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

[Rusty Russell <rusty@xxxxxxxxxxxxxxx>]

On Mon, 2007-06-04 at 15:43 +0200, Jens Axboe wrote:
> On Mon, Jun 04 2007, Carsten Otte wrote:
> > Jens Axboe wrote:
> > >On Fri, Jun 01 2007, Carsten Otte wrote:
> > >>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.


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