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

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Message-ID: <1271278661.16881.1761.camel@edumazet-laptop>
Date:	Wed, 14 Apr 2010 22:57:41 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	hadi@...erus.ca, Stephen Hemminger <shemminger@...tta.com>,
	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 13:45 -0700, Tom Herbert a écrit :
> > Only if more than one flow is involved.
> >
> > And if you have many flows, chance they will spread several queues...
> >
> 
> But use too many queues and the efficiency of NAPI drops and cost of
> device interrupts becomes dominant, so that the overhead from
> additional hard interrupts can surpass the overhead of doing RPS and
> the IPIs.  I believe we are seeing this is in some of our results
> which shows that a combination of multi-queue and RPS can be better
> than just multi-queue (see rps changelog).  Again, I'm not claiming
> that is generally true, but there are a lot of factors to consider.
> --

RPS can be tuned (Changli wants a finer tuning...), it would be
intereting to tune multiqueue devices too. I dont know if its possible
right now.

On my Nehalem machine (16 logical cpus), its NetXtreme II BCM57711E
10Gigabit has 16 queues. It might be good to use less queues according
to your results on some workloads, and eventually use RPS on a second
layering.




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