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

Il 04/09/2012 14:48, Michael S. Tsirkin ha scritto:

quoted

quoted

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@redhat.com>

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