[Simh] Losing Time In SIMH

[Johnny Billquist]

On 2017-01-31 06:34, Sergey Oboguev wrote:
> It has been a long while since I touched SIMH, but when I did in the
> 3.9.x timeframe (and I do not know if the design has changed since
> then), I made the following note:
>
> /... the tendency of SIMH idea of system time (such as OpenVMS system
> time, when OpenVMS is executed on SIMH) to fall behind wall clock time.
> This “falling behind” happens because of a number of [...] reasons: host
> timer events are fired later then requested; it takes time for the
> hibernating thread to resume and start tracking time; accumulating
> ///rounding /errors ///in /time data obtained from the host, SIMH timing
> mechanisms dependence on the VCPU calibration (i.e. instructions per
> second rate) which is subject to variations because of a concurrent host
> load and also inherently – because of the inherent variations in the
> composition of the executed instruction set, page faults, synchronous IO
> operations executed on the VCPU thread etc.; because of OpenVMS
> suspension while SIMH console is in use or ROM console is in use;
> because of OpenVMS suspension while host system was suspended and
> similar reasons./

I would suspect the cause is because imprecise time handling in a user 
process on the host OS. However, that can be managed and corrected for, 
so it depends on how simh actually have implemented this in the end.
If simh is clever, it checks the wall clock from time to time, to see if 
it is drifting, and increases the rate of the simulated clock in that 
case. What it cannot do is just fire several clock interrupts in a row, 
since most hardware do not handle multiple interrupts from one source 
queued up, nor clock interrupts happening when the interrupt handler is 
already running to process the clock interrupt.

I guess Mark or Bob could answer how simh deals with time slippage due 
to host OS scheduling effects.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
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