linux-kernel - Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

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Message-ID: <20120904150323.GA15116@redhat.com>
Date:	Tue, 4 Sep 2012 18:03:23 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	kvm@...r.kernel.org, rusty@...tcorp.com.au, jasowang@...hat.com,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

On Tue, Sep 04, 2012 at 04:55:56PM +0200, Paolo Bonzini wrote:
> Il 04/09/2012 16:47, Michael S. Tsirkin ha scritto:
> >> >  static void virtscsi_init_vq(struct virtio_scsi_vq *virtscsi_vq,
> >> > -			     struct virtqueue *vq)
> >> > +			     struct virtqueue *vq, bool affinity)
> >> >  {
> >> >  	spin_lock_init(&virtscsi_vq->vq_lock);
> >> >  	virtscsi_vq->vq = vq;
> >> > +	if (affinity)
> >> > +		virtqueue_set_affinity(vq, virtqueue_get_queue_index(vq) -
> >> > +				       VIRTIO_SCSI_VQ_BASE);
> >> >  }
> >> >  
> > This means in practice if you have less virtqueues than CPUs,
> > things are not going to work well, will they?
> 
> Not particularly.  It could be better or worse than single queue
> depending on the workload.

Well interrupts will go to CPU different from the one
that sends commands so ...

> > Any idea what to do?
> 
> Two possibilities:
> 
> 1) Add a stride argument to virtqueue_set_affinity, and make it equal to
> the number of queues.
> 
> 2) Make multiqueue the default in QEMU, and make the default number of
> queues equal to the number of VCPUs.
> 
> I was going for (2).
> 
> Paolo

3. use per target queue if less targets than cpus?

-- 
MST

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