[RFC] virtio: use mandatory barriers for remote processor vdevs

[Benjamin Herrenschmidt]

On Sun, 2011-12-11 at 14:25 +0200, Michael S. Tsirkin wrote:

> Forwarding some results by Amos, who run multiple netperf streams in
> parallel, from an external box to the guest.  TCP_STREAM results were
> noisy.  This could be due to buffering done by TCP, where packet size
> varies even as message size is constant.
> 
> TCP_RR results were consistent. In this benchmark, after switching
> to mandatory barriers, CPU utilization increased by up to 35% while
> throughput went down by up to 14%. the normalized throughput/cpu
> regressed consistently, between 7 and 35%
> 
> The "fix" applied was simply this:

What machine & processor was this  ?

Cheers,
Ben.

> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 3198f2e..fdccb77 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -23,7 +23,7 @@
>  
>  /* virtio guest is communicating with a virtual "device" that actually runs on
>   * a host processor.  Memory barriers are used to control SMP effects. */
> -#ifdef CONFIG_SMP
> +#if 0
>  /* Where possible, use SMP barriers which are more lightweight than mandatory
>   * barriers, because mandatory barriers control MMIO effects on accesses
>   * through relaxed memory I/O windows (which virtio does not use). */
> 
> 
>