Subject: Re: [PATCH v3] virtio-net: lower min ring num_free for efficiency

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 2 Sep 2019 11:01:48 -0400

On Tue, Aug 20, 2019 at 02:51:23AM +0000, ? jiang wrote:
> This change lowers ring buffer reclaim threshold from 1/2*queue to budget
> for better performance. According to our test with qemu + dpdk, packet
> dropping happens when the guest is not able to provide free buffer in
> avail ring timely with default 1/2*queue. The value in the patch has been
> tested and does show better performance.
> 
> Test setup: iperf3 to generate packets to guest (total 30mins, pps 400k, UDP)
> avg packets drop before: 2842
> avg packets drop after: 360(-87.3%)
> 
> Further, current code suffers from a starvation problem: the amount of
> work done by try_fill_recv is not bounded by the budget parameter, thus
> (with large queues) once in a while userspace gets blocked for a long
> time while queue is being refilled. Trigger refills earlier to make sure
> the amount of work to do is limited.
> 
> Signed-off-by: jiangkidd <[email-masked]>
> Acked-by: Jason Wang <[email-masked]>
> Acked-by: Michael S. Tsirkin <[email-masked]>

Dave, could you merge this please?

Either net or net-next at your discretion.

> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 0d4115c9e20b..bc08be7925eb 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1331,7 +1331,7 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  		}
>  	}
>  
> -	if (rq->vq->num_free > virtqueue_get_vring_size(rq->vq) / 2) {
> +	if (rq->vq->num_free > min((unsigned int)budget, virtqueue_get_vring_size(rq->vq)) / 2) {
>  		if (!try_fill_recv(vi, rq, GFP_ATOMIC))
>  			schedule_delayed_work(&vi->refill, 0);
>  	}
> -- 
> 2.11.0