Re: [PATCH] virtio_net tx performance fix

Hi Dor,

How are you measuring performance?  The numbers I've gotten with netperf 
before and after your patch are:

tx - 647.27mbit
rx - 89.22

tx - 27.82
rx - 79.93

So this patch is pretty much killing performance for netperf.

Dor Laor wrote:

There was a problem with the location of the notify call in
add_buff function:
When VRING_USED_F_NO_NOTIFY is set, the host does not kick the
guest when packets were transmitted, as a result the guest runs
out of tx buffers sometimes.

But even if F_NO_NOTIFY is set, if the tx buffer is full, we notify the 
guest, so this prevents that from happening.

 This is fine but the problem lies
when add_buf fails, it called notify and the host sends all the
pending tx pkts. When enable_cb was called, more_used(vq) returned
false so eventually the skb was dropped.
  

I'm having a tough time following this part.  If add_buf fails, we 
notify unconditionally (which is, I think what we want).  I'm not sure 
how that relates to a packet getting dropped though.

Regards,

Anthony Liguori