netdev - Re: bnx2/BCM5709: why 5 interrupts on a 4 core system (2.6.33.3)

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Date:	Tue, 18 May 2010 16:22:00 +0200
From:	Krzysztof Olędzki <ole@....pl>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Michael Chan <mchan@...adcom.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: bnx2/BCM5709: why 5 interrupts on a 4 core system (2.6.33.3)

On 2010-05-16 23:26, Eric Dumazet wrote:

<CUT>

>> My normal workload is TCP and UDP based so if it is only ICMP then there
>> is no problem. Actually I have noticeably more UDP traffic than an
>> average network, mainly because of LWAPP/CAPWAP, so I'm interested in
>> good performance for both TCP and UDP.
>>
>> During my initial tests ICMP ping showed the same behavior like UDP/TCP
>> with iperf, so I sticked with it. I'll redo everyting with UDP and TCP
>> of course. :)
>>
>>>> BTW: With a normal router workload, should I expect big performance drop
>>>> when receiving and forwarding the same packet using different CPUs?
>>>> Bonding provides very important functionality, I'm not able to drop it. :(
>>>>
>>>
>>> Not sure what you mean by forwarding same packet using different CPUs.
>>> You probably meant different queues, because in normal case, only one
>>> cpu is involved (the one receiving the packet is also the one
>>> transmitting it, unless you have congestion or trafic shaping)
>>
>> I mean to receive it on a one CPU and to send it on a different one. I
>> would like to assing different vectors (eth1-0 .. eth1-4) to different
>> CPUs, but with bnx2x+bonding packets are received on queues 1-4 (eth1-1
>> .. eth1-4) and sent from queue 0 (eth1-0). So, for a one packet, two
>> different CPUs will be involved (RX on q1-q4, TX on q0).
>
> As I said, (unless you use RPS), one forwarded packet only uses one CPU.
> How tx queue is selected is another story. We try to do a 1-1 mapping.

OK, but with multi-queue NIC, I can assign each queue to a different 
CPU. So, while forwarding packets from a flow, I would like to assign 
the same queue on both input and output.

>>> If you have 4 cpus, you can use following patch and have a transparent
>>> bonding against multiqueue.
>>
>> Thanks! If I get it right: with the patch, packets should be sent using
>> the same CPU (queue?) that was used when receiving?
>
> Yes, for forwarding loads.
>
> (You might use 5 or 8 instead of 4, because its not clear to me if bnx2
> has 5 txqueues or 4 in your case)

Thank you. What happens if I set it to a lower/bigger value, than 
avaliable txqueues in a NIC?

Best regards,

			Krzysztof Olędzki
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