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

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Date:	Wed, 14 Apr 2010 20:04:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	hadi@...erus.ca, netdev@...r.kernel.org, robert@...julf.net,
	David Miller <davem@...emloft.net>,
	Changli Gao <xiaosuo@...il.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: rps perfomance WAS(Re: rps: question

Le mercredi 14 avril 2010 à 10:31 -0700, Tom Herbert a écrit :
> The point of RPS is to increase parallelism, but the cost of that is
> more overhead per packet.  If you are running a single flow, then
> you'll see latency increase for that flow.  With more concurrent flows
> the benefits of parallelism kick in and latency gets better.-- we've
> seen the break even point around ten connections in our tests.  Also,
> I don't think we've made the claim that RPS should generally perform
> better than multi-queue, the primary motivation for RPS is make single
> queue NICs give reasonable performance.
> 

Yes, multiqueue is far better of course, but in case of hardware lacking
multiqueue, RPS can help many workloads, where application has _some_
work to do, not only counting frames or so...

RPS overhead (IPI, cache misses, ...) must be amortized by
parallelization or we lose.

A ping test is not an ideal candidate for RPS, since everything is done
at softirq level, and should be faster without RPS...



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