On Wed, Feb 02, 2011 at 10:09:14PM -0800, Shirley Ma wrote:
> On Thu, 2011-02-03 at 07:59 +0200, Michael S. Tsirkin wrote:
> > > Let's look at the sequence here:
> > >
> > > guest start_xmit()
> > > xmit_skb()
> > > if ring is full,
> > > enable_cb()
> > >
> > > guest skb_xmit_done()
> > > disable_cb,
> > > printk free_old_xmit_skbs <-- it was between more than 1/2
> > to
> > > full ring size
> > > printk vq->num_free
> > >
> > > vhost handle_tx()
> > > if (guest interrupt is enabled)
> > > signal guest to free xmit buffers
> > >
> > > So between guest queue full/stopped queue/enable call back to guest
> > > receives the callback from host to free_old_xmit_skbs, there were
> > about
> > > 1/2 to full ring size descriptors available. I thought there were
> > only a
> > > few. (I disabled your vhost patch for this test.)
> >
> >
> > The expected number is vq->num - max skb frags - 2.
>
> It was various (up to the ring size 256). This is using indirection
> buffers, it returned how many freed descriptors, not number of buffers.
>
> Why do you think it is vq->num - max skb frags - 2 here?
>
> Shirley
well queue is stopped which happens when
if (capacity < 2+MAX_SKB_FRAGS) {
netif_stop_queue(dev);
if (unlikely(!virtqueue_enable_cb(vi->svq))) {
/* More just got used, free them then recheck.
* */
capacity += free_old_xmit_skbs(vi);
if (capacity >= 2+MAX_SKB_FRAGS) {
netif_start_queue(dev);
virtqueue_disable_cb(vi->svq);
}
}
}
This should be the most common case.
I guess the case with += free_old_xmit_skbs is what can get us more.
But it should be rare. Can you count how common it is?
--
MST
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