Jobshop Simulation Model

• JobShopInput.txt
• JobShopOutput.txt

An EAS (entity attribute and set) description is in

• JobShopEAS.xls

The header files for the model are in:

• Accumulated_Tallied_Variables.h
• Events.h
• JobShop.h

The CPP files of the model are the following:

• Acc_Tal_Variables.cpp
• Initialize.cpp
• JobShopEntities.cpp
• Main.cpp
• Time_keeper.cpp

A zipped version of all these files can be downloaded from

• alljs.zip

General Description of Model

Each job in the JobShop model is of one or another “standard type”. Sales--that is, new orders--of jobs of a standard type arrive according to an exponential process whose mean time between arrivals is specified by the user. Each job of the type is processed through a set of steps referred to as the “routing” of the type. Each step of this routing is characterized by what machine group is required; how much process time is required; and what is the priority of the job at this step in the routing. At each step, the job needs one machine from the specified machine group for “process-time” length.

Input File

The first line of the Input file reads

length of simulation (in decimal days): 100.0

The program skips over the text up to the colon. The text is for documentation only. The program expects the LengthOfRun variable to appear after the colon. Similarly with the remainder of the input file, text is for documentation and colons mark the beginning of numbers which are expected in a fixed sequence.

Following the length of simulation, the Input routines read the number of machine groups; the number of machines in each machine group; the number of standard job types; and, for each standard job type, the mean time between sales, and the number of steps in its routing. Then, for each step in its routing, it reads the machine group required, the process time required, the priority of the job at that step, and the number of jobs initially in the shop at that step.

The input file is processed by the ReadInputData() routine in the Initialize.cpp file.

Simulation Output

The file JobShopOutputs.txt presents shop utilization and performance reports. This includes the average, standard deviation and maximum of queue size for each machine group; the average, standard deviation and maximum number of idle machines for each machine group; and the average, standard deviation and maximum time in shop for jobs of each standard type. These reports are produced by

TimeKeeper::EndSimulation().

While the simulation is running, the model writes the following information to the screen:

a copy of the input data;
the message “start simulation”;
messages: “day 10”, “day 20”, …; and
a message declaring end of simulation.

Main timing loop

In the routine Time_keeper::Simulate() in the file “Time_keeper.cpp, the section of code following the comment “//Main Timing loop” can be used as a pattern for any event driven simulation. The simulation contains one entity (object) of type (class) time_keeper, namely the global object, Timer. As noted in the JobShop EAS description, the time_keeper owns a set called Calendar. See the events.h file for how this is told to C++. Note there that time_keeper also has “methods” called Cause, Simulate and End _Simulation. The Cause statement in the time_keeper file does little more than file a coming event onto the Calendar according to its time of occurrence.

The main routine, which gets control first, opens the Input and Output files, invokes the ReadInputData routine and then Timer.Simulate(). Among other things, the ReadInputData causes--that is, files into the calendar of coming events--the first sale of each standard type, and end of process coming-event notices for each job in process when the simulation starts. The main timing loop removes the first coming event notice from the calendar, and then switches according to event type. Each case in the switch calls on the appropriate event routine. This switch can be modified to accommodate additional or other events in an event driven simulation.

Accumulated and Tallied Variables

The Jobshop simulator includes classes called Accumulated_variable and Tallied_variable. See the EAS description or the header file Accumulated_Tallied_Variables.h. The methods used by objects of these classes are located in the file Acc_Tal_Variables.cpp.

These classes and their methods may be used generally in simulation models. As an example of the use of these classes note, in the file JobShop.h, that class Machine_Group has an attribute, Free_machines, declared to be an accumulated_variable. To update the value of Free_machines one must use either the update or increment methods of accumulated_variable. For example, the following coding is found in the EndProcess::DispatchMachine() routine within the JobShopEntities.cpp file.

if (MGPtr Queue_size.Value()= = 0)
{

MGPtr Free_machine.Increment (1.0);
return;
}
else
{

MGPtr Queue.size.Increment (-1.0);

}

In addition to updating value, the Increment and Update methods update a time-weighted Sum and SumSq. They also note the minimum and maximum values which the variable has taken on. The values of the arguments to Update and Increment are double; those to Increment may be positive or negative. The initial value of an accululated_variable must be set by the Initialize method. This calls on the reset method which, among other things, stores the current value of SimTime as the StartTime attribute of the Accumulated_variable. Thus, while accumulated variables can be initialized before the start of the simulation, SimTime must be set already.

The current value or the accumulated sum, sumSq, lastUpdateTime, startTime, min and max can be accessed by public methods named Value(), Sum(),…, as in the Time_keeper::EndSimulation() routine in the Time_keeper.cpp file.

Tallied variables are similar to accumulated variables except they produce ordinary averages rather than time-weighted averages. See their class definition.

Jobshop Processing

With generic accumulate and update routines at our disposal, under the control of a typical timing routine, other than the initialize and report routines, not much needs to be programmed to simulate the Jobshop. Specifically, the JobShopEntities.cpp file contains the following short routines:

(a) Constructors which initialize machine_groups and jobs.

(b) The NewSale event routine. This causes the next occurrence of a NewSale
and calls upon the AdvanceJob() routine to process the new job.

(c) The endOfProcessing() event routine. For the job step just completed this
event routine disposes of the job being processed using the AdvanceJob() routine and the machine doing the processing, using the DispatchMachine() routine.

(d) The AdvanceJob() routine proceeds depending on whether the job is a
new job (place = = NULL) or an existing job. In the latter case it proceeds depending on whether the last step in the routing has or has not been completed.

(e) The DispatchMachine() routine proceeds according to whether or not ther there is a job waiting for it in queue.

See the routines themselves for details.