Subject: Re: [PATCH] virtio: Avoid virtio_net TX queue over run

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 17 Mar 2011 07:05:38 +0200

On Wed, Mar 16, 2011 at 05:09:55PM -0700, Shirley Ma wrote:
> This patch addresses small message size performance in a situation the
> KVM guest virtio_net TX queue overrun. This patch adds a new API in
> virtio_ring for ring capacity check; and remove KVM guest virtio_net TX
> queue send completion interrupts completely. The test has shown that 
> whenever the queue is overrun, it's much better to drop a few packets
> than stopping TX queue and waiting for host to notify the guest to wake
> up the TX queue again, the small messages size performance gain for
> single TCP_STREAM BW could be up to 200%-300% and better than bare
> metal, and no regression has been found in other situation.
> 
> Performance data for 10GbE,
> 
> KVM guest to local host:
> ------------------------
> Message size	2.6.38-rc8 	2.6.38-rc8+patch
> 1024		1770.61		4528.37
> 2048		2702.30		7110.95
> 4096		5256.84		10104.76
> 8192		7543.66		10945.93
> 16K		10500.47	10783.50
> 64K		13718.62	13640.80
> 
> KVM guest to remote host:
> --------------------------
> Message size	Bare Metal  2.6.38-rc8	2.6.38-rc8+patch
> 1024		1802.67		2381.41		5599.15	
> 2048		4317.87		4094.12		9241.86
> 4096		6266.15		5231.24		9321.87
> 8192		8409.17		7952.74		9265.45
> 16K		9351.63		8260.68		8310.29
> 64K		9347.94		9103.75		9094.38 
> 
> Thanks
> Shirley

Could you add CPU utilization data pls? I also wonder what does this do
to UDP?  Won't a lot of packets be dropped?

-- 
MST