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

[Michael S. Tsirkin]

On Tue, Sep 04, 2012 at 03:49:42PM +0200, Paolo Bonzini wrote:
> Il 04/09/2012 14:48, Michael S. Tsirkin ha scritto:
> >> > This patch adds queue steering to virtio-scsi.  When a target is sent
> >> > multiple requests, we always drive them to the same queue so that FIFO
> >> > processing order is kept.  However, if a target was idle, we can choose
> >> > a queue arbitrarily.  In this case the queue is chosen according to the
> >> > current VCPU, so the driver expects the number of request queues to be
> >> > equal to the number of VCPUs.  This makes it easy and fast to select
> >> > the queue, and also lets the driver optimize the IRQ affinity for the
> >> > virtqueues (each virtqueue's affinity is set to the CPU that "owns"
> >> > the queue).
> >> > 
> >> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > I guess an alternative is a per-target vq.
> > Is the reason you avoid this that you expect more targets
> > than cpus? If yes this is something you might want to
> > mention in the log.
> 
> One reason is that, even though in practice I expect roughly the same
> number of targets and VCPUs, hotplug means the number of targets is
> difficult to predict and is usually fixed to 256.
> 
> The other reason is that per-target vq didn't give any performance
> advantage.  The bonus comes from cache locality and less process
> migrations, more than from the independent virtqueues.
> 
> Paolo

Okay, and why is per-target worse for cache locality?

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