be useful as an intermediate step, e.g. for testing
guest code?

-- 
MST