Re: [PATCH V6 4/5] virtio-scsi: introduce multiqueue support

[Venkatesh Srinivas]

On Wed, Mar 20, 2013 at 03:01:23PM +0800, Wanlong Gao wrote:
> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>
> 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).
>
> The speedup comes from improving cache locality and giving CPU affinity
> to the virtqueues, which is why this scheme was selected.  Assuming that
> the thread that is sending requests to the device is I/O-bound, it is
> likely to be sleeping at the time the ISR is executed, and thus executing
> the ISR on the same processor that sent the requests is cheap.
>
> However, the kernel will not execute the ISR on the "best" processor
> unless you explicitly set the affinity.  This is because in practice
> you will have many such I/O-bound processes and thus many otherwise
> idle processors.  Then the kernel will execute the ISR on a random
> processor, rather than the one that is sending requests to the device.
>
> The alternative to per-CPU virtqueues is per-target virtqueues.  To
> achieve the same locality, we could dynamically choose the virtqueue's
> affinity based on the CPU of the last task that sent a request.  This
> is less appealing because we do not set the affinity directly---we only
> provide a hint to the irqbalanced running in userspace.  Dynamically
> changing the affinity only works if the userspace applies the hint
> fast enough.

Looks good! Tested as V5.

Tested-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxx>

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