Hello,

I came across this paper, which may be interesting for NetBSD too:

"Running and tuning OpenBSD network servers in a production environment"

(http://openbsd.underlinux.com.br/papers/tuning.pdf)

In section "IRQ" there is an interesting claim that sharing interrupts
between netwok cards could improve performance. (I would think the
opposite is true.) Quoting the paragraph for your convenience:

"Every interrupt requires a context switch, from
the process running when the IRQ took place,
to the interrupt handler. As a number of things
must be done upon entering the interrupt han-
dler, a large quantity of interrupts can result
in excess time required for context switching.
One non-obvious way to reduce this load is to
share interrupts between the network adapters,
something permitted on the PCI bus. As many
people are not even aware of the the possibility
of interrupt sharing, and the benefits are not
obvious, let's look at this a little closer.

With separate adapters on separate interrupt
lines, when the first interrupt comes in, a con-
text switch to the interrupt handler takes place.
If another interrupt comes in from the other
adapter while the first interrupt is still being
handled, it will either interrupt the first han-
dler, or be delayed until the first handler has
completed, depending on priority, but regard-
less, two additional context switches will take
place { one into the second handler, one back
out.

In the case of the PCI and EISA busses, in-
terrupts are level triggered, not edge triggered,
which makes interrupt sharing possible. As
long as the interrupt line is held active, a device
needs servicing, even if the first device which
triggered the interrupt has already been ser-
viced. So, in this case, when the first adapter
triggers the interrupt, there will be a context
switch to the handler. Before the handler re-
turns, it will see if any other devices need ser-
vicing, before doing a context switch back to
the previous process.

In a busy environment, when many devices are
needing service, saving these context switches
can signicantly improve performance by per-
mitting the processor to spend more time pro-
cessing data, rather than switching between
tasks.  (...) "

Now, a couple of questions:

- Is this true for NetBSD too?

- Isn't this a symptom of bad design? If there are different interrupt
  lines for different devices, the interrupt handler can immediately
  see which device caused the interrupt. If two devices share the same
  interrupt, the interrupt code must first look which device caused it.
  I would also guess that with different interrupts, it could be
  easier to run two interrupt handlers concurrently on two processors.
  What's exactly the situation with this?

Thanks	Pavel