Subject: Re: 8% performance improved by change tap interact with kernel stack

From: Qin Chuanyu <[email-masked]>
Date: Tue, 28 Jan 2014 17:14:46 +0800

On 2014/1/28 16:34, Michael S. Tsirkin wrote:
> On Tue, Jan 28, 2014 at 04:14:12PM +0800, Qin Chuanyu wrote:
>> according perf test result，I found that there are 5%-8% cpu cost on
>> softirq by use netif_rx_ni called in tun_get_user.
>>
>> so I changed the function which cause skb transmitted more quickly.
>> from
>> 	tun_get_user	->
>> 		 netif_rx_ni(skb);
>> to
>> 	tun_get_user	->
>> 		rcu_read_lock_bh();
>> 		netif_receive_skb(skb);
>> 		rcu_read_unlock_bh();
>>
>> The test result is as below:
>> 	CPU: Intel(R) Xeon(R) CPU E5620  @ 2.40GHz
>> 	NIC: intel 82599
>> 	Host OS/Guest OS:suse11sp3
>> 	Qemu-1.6
>> 	netperf udp 512(VM tx)
>> 	test model: VM->host->host
>>
>> 	modified before : 2.00Gbps 461146pps
>> 	modified after  : 2.16Gbps 498782pps
>>
>> 8% performance gained from this change,
>> Is there any problem for this patch ?
>
> I think it's okay - IIUC this way we are processing xmit directly
> instead of going through softirq.
> Was meaning to try this - I'm glad you are looking into this.
>
> Could you please check latency results?
>
netperf UDP_RR 512
test model: VM->host->host

modified before : 11108
modified after  : 11480

3% gained by this patch