Re: [kvm-devel] [PATCH RFC 3/3] virtio infrastructure: example block driver

On Mon, 2007-06-04 at 15:43 +0200, Jens Axboe wrote:

On Mon, Jun 04 2007, Carsten Otte wrote:

quoted

Jens Axboe wrote:

quoted

On Fri, Jun 01 2007, Carsten Otte wrote:

quoted

With regard to compute power needed, almost none. The penalty is 
latency, not overhead: A small request may sit on the request queue to 
wait for other work to arrive until the queue gets unplugged. This 
penality is compensated by the benefit of a good chance that more 
requests will be merged during this time period.
If we have this method both in host and guest, we have twice the 
penalty with no added benefit.

I don't buy that argument. We can easily expose the unplug delay, so you
can kill it at what ever level you want. Or you could just do it in the
driver right now, but that is a bit hackish.

That would be preferable if the device driver can chose the unplug 
delay, or even better it could be (guest)sysfs tuneable.

Right. We probably should make it sysfs configurable in any case, right
now it's a (somewhat) policy decision in the kernel with the delay and
unplug depth.

The danger is that it's just another knob noone knows how to use.
Perhaps simply setting it to 0 for the noop scheduler will cover all
known cases?

Cheers,
Rusty.