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

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Date:	Sat, 17 Apr 2010 11:23:49 +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 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.

Anyway, a big part of this 0.74 us overhead comes from get_rps_cpu()
itself, computing skb->rxhash and all. We should make a review of how
many cache lines we exchange per skb, and try to reduce this number.



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