[Simh] SLowing up simh

[Sterling Garwood]

I've been playing around with a nanosleep I put in pdp11_cpu.c in the  
ifetch loop.
So far I've been playing with what delay to use ... on a 1.3 GHZ  
iBook I've about settled on 250 nanoseconds... using that I still get  
good performance
from RSTS and the processor in my iBook only runs 50% busy (and a lot  
cooler.....)


On Sep 28, 2005, at 10:15 PM, Greg Johnson wrote:

> I've been kind of thinking about this since I would like to keep my  
> VAX
> simulator running all the time but, on a notebook, the host cpu can  
> get
> quite hot...  In fact I think my notebook can second as a hair dryer
> when I get the vax simulator running...
>
> I would think we could have some sort of 'idle loop' detection in the
> main cpu simulation processing loop.  Something where the specific cpu
> code would call a 'ok nothing interesting happened during this cycle'
> routine.  After a certain number of these cycles the processing loop
> would start entering short sleeps between cycles -- freeing up your
> host's cpu for other things.  This way if the simulated cpu is busy it
> gets the necessary cycles it needs while if it is doing 'nothing' it
> dozes off between cycles.  Once something happens the processing picks
> up again....
>
> Some simulated device interrupts might be considered 'idle' (for  
> example
> a line frequency clock interrupt).  While others (like input from the
> keyboard or ethernet) would not and would 'wake up' the simulator.
>
> Would need two configurable values - the number of 'idle' cycles  
> before
> the simulator gets 'sleepy' and the amount of time it sleeps  
> between cycles.
>
> Would not have to make the change in all the different simulated
> processors at the same time since, if the 'unchanged' specific cpu
> simulator code does not call the 'idle' routine it will still work the
> way it does now.
>
> --- Greg
>
> Tim Riker wrote:
>
>> Sterling Garwood wrote:
>>
>>
>>> is there a way to slow down the simulation? I am running PDP11
>>> simulations using RSTS and RT-11 and the simulator never seems  
>>> to  drop
>>> into a real wait state (my real processor stays pegged out even  if
>>> nothing is going on in RT-11). Since I am running on Mac OS X on  an
>>> iBook I'd like to trade off simulation speed for battery life  
>>> and  less
>>> heat. I have a 1.3 GHz laptop so a bit of a slow up of simh   
>>> wouldn't be
>>> too bad :-). My assumption is that either (or both) RSTS  and RT-11
>>> actually use a wait state.
>>> I looked at the code and thought about putting a nanosleep just   
>>> before
>>> IFetch but I thought I'd ask before I do that.
>>>
>>
>>
>> This has been discussed before, but nothing has yet been done.
>>
>> I patched my local simh to use select on stdin before doing the  
>> read and
>> I have it wait in the select briefly. This works fairly well as simh
>> still respond immediately to a keypress. It does run significantly
>> slower though,
>>
>> I assert that we could somehow watch for real tight loops in the
>> simulator and adjust the sleep times based on whether we are in  
>> one or
>> not. That way trivial loops in the hosted machine would result it  
>> more
>> cpu being given to the host. This would need some tweaking.
>>
>> I would think we could at least start with a configurable wait  
>> time that
>> defaults to 0. This way the default behavior would mirror the  
>> current,
>> and folks that want to slow things down, could without having to  
>> patch
>> and recompile.
>>
>>
>
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