Re: [PATCH] virtio_net tx performance fix

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

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



-------------------------------------------------------------------------
This SF.net email is sponsored by: Microsoft
Defy all challenges. Microsoft(R) Visual Studio 2008.
http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/