Re: [PATCH] virtio: make udp more efficient by avoiding indirect desc

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 11, 2014 at 10:58:52PM +0800, Qin Chuanyu wrote:
> udp packet use 2 buffers at least, one for vnet_hdr and
> one for skb->data.

Not really, we use 1 buffer now with vnet_hdr inline with data.

> we could change the threshold from 2 to 3, so the udp packet
> which data buff only using single desc will gain from this.
> the guest would avoid from allocating memory dynamically.
> the host would avoid from translating indirect desc.
> 
> Signed-off-by: Chuanyu Qin <qinchuanyu@huawei.com>

Optimization patch without any performance data?
Such a change would need much more testing than that:
would have to try various workloads with -net and -blk at least.

> ---
>  drivers/virtio/virtio_ring.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 28b5338..88d008f 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -220,7 +220,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> 
>  	/* If the host supports indirect descriptor tables, and we have multiple
>  	 * buffers, then go indirect. FIXME: tune this threshold */
> -	if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
> +	if (vq->indirect && total_sg > 2 && vq->vq.num_free) {
>  		head = vring_add_indirect(vq, sgs, next, total_sg, total_out,
>  					  total_in,
>  					  out_sgs, in_sgs, gfp);
> -- 
> 1.7.3.1.msysgit.0