em network issues

[Doug Ambrisko]

John Polstra writes:
| On 19-Oct-2006 Scott Long wrote:
| > The performance measurements that Andre and I did early this year showed
| > that the INTR_FAST handler provided a very large benefit.
| 
| I'm trying to understand why that's the case.  Is it because an
| INTR_FAST interrupt doesn't have to be masked and unmasked in the
| APIC?  I can't see any other reason for much of a performance
| difference in that driver.  With or without INTR_FAST, you've got
| the bulk of the work being done in a background thread -- either the
| ithread or the taskqueue thread.  It's not clear to me that it's any
| cheaper to run a task than it is to run an ithread.
| 
| A difference might show up if you had two or more em devices sharing
| the same IRQ.  Then they'd share one ithread, but would each get their
| own taskqueue thread.  But sharing an IRQ among multiple gigabit NICs
| would be avoided by anyone who cared about performance, so it's not a
| very interesting case.  Besides, when you first committed this
| stuff, INTR_FAST interrupts were not sharable.
| 
| Another change you made in the same commit (if_em.c revision 1.98)
| greatly reduced the number of PCI writes made to the RX ring consumer
| pointer register.  That would yield a significant performance
| improvement.  Did you see gains from INTR_FAST even without this
| independent change?

Something that we've fixed locally in atleast one version is:
     1)	Limit the loop in em_intr to 3 iterations
     2)	Pass a valid value to em_process_receive_interrupts/em_rxeof
	a good value like 100 instead of -1.  Since this is the count
	for how many time to iterate over the rx stuff.  Seems this
	got lost in the some change of APIs.
     3)	In em_process_receive_interrupts/em_rxeof always decrement
	the count on every run through the loop.  If you notice
	count is an is an int that starts at the passed in value
	of -1.  It then count-- until count==0.  Doing -1, -2, -3
	takes awhile until the int rolls over to 0.   Passing 100
	limits it more :-)  So this can run 3 * 100 versuses
	infinite * int roll over assuming we don't skip a count--.
Doing these changes made our multiple em based machines a lot happier
when slammed with traffic without starving other things that shared
interrupts like other em cards (especially in 4.X).  Interrupt handler 
should have limits of how long they should be able to run then let 
someone else go.  We use this in 6.X as well and haven't had any problems 
with our config's that use this.  We haven't tested much without these
since we need to fix other issues and this is now a non-issue for us.

I haven't pushed this more since I first found issue 1 and the concept
was rejected ... my machine hung in the interrupt spin loop :-(

If someone wants to examine/play with it more then that's great.
These issues (I think they are bugs) have been in there a while.

That's my 2 cents.

Doug A.