Subject: Re: behaviour question for igb on nehalem box

From: "Chris Friesen" <[email-masked]>
Date: Fri, 09 Oct 2009 16:31:59 -0600

On 10/09/2009 02:22 PM, Brandeburg, Jesse wrote:
> On Fri, 9 Oct 2009, Chris Friesen wrote:
>> I've got some general questions around the expected behaviour of the
>> 82576 igb net device.  (On a dual quad-core Nehalem box, if it matters.)

> the hardware you have only supports 8 
> queues (rx and tx) and the driver is configured to only set up 4 max.

The datasheet for the 82576 says 16 tx queues and 16 rx queues.  Is that
a typo or do we have the economy version?

>> My second question is around how the rx queues are mapped to interrupts.
>>  According to /proc/interrupts there appears to be a 1:1 mapping between
>> queues and interrupts.  However, I've set up at test with a given amount
>> of traffic coming in to the device (from 4 different IP addresses and 4
>> ports).  Under this scenario, "ethtool -S" shows the number of packets
>> increasing for only rx queue 0, but I see the interrupt count going up
>> for two interrupts.
> 
> one transmit interrupt and one receive interrupt?

No, two rx interrupts.  (Can't remember if the tx interrupt was going up
as well or no...was only looking at rx.)

> RSS will spread the 
> receive work out in a flow based way, based on ip/xDP header.  Your test 
> as described should be using more than one flow (and therefore more than 
> one rx queue) unless you got caught out by the default arp_filter 
> behavior (check arp -an).

I was surprised as well since it didn't match what I expected.  What's
the story around the arp_filter?  I just logged onto the test box and
"arp -an" gives:

? (47.135.251.129) at 00:00:5E:00:01:08 [ether] on eth0

but I'm not sure that's worth anything since someone is running a test
and it's currently using all four rx queues and all four rx interrupt
counts are increasing.  I'll have to see if they changed anything.

> Hope this helps,

That's great, thanks.

Chris

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