[Simh] vax idle for NetBSD

[Paul Koning]

> On Dec 11, 2018, at 12:01 PM, Mark Pizzolato <Mark at infocomm.com> wrote:
> 
> ...
> Meanwhile, something else has changed relatively recently that doesn't 
> seem to actually execute this code when the system starts.  Specifically, 
> I just observed:
>  1) Boot NetBSD 8.0
>  2) Boot took 100 seconds to get to a "login: " prompt.
>  3) I would normally expect that the system was pretty much idle at this point and the 100 seconds would have exceeded the 20 second calibration window the idle code needs to have elapsed before the system actually idles.
>  4) The system ran for another 80 seconds before idling actually kicked in.
> 
> I have no idea what the system was doing during this 80 second period, but that is not how things worked when the code above was initially added to the NetBSD kernel.
> 
> It is good that eventually the above code is engaged, so we don't really need to change anything else, but it would be interesting to understand what the system is actually doing...
> 
> - Mark

I remember seeing unexpectedly long delays to idle in other simulators too.  PDP11 quite reliably kicks into idle quickly, but on a simulator I'm working on (ELX8) which also has an explicit idle instruction it would take a minute or so.  The one difference I can think of is the clock period, 16 ms for PDP11 (given simulated line clock) but 10 ms on ELX8.

	paul