Interrupts + Polling mode (similar to Linux's NAPI)

[Luigi Rizzo]

On Fri, Mar 27, 2009 at 12:47:45AM +0000, Andrew Brampton wrote:
> Hi,
> Linux has a feature called NAPI, which amongst other things has this
> Interrupt initiated polling mode. Whilst the network traffic is quiet
> the network interfaces use interrupts, however as soon as the load
> becomes higher polling kicks in and stays like that until the load
> drops again.
> 
> I know that FreeBSD can do interrupts or polling, but not together. I
> think that that NAPI pretty neat as it provides the benefits of both
> interrupts and polling, namely low CPU load (when the network is not
> busy), and high performance. I was wondering if anyone has considered
> this approach in FreeBSD? If not why not? Is there some reason why the
> binary FreeBSD approach is better? Or is it just that no one has
> dedicated the time and effort to implement this feature?

The load of polling is pretty low (within 1% or so) even with
polling. The advantage of having interrupts is faster response
to incoming traffic, not CPU load.

There is nothing difficult in having both active, except figuring
out a good logic for when to disable polling on an interface
that has been quiet for a while.

I don't know what is the status of polling these days -- when i
wrote it, the architecture was designed for UP kernels, and I
don't know if/how it has been revised to deal efficiently with
the SMP kernels we have now (in other words: one or multiple
polling loops, interaction with interrupt threads, etc.)

	cheers
	luigi