Re: virtio_net backport performance

[Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>]

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.

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 "	}";

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

Thanks!
Rusty.

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