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# Simulation with Real Time Synchronization

Authors: | Klaus Muller <Muller@users.sourceforge.net> Tony Vignaux <Vignaux@users.sourceforge.net>
SimPy release: | 2.3
Web-site: | http://simpy.sourceforge.net/
Python-Version: | 2.6+
Date: | December 2011

Contents

- Simulation with Real Time Synchronization
  - Acknowledgement
  - Synchronizing with wall clock time
  - Changing the execution speed during a simulation run
  - Limitations
  - The SimulationRT API
    - Structure
    - simulate
    - rtset

This manual describes SimulationRT, a SimPy module which supports synchronizing the execution of simulation models with real (wallclock) time.

## Acknowledgement

SimulationRT is based on an idea by Geoff Jarrad of CSIRO (Australia). He contributed a lot to its development and testing on Windows and Unix.

The code for the adjustment of the execution speed during the simulation run was contributed by Robert C. Ramsdell.

## Synchronizing with wall clock time

SimulationRT allows synchronizing simulation time and real (wallclock) time. This capability can be used to implement e.g. interactive game applications or to demonstrate a model’s execution in real time.

It is identical to Simulation, except for the simulate function which takes an additional parameter controlling real-time execution speed.

Here is an example:

    """ RealTimeFireworks.py """
    from SimPy.SimulationRT  import *
    from random import *
    import time
    ## Model components -----------------------------------------------------------
    class Launcher(Process):
    	def launch(self):
    		while True:
    			print "Launch at %2.2f; wallclock: %2.2f"%(now(),time.clock()-startTime)
    			yield hold,self,uniform(1,maxFlightTime)
    			print "Boom!!! Aaaah!! at %2.2f; wallclock: %2.2f"\
    			       %(now(),time.clock()-startTime)
    def model():
    	initialize()
    	for i in range(nrLaunchers):
    		lau=Launcher()
    		activate(lau,lau.launch())
    	simulate(real_time=True,rel_speed=1,until=20) ##unit sim time = 1 sec clock
    ## Experiment data  -----------------------------------------------------------
    nrLaunchers=2
    maxFlightTime=5.0
    startTime=time.clock()
    seed(1234567)
    ## Experiment -----------------------------------------------------------------
    model()

rel_speed is the ratio simulated time/wallclock time. rels_speed=1 sets the synchronization so that 1 simulation time unit is executed in approximately 1 second of wallclock time. Run under Python 2.6 on a Windows Vista-box (2.3 GHz), this output resulted over about 17.5 seconds of wallclock time:

    Launch at 0.00; wallclock: 0.00
    Launch at 0.00; wallclock: 0.00
    Boom!!! Aaaah!! at 3.04; wallclock: 3.03
    Launch at 3.04; wallclock: 3.03
    Boom!!! Aaaah!! at 3.90; wallclock: 3.90
    Launch at 3.90; wallclock: 3.90
    Boom!!! Aaaah!! at 5.47; wallclock: 5.47
    Launch at 5.47; wallclock: 5.47
    Boom!!! Aaaah!! at 6.50; wallclock: 6.50
    Launch at 6.50; wallclock: 6.50
    Boom!!! Aaaah!! at 7.35; wallclock: 7.35
    Launch at 7.35; wallclock: 7.35
    Boom!!! Aaaah!! at 9.88; wallclock: 9.87
    Launch at 9.88; wallclock: 9.87
    Boom!!! Aaaah!! at 10.71; wallclock: 10.70
    Launch at 10.71; wallclock: 10.70
    Boom!!! Aaaah!! at 11.84; wallclock: 11.84
    Launch at 11.84; wallclock: 11.84
    Boom!!! Aaaah!! at 12.17; wallclock: 12.16
    Launch at 12.17; wallclock: 12.16
    Boom!!! Aaaah!! at 13.78; wallclock: 13.78
    Launch at 13.78; wallclock: 13.78
    Boom!!! Aaaah!! at 14.63; wallclock: 14.62
    Launch at 14.63; wallclock: 14.62
    Boom!!! Aaaah!! at 17.30; wallclock: 17.29
    Launch at 17.30; wallclock: 17.29
    Boom!!! Aaaah!! at 17.32; wallclock: 17.31
    Launch at 17.32; wallclock: 17.31

Clearly, the wallclock time does not deviate significantly from the simulation time.

## Changing the execution speed during a simulation run

By calling method rtset with a parameter, the ratio simulated time to wallclock time can be changed during a run.

Here is an example:

    """variableTimeRatio.py
    Shows the SimulationRT capability to change the ratio simulation
    time to wallclock time during the run of a simulation.
    """
    from SimPy.SimulationRT import *

    class Changer(Process):
        def change(self,when,rat):
            global ratio
            yield hold,self,when
            rtset(rat)
            ratio=rat
    class Series(Process):
        def tick(self,nrTicks):
            oldratio=ratio
            for i in range(nrTicks):
                tLastSim=now()
                tLastWallclock=wallclock()
                yield hold,self,1
                diffSim=now()-tLastSim
                diffWall=wallclock()-tLastWallclock
                print "now(): %s, sim. time elapsed: %s, wall clock elapsed: "\
                    "%6.3f, sim/wall time ratio: %6.3f"\
                    %(now(),diffSim,diffWall,diffSim/diffWall)
                if not ratio==oldratio:
                    print "At simulation time %s: ratio simulation/wallclock "\
                        "time now changed to %s"%(now(),ratio)
                    oldratio=ratio
    initialize()
    ticks=15
    s=Series()
    activate(s,s.tick(nrTicks=ticks))
    c=Changer()
    activate(c,c.change(5,5))
    c=Changer()
    activate(c,c.change(10,10))
    ratio=1
    print "At simulation time %s: set ratio simulation/wallclock time to %s"\
           %(now(),ratio)
    simulate(until=100,real_time=True,rel_speed=ratio)

The program changes the time ratio twice, at simulation times 5 and 10.

When run on a Windows Vista computer under Python 2.6, this results in this output:

    At simulation time 0: set ratio simulation/wallclock time to 1
    now(): 1, sim. time elapsed: 1, wall clock elapsed:  0.998, sim/wall time ratio:  1.002
    now(): 2, sim. time elapsed: 1, wall clock elapsed:  0.999, sim/wall time ratio:  1.001
    now(): 3, sim. time elapsed: 1, wall clock elapsed:  0.999, sim/wall time ratio:  1.001
    now(): 4, sim. time elapsed: 1, wall clock elapsed:  0.999, sim/wall time ratio:  1.001
    now(): 5, sim. time elapsed: 1, wall clock elapsed:  0.999, sim/wall time ratio:  1.001
    At simulation time 5: ratio simulation/wallclock time now changed to 5
    now(): 6, sim. time elapsed: 1, wall clock elapsed:  0.199, sim/wall time ratio:  5.027
    now(): 7, sim. time elapsed: 1, wall clock elapsed:  0.199, sim/wall time ratio:  5.025
    now(): 8, sim. time elapsed: 1, wall clock elapsed:  0.199, sim/wall time ratio:  5.026
    now(): 9, sim. time elapsed: 1, wall clock elapsed:  0.199, sim/wall time ratio:  5.026
    now(): 10, sim. time elapsed: 1, wall clock elapsed:  0.199, sim/wall time ratio:  5.024
    At simulation time 10: ratio simulation/wallclock time now changed to 10
    now(): 11, sim. time elapsed: 1, wall clock elapsed:  0.099, sim/wall time ratio: 10.108
    now(): 12, sim. time elapsed: 1, wall clock elapsed:  0.099, sim/wall time ratio: 10.105
    now(): 13, sim. time elapsed: 1, wall clock elapsed:  0.099, sim/wall time ratio: 10.102
    now(): 14, sim. time elapsed: 1, wall clock elapsed:  0.099, sim/wall time ratio: 10.104
    now(): 15, sim. time elapsed: 1, wall clock elapsed:  0.099, sim/wall time ratio: 10.104

## Limitations

This module works much better under Windows than under Unix or Linux, i.e., it gives much closer synchronization. Unfortunately, the handling of time in Python is not platform-independent at all. Here is a quote from the documentation of the time module:

    "clock()
    On Unix, return the current processor time as a floating point number expressed in seconds.
    The precision, and in fact the very definition of the meaning of ``processor time'' , depends
    on that of the C function of the same name, but in any case, this is the function to use for
    benchmarking Python or timing algorithms.

    On Windows, this function returns wall-clock seconds elapsed since the first call to this
    function, as a floating point number, based on the Win32 function QueryPerformanceCounter().
    The resolution is typically better than one microsecond.
    "

## The SimulationRT API

### Structure

Basically, SimulationStep has the same API as Simulation, but with:

> - a change in the definition of simulate, and
> - an additional method to change execution speed during a simulation run.

### simulate

Executes the simulation model.

Call:

> simulate(<optional parameters>)

Mandatory parameters:
  None.
Optional parameters:
  - until=0 : the maximum simulation (end) time (positive floating point number; default: 0)
  - real_time=False : flag to switch real time synchronization on or off (boolean; default: False, meaning no synchronization)
  - rel_speed=1 : ratio simulation time over wallclock time; example: rel_speed=200 executes 200 units of simulation time in about one second (positive floating point number; default: 1, i.e. 1 sec of simulation time is executed in about 1 sec of wallclock time)
Return value:
  Simulation status at exit.

### rtset

Changes the ratio simulation time over wall clock time.

Call:

> rtset(<new ratio>)

Mandatory parameters:
  None
Optional parameters:
  - rel_speed=1 : ratio simulation time over wallclock time; example: rel_speed=200 executes 200 units of simulation time in about one second (positive floating point number; default: 1, i.e. 1 sec of simulation time is executed in about 1 sec of wallclock time)
Return value:
  None

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