Maximum NIC interrupts

Nash Nipples trashy_bumper at
Wed Dec 26 08:10:31 PST 2007

Dear Jordi,

In theory, on a Gigabit link you get 1 000 000 000 bits * second.
By default you have the MTU set to 1500 bytes which makes ~12 000 bits.
1 000 000 000 / 12 000 = ~ 83 333 packets per second.
83 333 packets per second makes 0.083333 packets per microsecond.
1 / 0.08333 = 12.0 microseconds per packet. Thus one can interrupt CPU
at a rate of ~83 333 times per second. If you use lower packets sizes you 
might get even more funny numbers.

8000 is a quiet low number. The driver was developed by guys
at Intel. I don't see a reason to worry. 

By the way they have products with Interrupt Moderation.

The question is really amazing. Thanks, it have tickled me big time.



----- Original Message ----
From: Jordi Espasa Clofent <jordi.espasa at>
To: freebsd-net at
Sent: Wednesday, December 26, 2007 12:12:55 PM
Subject: Re: Maximum NIC interrupts

OK, I'll try to explain in another way.

While I've done network performance test I've monitored the IRQ rate, 
and, for example, it's a 7000/8000 interrupts per second in every NIC
use 2 NICs in a bridge). The question is

¿how can I know if this irq rate is too high or not? ¿how can I know
I'm closer to device limits, or kernel limits?

I want to say that I'm don't know if 8000 irq per second means a high 
IRQ use or a lower user.

I hope I've explained better at this time.

Jordi Espasa Clofent
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