netdev - Re: rps perfomance WAS(Re: rps: question

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Message-ID: <1271514445.16881.4746.camel@edumazet-laptop>
Date:	Sat, 17 Apr 2010 16:27:25 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	hadi@...erus.ca, Changli Gao <xiaosuo@...il.com>,
	Rick Jones <rick.jones2@...com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	robert@...julf.net, andi@...stfloor.org
Subject: Re: rps perfomance WAS(Re: rps: question

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.




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