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

On Tue, Sep 04, 2012 at 04:30:35PM +0200, Paolo Bonzini wrote:

Il 04/09/2012 16:21, Michael S. Tsirkin ha scritto:

quoted

quoted

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.

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

Because per-target doesn't have IRQ affinity for a particular CPU.

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.

But if you have many such I/O-bound processes, the kernel will execute
the ISR on a random processor, rather than the one that is sending
requests to the device.

Paolo

I see, another case where our irq balancing makes bad decisions.
You could do it differently - pin irq to the cpu of the last task that
executed, tweak irq affinity when that changes.
Still if you want to support 256 targets vector per target
is not going to work.

Would be nice to add this motivation to commit log I think.

-- 
MST