Subject: Re: [PATCH] vhost-net: add limitation of sent packets for tx polling

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 3 Apr 2018 14:59:03 +0300

On Tue, Apr 03, 2018 at 08:08:26AM +0000, haibinzhang(张海斌) wrote:
> handle_tx will delay rx for a long time when tx busy polling udp packets
> with small length(e.g. 1byte udp payload), because setting VHOST_NET_WEIGHT
> takes into account only sent-bytes but no single packet length.
> 
> Tests were done between two Virtual Machines using netperf(UDP_STREAM, len=1),
> then another machine pinged the client. Result shows as follow:
> 
> Packet#       Ping-Latency(ms)
>               min     avg     max
> Origin      3.319  18.489  57.503
> 64          1.643   2.021   2.552
> 128         1.825   2.600   3.224
> 256         1.997   2.710   4.295
> 512*        1.860   3.171   4.631
> 1024        2.002   4.173   9.056
> 2048        2.257   5.650   9.688
> 4096        2.093   8.508  15.943
> 
> 512 is selected, which is multi-VRING_SIZE

There's no guarantee vring size is 256.

Could you pls try with a different tx ring size?

I suspect we want:

#define VHOST_NET_PKT_WEIGHT(vq) ((vq)->num * 2)

> and close to VHOST_NET_WEIGHT/MTU.

Puzzled by this part.  Does tweaking MTU change anything?

> To evaluate this change, another tests were done using netperf(RR, TX) between
> two machines with Intel(R) Xeon(R) Gold 6133 CPU @ 2.50GHz. Result as follow
> does not show obvious changes:
> 
> TCP_RR
> 
> size/sessions/+thu%/+normalize%
>    1/       1/  -7%/        -2%
>    1/       4/  +1%/         0%
>    1/       8/  +1%/        -2%
>   64/       1/  -6%/         0%
>   64/       4/   0%/        +2%
>   64/       8/   0%/         0%
>  256/       1/  -3%/        -4%
>  256/       4/  +3%/        +4%
>  256/       8/  +2%/         0%
> 
> UDP_RR
> 
> size/sessions/+thu%/+normalize%
>    1/       1/  -5%/        +1%
>    1/       4/  +4%/        +1%
>    1/       8/  -1%/        -1%
>   64/       1/  -2%/        -3%
>   64/       4/  -5%/        -1%
>   64/       8/   0%/        -1%
>  256/       1/  +7%/        +1%
>  256/       4/  +1%/        +1%
>  256/       8/  +2%/        +2%
> 
> TCP_STREAM
> 
> size/sessions/+thu%/+normalize%
>   64/       1/   0%/        -3%
>   64/       4/  +3%/        -1%
>   64/       8/  +9%/        -4%
>  256/       1/  +1%/        -4%
>  256/       4/  -1%/        -1%
>  256/       8/  +7%/        +5%
>  512/       1/  +1%/         0%
>  512/       4/  +1%/        -1%
>  512/       8/  +7%/        -5%
> 1024/       1/   0%/        -1%
> 1024/       4/  +3%/         0%
> 1024/       8/  +8%/        +5%
> 2048/       1/  +2%/        +2%
> 2048/       4/  +1%/         0%
> 2048/       8/  -2%/         0%
> 4096/       1/  -2%/         0%
> 4096/       4/  +2%/         0%
> 4096/       8/  +9%/        -2%
> 
> Signed-off-by: Haibin Zhang <[email-masked]>
> Signed-off-by: Yunfang Tai <[email-masked]>
> Signed-off-by: Lidong Chen <[email-masked]>
> ---
>  drivers/vhost/net.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8139bc70ad7d..13a23f3f3ea4 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -44,6 +44,10 @@ MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
>   * Using this limit prevents one virtqueue from starving others. */
>  #define VHOST_NET_WEIGHT 0x80000
>  
> +/* Max number of packets transferred before requeueing the job.
> + * Using this limit prevents one virtqueue from starving rx. */
> +#define VHOST_NET_PKT_WEIGHT 512
> +
>  /* MAX number of TX used buffers for outstanding zerocopy */
>  #define VHOST_MAX_PEND 128
>  #define VHOST_GOODCOPY_LEN 256
> @@ -473,6 +477,7 @@ static void handle_tx(struct vhost_net *net)
>  	struct socket *sock;
>  	struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
>  	bool zcopy, zcopy_used;
> +	int sent_pkts = 0;
>  
>  	mutex_lock(&vq->mutex);
>  	sock = vq->private_data;
> @@ -580,7 +585,8 @@ static void handle_tx(struct vhost_net *net)
>  		else
>  			vhost_zerocopy_signal_used(net, vq);
>  		vhost_net_tx_packet(net);
> -		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
> +		if (unlikely(total_len >= VHOST_NET_WEIGHT) ||
> +		    unlikely(++sent_pkts >= VHOST_NET_PKT_WEIGHT)) {
>  			vhost_poll_queue(&vq->poll);
>  			break;
>  		}
> -- 
> 2.12.3
>