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:

quoted

quoted

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

quoted

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

quoted

1/2 to full ring size descriptors available. I thought there were

only a

quoted

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