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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 28 Jan 2014 10:34:59 +0200

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?

-- 
MST