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

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Message-ID: <5046167C.3070109@redhat.com>
Date:	Tue, 04 Sep 2012 16:55:56 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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

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.

> 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
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