Re: [PATCH 2/9] kvm: qemu: Fix virtio_net tx timer

[Mark McLoughlin <markmc@redhat.com>]

On Sat, 2008-07-26 at 12:48 +0300, Avi Kivity wrote:
> Mark McLoughlin wrote:
> > The current virtio_net tx timer is 2ns, which doesn't
> > make any sense. Set it to a more reasonable 150us
> > instead.
> >
> > Signed-off-by: Mark McLoughlin <markmc@redhat.com>
> > ---
> >  qemu/hw/virtio-net.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/qemu/hw/virtio-net.c b/qemu/hw/virtio-net.c
> > index 2e57e5a..31867f1 100644
> > --- a/qemu/hw/virtio-net.c
> > +++ b/qemu/hw/virtio-net.c
> > @@ -26,7 +26,7 @@
> >  #define VIRTIO_NET_F_MAC	5
> >  #define VIRTIO_NET_F_GS0	6
> >  
> > -#define TX_TIMER_INTERVAL (1000 / 500)
> > +#define TX_TIMER_INTERVAL (150000) /* 150 us */
> >  
> 
> Ouch.

Well, not so much - and I should have explained why.

Even though virtio-net is requesting a 2ns tx timer, it actually gets
limited to MIN_TIMER_REARM_US which is currently 250us.

However, even though the timer itself will only fire after 250us,
expire_time is only set to +2ns, so we'll get the timeout callback next
time qemu_run_timers() is called from the mainloop.

I think this might account for a lot of the jitter in the throughput
numbers - the effective tx window size is anywhere between 2ns and 250us
depending on e.g. whether there is rx data available on the tap fd.

Cheers,
Mark.