igb interrupt moderation

Michael Tüxen Michael.Tuexen at lurchi.franken.de
Sun Jan 3 13:55:23 UTC 2010

Hi Barney, Hi Jack,

some comments and some more questions inside...

Best regards

On Jan 2, 2010, at 8:42 PM, Barney Cordoba wrote:

> Jack,
> I'm trying to get some clarification on differences I'm finding between
> the 82575 and 82576 parts with respect to interrupt moderation. The spec
> I have for the 82576 (82576_Datasheet_v2p1.pdf) indicates that the 
I'm only commenting 82576. You can get rev 2.41 from intels website...
> ITR algorithm is different than the one used (I don't have one of the
> secret copies of the 82575 spec). The algorithm shown is
> interrupts/sec = 1/(2 * 10-6sec x interval) (page 295, Section 7.3.4)
> which is clearly wrong from practice. I have an 82576 (device id 10C9)
If you look at section 8.8.12, you find other formulas...
Jack: Which ones are correct?
> if I use the 125d setting in the example get just under 32000 interrupts
> per second. Clearly your code doesnt implement this, nor do you have
> different settings for the 82575 and 82576 parts. So I assume that the 
> same formula for the em parts hold for the igb parts, and that the 
> datasheet is wrong?
> There does seem to be a slight difference. The setting that gets 1000
> ints/second on the 82575 generates about 1020 on the 82576. Not a big
> deal but I wonder why there's a difference? Is the reference clock for
> these something that may not be fixed and could vary from board to 
> board? Note that both devices are on the same MB.
> Also, it seems that settings to EITR over 32767 wrap on the 82576 (for
> example writing 32768 to EITR is the same as writing a 1). So the  minimum setting on the 82576 is around 125 ints/second. The 82575 can accept 
> values up the 65535 before wrapping. 
Hmm, looking at the table in 8.8.12 would suggest:
Setting it to one sets a reserved bit, but does not change the interval.
Setting it to 2^15 should set the LLI_EN bit, but does not change in interval.

Jack is setting the register to
igb_low_latency: 128
igb_ave_latency: 450
igb_bulk_latency: 1200

This would result in intervals of:
igb_low_latency: 32
igb_ave_latency: 112
igb_bulk_latency: 300
Jack: What are the corresponding interrupt rates? The spec provides different
      formulas and talks about a 1us, 2us or 8us counter. Not sure what is right...
Jack: Why are you setting bit1 (which is reserved) in the case igb_ave_latency?

And another question for Jack:
In igb_update_aim() you do
	if (olditr != newitr) {
		/* Change interrupt rate */
		rxr->eitr_setting = newitr;
		E1000_WRITE_REG(&adapter->hw, E1000_EITR(rxr->me),
		    newitr | (newitr << 16));
So why are setting the higher bits of the EITR? You are setting
igb_low_latency: the LL Counter becomes 0, the moderation counter becomes 16
igb_ave_latency: the LL Counter becomes 2, the moderation counter becomes 56
igb_bulk_latency: the LL Counter becomes 16, the moderation counter becomes 148

I really do not understand these settings. Maybe the spec is wrong? Or you do mean
	if (olditr != newitr) {
		/* Change interrupt rate */
		rxr->eitr_setting = newitr;
		E1000_WRITE_REG(&adapter->hw, E1000_EITR(rxr->me), newitr);
Or do you want to preserve the counters, set the CNT_INGR bit and mean
	if (olditr != newitr) {
		/* Change interrupt rate */
		rxr->eitr_setting = newitr;
		E1000_WRITE_REG(&adapter->hw, E1000_EITR(rxr->me), 0x80000000 | newitr);

Could you clarify that?
> The 82576 document doesn't have a map of the register that I can find, so
> Im curious as to whether these observations are something I can assume is
> true across all parts and motherboards/cards, or is there some
> implementation variance that will cause these to only apply to the ones
> I happen to be testing?
> Thanks,
> Barney
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