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

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Message-ID: <z2y65634d661004141345o9bdcf759sf266866931823baf@mail.gmail.com>
Date:	Wed, 14 Apr 2010 13:45:37 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.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

> 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.
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