linux-kernel - Re: [PATCH v2] virtio-net: lower min ring num

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Date:   Sun, 18 Aug 2019 03:10:35 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     ? jiang <jiangkidd@...mail.com>
Cc:     "jasowang@...hat.com" <jasowang@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kafai@...com" <kafai@...com>,
        "songliubraving@...com" <songliubraving@...com>,
        "yhs@...com" <yhs@...com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "jiangran.jr@...baba-inc.com" <jiangran.jr@...baba-inc.com>
Subject: Re: [PATCH v2] virtio-net: lower min ring num_free for efficiency

On Thu, Aug 15, 2019 at 09:42:40AM +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%)
> 
> Signed-off-by: jiangkidd <jiangkidd@...mail.com>

To add to that:

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.

With this addition to the log:

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
>  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