What is rx_processing_limit sysctl for Intel igb NIC driver?

[Andy Young]

I am tuning our server that has an Intel 82576 gigabit NIC using the igb
driver. I see a lot of posts on the net where people bump the
rx_processing_limit sysctl from the default value of 100 to 4096. Can
anyone tell me what this is intended to do?

Thanks!

Andy

[John Baldwin]

Post by Andy Young
I am tuning our server that has an Intel 82576 gigabit NIC using the igb
driver. I see a lot of posts on the net where people bump the
rx_processing_limit sysctl from the default value of 100 to 4096. Can
anyone tell me what this is intended to do?

If you have multiple devices sharing an IRQ with igb (and thus are not using
MSI or MSI-X), it forces the driver to more-or-less cooperatively schedule
with the other interrupts on the same IRQ. However, since igb uses a fast
interrupt handler and a task on a dedicated taskqueue in the non-MSI case now,
I think it doesn't even do that. It should probably be set to -1 (meaning
unlimited) in just about all cases now.

[Nikolay Denev]

Post by John Baldwin

Post by Andy Young
I am tuning our server that has an Intel 82576 gigabit NIC using the igb
driver. I see a lot of posts on the net where people bump the
rx_processing_limit sysctl from the default value of 100 to 4096. Can
anyone tell me what this is intended to do?

If you have multiple devices sharing an IRQ with igb (and thus are not using
MSI or MSI-X), it forces the driver to more-or-less cooperatively schedule
with the other interrupts on the same IRQ. However, since igb uses a fast
interrupt handler and a task on a dedicated taskqueue in the non-MSI case now,
I think it doesn't even do that. It should probably be set to -1 (meaning
unlimited) in just about all cases now.
--
John Baldwin
_______________________________________________
http://lists.freebsd.org/mailman/listinfo/freebsd-hardware

And setting it to -1 gave a nice performance improvement in some tests that I did recently.
AFAIR only after setting this to -1 I was able to reach 10gig speed using iperf on two directly
connected machines with ix(4) 82599_______________________________________________
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