Re: [PATCH 3/9] kvm: qemu: Remove virtio_net tx ring-full heuristic

[Mark McLoughlin <markmc@redhat.com>]

On Fri, 2008-07-25 at 10:30 +1000, Rusty Russell wrote:
> On Friday 25 July 2008 09:22:53 Dor Laor wrote:
> > Mark McLoughlin wrote:
> > >  	vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
> > >  	qemu_del_timer(n->tx_timer);
> > >  	n->tx_timer_active = 0;
> >
> > As stated by newer messages, we should handle the first tx notification
> > if the timer wasn't active to shorten latency.
> > Cheers, Dor
> 
> Here's what lguest does at the moment.  Basically, we cut the timeout a tiny 
> bit each time, until we get *fewer* packets than last time.  Then we bump it 
> up again.
> 
> Rough, but seems to work (it should be a per-device var of course, not a 
> static).
> 
> @@ -921,6 +922,7 @@ static void handle_net_output(int fd, st
>  	unsigned int head, out, in, num = 0;
>  	int len;
>  	struct iovec iov[vq->vring.num];
> +	static int last_timeout_num;
>  
>  	if (!timeout)
>  		net_xmit_notify++;
> @@ -941,6 +943,14 @@ static void handle_net_output(int fd, st
>  	/* Block further kicks and set up a timer if we saw anything. */
>  	if (!timeout && num)
>  		block_vq(vq);
> +
> +	if (timeout) {
> +		if (num < last_timeout_num)
> +			timeout_usec += 10;
> +		else if (timeout_usec > 1)
> +			timeout_usec--;
> +		last_timeout_num = num;
> +	}

Yeah, I gave this a try in kvm and in the host->guest case the timeout
just grew and grew. In the guest->host case, it did stabilise at around
50us with high throughput.

Basically, I think in the host->guest case the number of buffers was
very variable so the timeout would get bumped by 10, reduced by a small
amount, bumped by 10, reduced by a small amount, ...

But, I agree the general principal seems about right; just needs some
tweaking.

Cheers,
Mark.