Re: rps perfomance WAS(Re: rps: question

Eric Dumazet
2010-04-17T14:27:45Z

Le samedi 17 avril 2010 à 11:23 +0200, Eric Dumazet a écrit :
> Le samedi 17 avril 2010 à 01:43 -0700, Tom Herbert a écrit :
> > > So the cost of queing the packet into our own queue (netif_receive_skb
> > > -> enqueue_to_backlog) is about 0.74 us (74 ms / 100000)
> > >
> > > I personally think we should process packet instead of queeing it, but
> > > Tom disagree with me.
> > >
> > You could do that, but then the packet processing becomes HOL blocking
> > on all the packets that are being sent to other queues for
> > processing-- remember the IPIs is only sent at the end of the NAPI.
> > So unless the upper stack processing is <0.74us in your case, I think
> > processing packets directly on the local queue would improve best case
> > latency, but would increase average latency and even more likely worse
> > case latency on loads with multiple flows.

Tom, I am not sure what you describe is even respected for NAPI devices.
(I hope you use napi devices in your company ;) )

If we enqueue a skb to backlog, we also link our backlog napi into our
poll_list, if not already there.

So the loop in net_rx_action() will make us handle our backlog napi a
bit after this network device napi (if time limit of 2 jiffies not
elapsed) and *before* sending IPIS to remote cpus anyway.