Re: virtio_net backport performance

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

Rusty Russell wrote:
> On Saturday 05 January 2008 09:24:40 Anthony Liguori wrote:
>   
>> Hey Rusty et al,
>>
>> I've got automatic backports of the virtio modules[1] working back to
>> 2.6.18.  Everything seems okay except that for any kernel with the older
>> NAPI api, performance is extremely bad.  I get about 1gbit on TX with
>> 2.6.24 but I get somewhere around 20mbit on 2.6.22.
>>     
>
> OK, I tested this backport and immediately got oopses.  Revealed some 
> interesting races in net driver (again, it's that damn callback disable 
> causing problems).  New queue fixes these, but get awful performance:
> 'dd bs=1M count=1000 if=/dev/zero | nc 172.20.0.1 8889' takes almost 30 
> seconds.
>   

rx performance is pretty abysmal but tx performance seems respectable.  
I'm getting strange results with netperf but my guess is that we're 
getting around 800mbit tx but only 25mbit rx.

tx is right around what it should be but rx is about an order of 
magnitude off.

> Found one bug in your code tho: if enable_cb returns false, it means the queue 
> has *not* been enabled:
>
> --- hack-module.awk.~1~	2008-01-06 10:49:16.000000000 +1100
> +++ hack-module.awk	2008-01-07 19:08:40.000000000 +1100
> @@ -49,7 +49,6 @@
>      print "	netif_rx_complete(vi->dev);";
>      print "";
>      print "	if (!no_work && netif_rx_reschedule(vi->dev, received)) {";
> -    print "		vi->rvq->vq_ops->disable_cb(vi->rvq);";
>      print "		skb = NULL;";
>      print "		goto again;";
>      print "	}";
>   

Thanks, I've applied this to the backport tree.

> Will continue looking for performance regression tomorrow (actually, better 
> check my changes haven't introduced it in non-backports first!).
>   

Excellent, thanks!

Regards,

Anthony Liguori

> Thanks!
> Rusty.
>   


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