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

[Jens Axboe <jens.axboe-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

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.

-- 
Jens Axboe


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