netdev - Re: RPS runs with multiqueue NIC

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Message-ID: <1298353563.3360.6.camel@edumazet-laptop>
Date:	Tue, 22 Feb 2011 06:46:03 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jon Zhou <Jon.Zhou@...u.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RPS runs with multiqueue NIC

Le lundi 21 février 2011 à 19:07 -0800, Jon Zhou a écrit :
> Hi
> 
> What will happen if RPS runs on kernel 2.6.36.4 machine where a multiqueue NIC intel x520 installed
> Will it decrease or improve performance?

It depends on the workload.

For typical TCP flows, hardware multiqueue is generally better,
especially if the NIC supports the ethtool -X command (so that you can
tune effective number of cpus dedicated to the softirq processing).
Packet are delivered directly to the CPU target, without taking an extra
IPI.

For UDP workloads, adding RPS might be a win, because many NIC dont take
into account ports numbers in their hash computation, while
net/core/dev.c hash function does. If you receive trafic coming from a
single remote machine, RPS helps to spread flows between several cpus.

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