netdev - Re: RSS is not efficient when forwarding (ixgbe)

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Message-ID: <21436.11067.667194.899369@fisica.ufpr.br>
Date:	Tue, 8 Jul 2014 14:32:43 -0300
From:	Carlos Carvalho <carlos@...ica.ufpr.br>
To:	Flavio Leitner <fbl@...hat.com>
Cc:	"Skidmore\, Donald C" <donald.c.skidmore@...el.com>,
	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: RSS is not efficient when forwarding (ixgbe)

Flavio Leitner (fbl@...hat.com) wrote on 8 July 2014 14:21:
 >On Tue, Jul 08, 2014 at 02:09:13PM -0300, Carlos Carvalho wrote:
 >> Flavio Leitner (fbl@...hat.com) wrote on 7 July 2014 21:28:
 >>  >On Mon, Jul 07, 2014 at 04:33:24PM +0000, Skidmore, Donald C wrote:
 >>  >> 
 >>  >> 
 >>  >> > 
 >>  >> > It's a router forwarding traffic from one interface to another, so I guess it's
 >>  >> > only the kernel. BTW, no firewall.
 >>  >> > 
 >>  >> > Flow Director needs to be enabled and I am using defaults.
 >>  >> 
 >>  >> Flow Director in ATR mode is on by default for ixgbe.  So like Tom mentioned the driver will create hash buckets for egress packets.  You could try disabling ATR and just use RSS.  Which would probably be the right thing to do any way since Flow Director isn't very useful for routing scenarios.  
 >>  >
 >>  >That was it.
 >> 
 >> We have a similar setup and similar problem. How do we disable ATR? I
 >> tried to set ntuple off but this almost zeroed traffic. I also tried
 >> to change rx-flow-hash but ethtool says it's not possible. The docs
 >> say that one can disable ATR by setting AtrSampleRate to 0 but this
 >> parameter doesn't exist in 3.14.10.
 >> 
 >> So, how do we disable ATR and keep RSS?
 > 
 >Keep in mind that this is actually 2 problems. One is enabling the
 >NIC to receive the streams in all queues for this scenario (setting
 >ntuple off and restarting the traffic works for me). The second problem
 >is having all the queue interrupts spread among the CPUs. That's what
 >does irqbalance, tuna, etc...

Spreading the interrupts among the cpus is not the issue for us. The
problem is that the number of interrupts is *very* different among the
irq's, so no matter how I distribute them among cores there will
always be a few that get saturated while 70% of the machine capacity
remains idle. Your case seems to be the extreme of ours, where all
the flux goes to a single irq.

The problem is in how the NIC distributes traffic among the irq's in
the router. Traffic comes almost only from a single machine and
spreads through several thousand destinations in the internet. That's
why I tried to set the receiving hash mode to the destination IP, but
the NIC or driver refuses. So how do I even out the frequency of irq's?
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