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

[haibinzhang(åææ)]

>On Tue, Apr 03, 2018 at 08:08:26AM +0000, haibinzhang 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?

The MTU of ethernet is 1500, so VHOST_NET_WEIGHT/MTU equals 0x80000/1500=350.
Then sent-bytes cannot reach VHOST_NET_WEIGHT in one handle_tx even with 1500-bytes 
frame if packet# is less than 350. So packet# must be bigger than 350.
512 meets this condition and is also DEFAULT VRING_SIZE aligned.

>
>> 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 <haibinzhang@xxxxxxxxxxx>
>> Signed-off-by: Yunfang Tai <yunfangtai@xxxxxxxxxxx>
>> Signed-off-by: Lidong Chen <lidongchen@xxxxxxxxxxx>
>> ---
>>  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
>>