Lesson 5 - The GENERATE and TERMINATE Blocks: Transaction Entities

 

A GPSS Transaction is an object with a set of attributes. Each Transaction is distinct in that it can be identified by a unique Transaction Number. Transactions created in a simulation are numbered consecutively starting with 1. In addition, you can change the attributes associated with each Transaction. Attributes are in the form of priority, scheduled time, Assembly Set, and "Transaction Parameters", which are given user ASSIGNed names or numbers. Transaction Parameters are like the letters in the mail carrier’s bag that are addressed to one person. The Parameters carry information about the Transaction that is unique to that Transaction only. You can use Snapshot Windows to look at information that is carried in these Parameters. Transactions pass from Block to Block in a simulation, acting on, and being acted upon by other GPSS Entities.

Unlike general purpose programming languages, each Transaction in GPSS maintains a place marker. Whereas there is only one place in a computer program to be remembered, in a GPSS simulation there may be thousands, with thousands of Transactions located in many different Blocks. Transactions do not necessarily start at the top of the program. A GPSS simulation may have many GENERATE statements, each of which creates many Transactions, each of which has a place marker.

Transactions can be created in a batch, or at spaced out time intervals by a GENERATE Block. All simulations must have at least one GENERATE Block. Let’s consider the case where a new Transaction is created every 60 time units. The specific unit of time does not matter as long as we are consistent throughout. Let’s say that one time unit represents a second. Then, the appropriate form of the GENERATE statement for a Block which creates a new Transaction every minute is:

GENERATE 60 ;Create a new Transaction

Since GPSS World has a floating-point clock, we could also represent generation of Transactions at one minute intervals by

GENERATE 1 ;Create a new Transaction

Then if we needed to represent a one second interval it could be represented by 1/60 of a minute or 0.01666667. You should choose the representation that is most meaningful in your model. If you use an expression here, it must be parenthesized.

Start a GPSS World Session as we have done before.

CHOOSE     File / New

and since the model is already selected

SELECT     OK

and type the GENERATE statement from the previous page in the Model Window that appears. Then, translate your one line model. Start by pressing the Tab Key.

PRESS     [Tab]

TYPE     GENERATE

PRESS     [Tab]

TYPE     60

PRESS     [Tab]

TYPE    ;Create a new Transaction

We have only used tabs so the model will line up. It is not necessary for the parts of your GPSS model line to be in certain columns, but it does make it easier to read. You can change the tabstops by editing the Settings of the Model Object. We will show you how, later.

Now,

CHOOSE     File / Save As

and in the dialog box in the highlighted area

TYPE     MyModel

and

SELECT     SAVE

Then,

CHOOSE     Command /Create Simulation

and

CHOOSE     Window / Simulation Window / Blocks Window

You should see your GENERATE Block in the Blocks Window. Blocks are always numbered starting with 1. This is the "Details View" of the Blocks Window where the most important information about each Block is listed in columns.

Let's turn off the "Details View".

CHOOSE     View / Entity Details

This "toggles" the window into and out of "Details View". In this non-details view, succeeding Blocks are generally placed beneath their predecessors. When the next Block won’t fit beneath its predecessor, it is moved up to the top of the next column to the right. The arrangement of the Blocks is also due to the size of the window and will change if you resize the window.

Let's move back to off the "Details View".

CHOOSE     View / Entity Details

TERMINATE Blocks remove Transactions from the simulation. When a Transaction enters a TERMINATE Block, the Transaction is destroyed. Also, there is an optional operand that will decrement a special number called the "Termination Count". When you type START 123, you are setting the Termination Count to 123. This controls how long the simulation should run by itself. If you place a 1 in operand A of a TERMINATE Block, every time a Transaction is destroyed by that Block, the Termination Count is reduced by 1. When the Termination Count becomes 0 or less, the simulation ends and, unless suppressed by Operand B of START, a Standard Report is written automatically. The Termination Count can be less than zero when the simulation ends if Operand A of the TERMINATE Block is greater than 1.

Let’s add a TERMINATE Block to our model. Click on the Title Bar of the MyModel.gps Model Window. In the Model Window start a new line after the GENERATE Block line. With the cursor at the end of the GENERATE Block line

PRESS     [Enter]

then

PRESS     [Tab]

TYPE     TERMINATE

PRESS     [Tab]

TYPE     1

PRESS     [Tab]

TYPE     ;Destroy a Transaction

CHOOSE     Command / Retranslate

Now, click on the Title Bar of the Blocks Window. Another way to get to the Blocks Window is to could click on the appropriate item in the window list under the Windows menu of the Main Window.

 

Figure 5—1. The Blocks Window with a two Block Model

 

These two Blocks represent a tiny simulation. They could represent a situation where a customer arrives at a store every minute, precisely on the minute. Not very likely, but later you will see how to change the model to match your specific situation. For now, let’s use this model to study what happens in a simulation.

The clipboard operations of Cut, Copy and Paste are available as you build your model. Large segments of blocks can be selected and then replicated, moved, or deleted this way. Feel free to experiment with this powerful feature. Multiple Block selection and the clipboard operations are discussed in the GPSS World Reference Manual.

Before we begin the simulation, you must know about two lists of Transactions called the Current Events Chain, and the Future Events Chain. To understand these chains, you must keep in mind the "system clock", which tells what the current simulated time is. Each Transaction has an attribute that indicates when it must attempt its next Block entry. If the current simulated clock time has not yet reached that time, the Transaction must be put aside until the simulation is ready for it. That’s what the Future Events Chain is for. The Future Events Chain is a holding area for Transactions that are scheduled to do something in the future.

Every time a Transaction enters a GENERATE Block, a second Transaction is created. This new Transaction does not enter the GENERATE Block right away. Instead, it is placed on the Future Events Chain according to the arrival time interval you specified in the GENERATE Block.

The Current Events Chain is the set of Transactions that still have Blocks to be entered at the current system clock time. Transactions are taken from the front of the Current Events Chain, one at a time, to enter as many Blocks as possible. When there are no more Transactions on the Current Events Chain, GPSS World will advance the system clock.

Let’s take a look at the Current Events Chain and the Future Events Chain in our Current Model. First set up a Stop Condition that will cause the simulation to stop on each attempted Block entry.

Make sure the Blocks Window has the focus. You should now see two blocks

CLICK ON     The Generate Block Icon

In the debugging icons in the Debug Toolbar at the top of the Blocks Window, you will see a place icon in the form of a stop light.

CLICK ON     The Place Icon in the Debug Toolbar at the Top of the Window

This will place a STOP on the Generate Block. The Simulation will stop on an attempted entry into the GENERATE Block. The STOP command is discussed in Chapter 6 of the GPSS World Reference Manual. Before we even start, let’s see what’s on the Current Events Chain and the Future Events Chain.

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

CHOOSE     Window / Simulation Snapshot / CEC Snapshot

Nothing on the CEC, but there is one Transaction on the FEC! That’s because the GENERATE Block was "primed", thereby creating its first Transaction when we issued the STOP Command. Once we issue a START Command, Transaction 1 will be moved to the Current Events Chain and the time will be set to 60.

Close each of the Snapshot Windows.

CLICK ON     The X-Upper Right of Each Window

These windows are not like the windows that you select under Window / Simulation Window. Snapshots are static views of the current condition and will not change as the simulation runs. They can be refreshed by halting the simulation and opening the window when you are ready for current information. Although you don't really have to halt the simulation, doing so keeps the information in the Snapshot Window consistent with the current state of the simulation.

Make sure you can see the entire Journal Window. Click on its Title Bar if you need to.

CHOOSE     Command / START

and in the dialog box, replace the 1

TYPE     100

SELECT     OK

This sets the Termination Count to 100, moves Transaction 1 to the Current Events Chain, and begins the simulation. But remember that we have set up a Stop Condition. That is why the simulation stops before the first Block entry, and writes a 2 line message in the Journal Window. We could also put a Stop Condition on any Block using the Stops menu in the Blocks Window. As we shall see shortly, current Stop Conditions can be viewed in a special Simulation Snapshot window.

Let’s make Transaction 1 enter the GENERATE Block. We’ll use the STEP 1 Command, which has been preloaded into function key p. It will be available until you change it.

PRESS     [F5]

Now, look at the Current Events Chain and the Future Events Chain using the same mouse clicks that we did a minute ago in the Window / Simulation Snapshot Menu.

Your windows should look like this.

 

Figure 5—2. CEC Snapshot-Unexpanded View.

 

Figure 5—3. FEC Snapshot

.

Now, look at the FEC Snapshot so you can see all the information about the Transaction waiting to enter the simulation. On the FEC the Time is the "Block Departure Time" (BDT) which is the system clock time when Transactions on the FEC are scheduled to come back into the simulation. The Next Block column shows the number of the Block to be entered next by each Transaction. Block number 1 is the GENERATE Block and Block number 2 is the TERMINATE Block. Transaction 2 is waiting to enter the GENERATE Block.

Now let’s look at the Expanded View of the CEC. With the focus on the CEC Snapshot Window

CLICK ON     The + in the CEC Window

 

Figure 5—4. CEC Snapshot. Expanded View.

Transaction 1 has entered the GENERATE Block, thereby creating Transaction 2, which goes onto the Future Events Chain. Transaction 1 is on the Current Events Chain, with M1, its time of entry into the simulation at 60 seconds, and Transaction 2 is on the Future Events Chain, ready to enter the GENERATE Block when the simulated clock time becomes 120 seconds. The GENERATE Block’s job is to bring in a new Transaction every 60 simulated seconds. You will see this information in the various views of these windows. Later we will see that Transaction Parameters can be easily accessed in these windows.

 

Close the Snapshot Windows as we did a few minutes ago.

Let’s see what time it is right now. Activate the Journal Window as we did above

CHOOSE     Command / SHOW

In the dialog box:

TYPE     AC1

CHOOSE     OK

AC1 is the SNA that returns the value of the absolute system clock. Transaction number 1 will move as far as it can in the Current Model at time 60. When it can move no further, it is taken off the Current Events Chain.

Let’s load the SHOW command into [F8].

CHOOSE     Edit / Settings

Then, page forward in the Model Settings Notebook to the Function Keys Page.

In the F8 box

TYPE     SHOW AC1

SELECT     OK

Transaction 1 is about to be destroyed by entering the TERMINATE Block. Let’s let it do so.

PRESS     [F5] 

The Transaction will move one Block in response to a STEP 1 command that has been caused by a single keystroke. Transaction 1 has been destroyed. The next Transaction to move in the simulation is Transaction 2, which has just been taken off the FEC.

Transaction 1 is gone. Since that depleted the Current Events Chain, there was nothing more to be done at time 60. GPSS World then advanced the clock to the next scheduled time of 120. Transaction 2 was then taken off the Future Events Chain and placed on the Current Events Chain. The Future Events Chain is now empty. However, Transaction 2 is about to enter the GENERATE Block. This will create Transaction 3 and schedule it for 60 time units into the future. Then Transaction 2 will go on its way. Let’s allow Transaction 2 to enter the GENERATE Block.

PRESS     [F5] 

Now, look at the chains.

CHOOSE     Window / Simulation Snapshot / CEC Snapshot

and with the CEC Snapshot Window having the focus

CLICK ON     The + Sign

then in the Main Window

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

Size the windows and move them one above the other. Then elongate them so you can see all the information. See? Transaction 3 is now on the Future Events Chain at time 180, and Transaction 2 is ready to enter the TERMINATE Block. When it does, it will be destroyed, and the simulation will have nothing more to do at time 120.

Oh yes! Remember that Transaction 1 already entered the TERMINATE Block. That means that the Termination Count was decremented by 1. Let’s check TG1, the SNA that returns the remaining Termination Count. In the Model Window menu

CHOOSE     Command / SHOW

then

TYPE     TG1

in the dialog box.

SELECT     OK

The value is written to the Journal Window and also to the Status Line at the bottom of the Main Window.

Of course! We set up the TERMINATE Block to subtract 1 from the Termination Count every time it is entered by a Transaction. Since it has been entered once, the Termination Count is now 99. If we would remove the Stop Condition, and let the simulation run, it would stop automatically when the Termination Count reached 0.

Now you’re on your own for a minute. Your assignment is to load other commands into the function keys and to become totally familiar with the creation and movement of Transactions to the Current Events Chain. Please take some time now to study these things. Use the Snapshot Windows to study these changes. When you’re done, please close up any Snapshot Windows that are open.

Welcome back. I hope you feel good about the Current Events Chain and the Future Events Chain.

Now, let’s open an Expression Window on the Clock, the Termination Count, and the Active Transaction number.

CHOOSE      Window / Simulation Window / Expression Window

in the dialog box, for the Label

TYPE     Clock

and for the Expression

TYPE     AC1

CLICK ON     View

CLICK ON     Memorize

View allows you to see the expression in the current Expression Window, and Memorize will allow you to save the information with the simulation so you will not have to enter the information the next time you want to display it in a Plot or Expression Window. You can also enter expressions that you wish to save with a model by using Edit / Settings and Clicking on the Expressions tab. You must save the model in order for expressions that are entered here to be saved These expressions will be memorized and will be available to be viewed the next time you create a simulation from this model.

Now, add the next expression. Type in the next Label and Expression in the dialog box deleting the previous entries.

TYPE     Act. Xact

and

TYPE     XN1

CLICK ON     View

CLICK ON     Memorize

and repeat the procedure for the third expression. In the dialog box for the Label and Expression,

TYPE     Term Ct.

and

TYPE     TG1

CLICK ON     View

CLICK ON     Memorize

Close the Edit Expression Window Dialog

SELECT     OK

You should now see the new Expression Window.

Now let’s speed things up a bit. Let's remove all Stop Conditions.

CHOOSE     Window / Simulation Snapshot / User Stops

CLICK ON     Remove All

SELECT     OK

Let’s look at the Blocks Window again.

CLICK ON     Anyplace on the Blocks Window

Now,

PRESS     [F5] 

several times to observe the effects of the movement of Transactions on the system clock and the Termination Count. Notice, in the Blocks Window, that each Block flashes after it has been entered by a Transaction, not before.

When you are ready, use either the Continue icon in the Debug Toolbar at the top of the Blocks Window or the loaded Function Key to continue the run.

CLICK ON     The Continue Icon in the Debug Toolbar at the Top of the Window

or

PRESS     [F2] 

The simulation will end soon and write a report. You can examine the report later, if you like.

We have covered a lot of ground. Let’s review. To schedule a Transaction to come into the simulation at some future time, GPSS World places the Transaction on a chain of Transactions called the Future Events Chain, or FEC. The FEC is ordered by increasing values of scheduled time. In other words, a Transaction scheduled at time 200 will precede a Transaction scheduled at time 300. When there is nothing more to do at any given simulated clock time, GPSS World takes Transaction(s) off the Future Events Chain that have the lowest scheduled time value. It is precisely this action that causes the system clock to advance. Since the system clock jumps over time durations where no events occur, it is often possible for a discrete event simulation to run much faster than the real process being simulated.

The Current Events Chain, or CEC, is used as a holding area for Transactions that still have things to do at the current simulated clock time. When there are no more Transactions on the CEC, it means that there is nothing more to do at the current simulated time instant. GPSS World then moves all Transactions scheduled at the lowest time on the FEC to the CEC, and advances the system clock to the new value.

Please note that when Transactions scheduled to reenter the simulation at the same time are moved from the FEC to the CEC, their order is randomized. This prevents certain unintended synchronization sequences from developing. You can prevent this randomization by setting time tie randomization to zero in the Random Page of the Model Settings Notebook. If you wish to view this setting

CHOOSE     Edit / Settings

to move to the Random Page.

An important rule to remember is that the simulated clock time advances every time a Transaction or Transactions must be taken from the Future Events Chain. As you can see, time in a digital computer simulation does not flow smoothly. A simulation usually consists of many Block entries at a given clock time, followed by a sudden jump to the next clock time. As far as the interpretation of the simulation goes, all GPSS Block entries occurring at a given clock time are occurring instantaneously! The quick jumps to each new time instant often allow simulations to run much faster than the real time process being modeled. A simulated time duration is always associated with new Transactions being taken from the FEC.

When you use the INTEGRATE Command to automatically maintain a continuous variable, a mathematical integration algorithm is employed between discrete time instants to update the continuous variables. Such mixed simulations proceed in alternating discrete and continuous phases. But more on this later, in the lesson on continuous modeling.

That’s all for now. Go ahead and end the GPSS World Session.

CLICK ON     The X-Upper Right of the Main Window

and answer the questions asked in the dialog boxes. We have added a TERMINATE Block since we saved the model. If you want to use the model in the next lesson, answer "No" no to the "Do you want to save changes ..." question for the Simulation Object (.sim) and the Report Object (.gpr), and "Yes" to the save question for the Model Object (.gps).

In the simple simulation we studied in this lesson, the Transactions that represented the customers did not do anything! They entered the store and disappeared in the same instant. To be realistic, we must have the customers spend time before departing. We obviously must have a way to cause a time delay in the simulation. This is discussed in the next lesson. See you then.

 

Lesson 6 - The ADVANCE Block

Before starting this lesson, let’s review how a simulation works. Remember that the Future Events Chain, the FEC, is a chain of Transactions in increasing order of scheduled times. When all the Transactions scheduled for the current time have done all they can do, the most imminent Transaction(s) are taken from the FEC, and the system clock is advanced. Such Transactions are kept in a first come first served (within priority) holding area called the Current Events Chain, or CEC.

The highest priority Transaction on the CEC is the Active Transaction. A simulation consists of the Active Transaction entering first one GPSS Block, then the next, until it can enter no more. When the Active Transaction can no longer move, it is removed from the CEC. Then another Transaction becomes the Active Transaction. If the CEC is empty, the next Active Transaction must come from the FEC, by moving one or more Transactions from the FEC, to the CEC. Remember, it is precisely the act of taking the most imminent Transaction(s) from the FEC that makes the system clock advance.

In the last lesson we learned how GPSS World simulates time durations by placing Transactions on the FEC, and moving them to the CEC at the end of the simulated duration. This was controlled by a GENERATE Block. In this lesson, you will learn how to use the ADVANCE Block to cause existing Transactions to experience simulated delays.

Let’s take another look at the simulation we used in the last lesson. It simulated the arrival of customers at a store. Recall that in the simulation, we represent the situation where customers arrive precisely 60 seconds apart.

Let’s begin. Start a GPSS World Session. See Lesson 1 if you are unsure of how to do this.

If you have not deleted MYMODEL.GPS from Lesson 5, open it now otherwise,

CHOOSE     File/ New

and since the model type is already selected

SELECT     OK

TYPE     GENERATE 60 ;Create a new Transaction

and on the next line

TYPE     TERMINATE 1 ;Destroy a Transaction

Save the model using Save As like we did in Lesson 5.

Create the simulation.

CHOOSE     Command / Create Simulation

Now, open the Blocks Window.

CHOOSE     Window / Simulation Window / Blocks Window

Now, set up a Stop Condition that will cause the simulation to stop on each attempted entry into the TERMINATE Block.

CLICK ON     The TERMINATE Block Icon

in the Blocks Window, then

CLICK ON     The Place Icon in the Debug Toolbar at the Top of the Window

to place a Stop condition on the TERMINATE Block and then return to the Journal / Simulation Window

CLICK ON     Anyplace on the Journal / Simulation Window.

then

CHOOSE     Command / START

and in the dialog box replace the 1,

TYPE     1000

SELECT     OK

This sets the Termination Count to 1000, and begins the simulation. But remember that we have set up a Stop Condition. That is why the simulation stops on the first attempt to enter the TERMINATE Block. Now, look at the Current Events Chain and the Future Events Chain.

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

and

CHOOSE     Window / Simulation Snapshot / CEC Snapshot

CLICK ON      The Plus Sign in the CEC Snapshot Window

Transaction 1 is on the Current Events Chain, and Transaction 2 is on the Future Events Chain, ready to enter the GENERATE Block when the simulated clock time becomes 120 seconds. Transaction number 1 does all its Block entries at time 60, and then the clock is advanced. Transaction 1 is about to be destroyed by entering the TERMINATE Block. Before we continue, close the FEC and CEC Snapshots.

CLICK ON      The X-Upper Right of the Main Window

Now, let’s continue. Make sure that the focus is on the Blocks Window.

PRESS     [F5]

to step one Block entry.

Let’s look at the simulated clock time.

CHOOSE      Command / SHOW

and in the dialog box

TYPE      AC1

The current time is 120.000, as we can see in the Status Line of the Main Window. This is the simulation entry time for Transaction 2. Let’s load the SHOW command into [F8].

CHOOSE      Edit / Settings

In the notebook go to the Function Keys page and to the right of F8

TYPE      SHOW AC1

SELECT      OK

Transaction 1 is gone. Since that depleted the Current Events Chain, the simulation jumped to the next scheduled time at 120 seconds. Transaction 2 was taken off the Future Events Chain and placed on the Current Events Chain, thereby advancing the system clock to 120.

Transaction 1 did not spend any time in the simulation. All its Block entries occurred at a single simulated instant, namely when the system clock was at 60 seconds. In the case of a simulation of customer arrivals, we would have to interpret this as a succession of customers arriving, but spending no time in the store. We must get each Transaction back onto the Future Events Chain to simulate the passage of time while the Transaction is still in the simulation. Then the Transaction would be taken off the Future Events Chain after the system clock had advanced further. The GPSS ADVANCE Block is what we need! Operand A of the ADVANCE Block is the time increment at which the Transaction should be scheduled to come off the Future Events Chain. Let’s set this up for Transaction 2 in our simulation. First, insert the ADVANCE Block after the GENERATE Block. In the Model Window, position the cursor at the end of the GENERATE Block line and

PRESS     [Enter]

or

CHOOSE      Edit / Insert Line

then

TYPE      ADVANCE 61 ;Cust browses 61 sec.

Now that you have added a new Block, you will have to retranslate the model. Make sure that the focus is on the Journal Window to keep track of messages as we Step through the simulation.

CLICK ON      Anyplace on the Journal / Simulation Window

and

CHOOSE      Command / Retranslate

In the simulation in the last lesson, Transaction 2 left the simulation at time 120, before Transaction 3 arrived (at 180). Now, however, Transaction 2 will stay in the simulation 61 time units. The ADVANCE Block will cause this to happen. This means that Transaction 3 will arrive before Transaction 2 leaves. Let's see if this is true. First let Transaction 2 enter the GENERATE Block. This will create, and schedule Transaction 3 to come in at time 180.

In the Blocks Window we had put a Stop Condition on the TERMINATE Block before we Retranslated. Stops work by block number and survive a Retranslation. The TERMINATE was Block number 2. Now, the ADVANCE Block is block 2 and the Stop will be on it. Start the simulation.

CHOOSE      Command / START

In the dialog box replace the 1,

TYPE      1000

SELECT      OK

The simulation will stop when Transaction 1 tries to enter the Advance Block.

PRESS     [F5]

to make the Transaction enter the ADVANCE Block.

Look at the CEC and the FEC as we did before by opening the appropriate Snapshot Windows. If you don’t remember how to do this, look back to the beginning of this chapter.

Do you see Transaction 1 on the FEC at time 121 as well as Transaction 2 on the CEC scheduled to move at time 120? Your windows should look like this.

 

Figure 6—1 FEC Snapshot.

 

Figure 6—2. CEC Snapshot-Expanded View.

 

PRESS     [F8]

and you will see (in the Status line of the Main Window) that the clock has been updated to time 120. Step twice more.

PRESS      [F5]

two times. Now, check the time again by pressing:

PRESS     [F8]

Notice that since Transaction 2 was the last Transaction on the Current Events Chain, the system clock was advanced immediately when Transaction 1 was taken from the FEC at time 121. The time is now 121. Once again, you should look at the FEC and CEC to understand which Transaction is on which chain. Notice that Transaction 2 is on the FEC, and will not be active until time 181. Let’s continue. Step two more times.

PRESS      [F5]

two times and then look at the location of the Transactions on the chains. When you stepped, you should have caused Transaction 1 to leave the ADVANCE Block and be TERMINATEd. Then, Transaction 3, which has been scheduled to enter the simulation at time 180, will enter the GENERATE Block. Show the current time.

PRESS      [F8]

Transaction 3 has entered the GENERATE Block creating Transaction 4. Transaction 3 will take its turn entering the ADVANCE Block next.

Please take time now to understand what is happening. Transaction 3 is about to be placed on the FEC behind Transaction 2 and 4, at time 241 (180+61). The current time is 180, and Transaction 2 will become active at time 181. For the simulated second between time 180 and 181 both Transactions will be on the FEC using up part of their 61 second browsing intervals. This is the first second of Transaction 3’s interval and the last second of Transaction 2’s interval. For this one simulated second, both customers are browsing in the store.

PRESS      [F5]

and

PRESS      [F5]

Now, notice that Transaction 2 has become active, and that the time is now 181. Transaction 2 is about to leave the simulation at this time since it will enter the TERMINATE Block next. It has just left the ADVANCE Block. Look at snapshots of the CEC and FEC as we did before to establish which Transactions are on which chain. When you have studied the SNAPSHOTS and feel comfortable with the location of the Transactions and their movement times,

PRESS      [F5]

PRESS      [F8]

Transaction 2 has left the simulation. It arrived at time 120, and left at time 181. That means we have simulated its browsing time to be 61 seconds. That’s exactly what we wanted the ADVANCE Block to do for us.

If we interpret Transaction 2 to represent "customer number 2", we conclude that the customer browsed in the store for 61 seconds, leaving 1 second after the arrival of customer number 3.

GPSS is much more than a programming language. Unlike programming languages, GPSS has many points of control. When you interrupt a program written in C or BASIC you need only know which statement is next. GPSS is different. There’s usually more than one Transaction in the simulation at any instant, and each one can be ready to enter a different GPSS Block! So, we see that unlike a programming language, which always knows which one statement is to be executed next, GPSS has a place marker for each Transaction. This allows you to have each customer in a simulation doing or waiting for a different thing. The use of multiple place markers is one of the reasons that GPSS is a more powerful simulation language than are programming languages like BASIC or FORTRAN or C. Each Transaction in GPSS knows which Block it is to enter next, and there may be thousands of Transactions.

Once again, we now leave you on your own. Your assignment is to continue to use the STEP, and SHOW commands and the Chain Snapshot Windows until you are quite comfortable with the action of the ADVANCE Block to represent a simulated time delay. Use pencil and paper to calculate when future events should be scheduled. Feel free to experiment. Try modifying the ADVANCE Block, but don’t forget you must retranslate after you make changes. You can make the simulation more realistic by using operand B of ADVANCE to randomize the delay times. If you want to venture this far, you should read about the ADVANCE, GENERATE, and TERMINATE Blocks in Chapter 7 of the GPSS World Reference Manual. Return to the tutorial when you are ready.

Welcome back. Did you see that the ADVANCE Block simply places a Transaction back on the FEC to simulate a time delay? It causes the delayed Transaction to be processed at precisely the right simulated time.

Remember that the simulated clock time advances every time a Transaction must be taken from the Future Events Chain. A simulation usually consists of many Block entries at a given clock time, followed by a sudden jump to the next clock time. As far as the interpretation of the simulation goes, all GPSS Block entries occurring at a given clock time are occurring instantaneously!

If you want to experiment some more with the ADVANCE Block, please do so.

For now,

CLICK ON     The X-Upper Right of the Main Window

You decide if you want to save the Simulation or the Model with the ADVANCE block for further experimentation. This will end the GPSS World Session.

Well, now we can simulate customers browsing in the store. But what about when customers want to engage a salesman? A salesman will certainly only talk to one customer at a time. If we do nothing other than add an ADVANCE Block, we will simulate the case where more than one customer has engaged the salesman at the same time. Remember how both Transaction 2 and Transaction 3 were using up their 61 second delays during the same simulated second (180-181) ?

If a customer needs a full 61 seconds of the salesman’s time, we must not start counting until the customer begins talking to the salesman. In other words, we cannot allow a Transaction to enter the ADVANCE Block until it has engaged the salesman. And we must stop any Transaction that tries to engage a busy salesman.

As you might expect, GPSS provides Blocks (SEIZE and RELEASE) to do this. We’ll discuss them in the next lesson. See you then.

 

Lesson 7 - The SEIZE and RELEASE Blocks: Facility Entities

Before starting this lesson, let’s review how a simulation works. Remember that the Future Events Chain, the FEC, is a chain of Transactions in increasing order of scheduled times. When all the Transactions scheduled for the current time have done all they can do, the most imminent Transaction is taken from the FEC, and the system clock is advanced. All transactions scheduled for the new time are removed. Such Transactions are kept in a first come first served (within priority) holding area called the Current Events Chain, or CEC. The top Transaction on the CEC is called the "Active Transaction". A simulation consists of a series of Active Transactions, each entering as many GPSS Blocks as possible. When one Active Transaction has nothing more to do, another Transaction is chosen to be the Active Transaction. Block entries at a single clock time occur instantaneously, with no simulated time duration.

Good. Now that that’s out of the way, let’s recall what happened in the last lesson. We simulated customers browsing in the store, but we did not cause them each to fully engage a salesman, one at a time. Let us assume that each customer must spend 61 seconds talking to a single salesman. To simulate this, we cannot let a Transaction enter an ADVANCE Block immediately, because that would start counting off the 61 second interval, even if the salesman were talking to another customer. If the salesman is already talking to a customer, we want any succeeding Transactions to be prevented from entering the ADVANCE Block. To do this, we can use a GPSS Entity called a Facility, and the associated Blocks, SEIZE and RELEASE. The Facilities Window is available for viewing Facility Entities.

A GPSS Facility Entity has several attributes, the most important of which is ownership. When a Facility is owned by a Transaction, it is said to be busy. Otherwise, it is said to be idle. Only one Transaction can own a Facility, and ownership is acquired by entering a SEIZE Block which names that Facility in its A operand. Transactions attempting to acquire ownership of a Facility by entering a SEIZE Block are denied entry into the SEIZE Block until the Facility is idle. This prevents the movement of Transactions that are waiting to gain ownership of the Facility. The would-be SEIZEing Transaction stops moving in the simulation until its turn for ownership comes up. Such Transactions are removed from the CEC, and placed on the Facility’s "Delay Chain" until they can assume ownership of the Facility.

An owning Transaction continues to move in the simulation, and owns the Facility until it enters a RELEASE Block naming that Facility in its A operand. As you can see, the SEIZE and RELEASE Blocks are exactly what we need to keep our "customer" Transactions from entering the ADVANCE Block before they have engaged the salesman. Only when a Transaction comes off the FEC and enters a RELEASE Block, does the next simulated customer get to talk to the salesman.

Let’s try out these new Blocks.

Start a GPSS World Session as we have before. See Lesson 1 if you are unsure of how to do this,

CHOOSE     File / Open

Since you know how to enter statements in building a model, we have provided this one for you. All you have to do is select Seizemod from the model list. You will see that the model has the following Blocks.

 

        GENERATE 20 ;Customer arrives

and

        SEIZE SALESMAN ;Customer engages salesman

and

        ADVANCE 41 ;Customer talks to salesman

and

        RELEASE SALESMAN ;Customer gives up salesman

and

        TERMINATE 1 ;Customer leaves

 

Create a simulation.

CHOOSE      Command / Create Simulation

Now, let's put a STOP on the GENERATE Block

CHOOSE       Window / Simulation Window / Blocks Window

Next,

CLICK ON       The GENERATE Block Icon

CLICK ON       The Place Icon in the Debug Toolbar at the Top of the Window

This will set up a Stop Condition that will cause the simulation to stop on each attempted entry into the GENERATE Block.

CHOOSE       Command / START

in the dialog box, replace the 1

TYPE       1000

and

SELECT       OK

or

PRESS     [Enter]

The Function key [F5] is also set to cause you to be able to step through the simulation with a single keystroke. Function key  [F8]  has been set to show us the clock time. Remember, you can set any of the function keys to SHOW expressions or System Numerical Attributes (SNAs).

Let’s move the first Transaction one step into the GENERATE Block.

PRESS     [F5] 

Look at the Journal Window. The message will tell you that Transaction 1 is about to enter the SEIZE Block. It will be able to do so because the salesman is idle. Let’s also view the Current and Future Events Chains.

CHOOSE     Window / Simulation Snapshot / CEC Snapshot

and in the CEC Snapshot Window,

CLICK ON     The + Sign

for a closer look at the individual Transaction on the chain.

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

After you have viewed the Events Chains, close them. If you want to print them first, select File / Print.

Transaction 1 is on the CEC ready to enter the SEIZE Block. Transaction 2 is on the FEC scheduled to enter the simulation at time 40.

Give the Blocks Window the focus.

CLICK ON     Any place on the Blocks Window

Now, let’s allow Transaction 1 to gain ownership of a Facility Entity used to represent the salesman.

PRESS      [F5] 

Transaction 1 is still the Active Transaction, i.e., first on the CEC. It is now the owner of the Facility that represents the salesman. Let’s check by opening the Facilities Window.

CHOOSE     Window / Simulation Window / Facilities Window

This is a representation of the Facility Entity that represents the salesman. Don’t worry about the details of this window, as we will study it later. For now, find the "Owner XN" heading and notice that Transaction 1 is the owner. Transaction 1 is ready to begin the 41 second interval during which the customer will be talking to the salesman, by entering the ADVANCE Block.

PRESS      [F8] 

and

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

PRESS      [F8] 

The ADVANCE Block has placed Transaction 1 on the FEC. The customer it represents will disengage the salesman at time 61. Notice that the current time is 40. Actually, we can read it off the Title Bar of the Snapshot window as well as in the Status Line.

Before we go any further, let’s look at the Blocks Window.

CLICK ON     Any place on the Blocks Window

to make it the Active Window.

Now, let’s move the current Transaction, number 2 one more Block.

PRESS      [F5] 

Transaction 2 is about to attempt to SEIZE the salesman at time 40. It will not be able to do so. Transaction 1 owns the salesman.

PRESS      [F5] 

and

PRESS      [F8] 

and

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

and

CHOOSE     Window / Simulation Snapshot / CEC Snapshot

What happened to Transaction 2? Where is it? Transaction 2 attempted to enter the SEIZE Block but was denied entry and placed on the Delay Chain of the salesman Facility. By refusing to let Transaction 2 enter the ADVANCE Block, we have not yet started on the 41 seconds that Transaction 2 must pass while owning the salesman Facility. Let’s look at the Facilities Window.

CLICK ON     Any place on the Facilities Window

 

Figure 7—1. Facilities Window. Detailed View.

 

You will see that there is one Transaction on the Delay Chain for the Facility.

CHOOSE     View / Entity Details

We are now looking at the non-detailed view of the salesman Facility. See the small column representing a queue containing one Transaction? Transaction 2 is on the Delay Chain of the Facility Entity that represents the salesman.

 

Figure 7—2. Facilities Window. Non-detailed View.

 

Let’s continue.

PRESS      [F5] 

PRESS      [F8] 

The removal of Transaction 2 from the CEC depleted it. Since Transaction 3 was next on the FEC, the system clock has been advanced to 60. Transaction 1 will be finished talking to the salesman at time 61, and will be ready to disengage by entering the RELEASE Block. Make the Blocks Window the active window

CLICK ON     Any place on the Blocks Window

PRESS      [F5] 

What happened? Where did Transaction 2 come from? Transaction 1 gave up ownership of the salesman Facility when it entered the RELEASE Block. This made the Facility idle. Since Transaction 2 was next on the Delay Chain of the salesman Facility, it was automatically given ownership and allowed to enter the SEIZE Block. Transaction 3 is now on the Delay Chain waiting its turn with the salesman. Now make the Facility Window active

CLICK ON     Any place on the Facilities Window

and return to the detailed view.

CHOOSE     View / Entity Details

 

Figure 7—3. Facilities Window. Detailed View.

 

See? Transaction 2 owns the Facility, and there is 1 Transaction waiting to own it.

Just a quick note here: You can minimize any of these windows when you don’t want them by clicking on the minimize button in the upper right corner of the window. Then you can quickly reopen the window by clicking on the window's icon in the lower left of the Main Window. However, always be aware that when on-line windows are minimized or open, they will slow the running of your simulation. An alternative is to use the Windows / Tile menu command to organize all your windows.

Transaction 1 has entered the RELEASE Block, thereby allowing Transaction 2 to enter the SEIZE Block. Transaction 2 will enter the ADVANCE Block when it gets its turn as the Active Transaction. Transaction 2 immediately takes possession of the Facility even though it is not the active Transaction. It does so by entering the SEIZE Block. This prevents a Transaction that might be transferred from elsewhere in the simulation to the SEIZE Block from jumping ahead of the next rightful owner, Transaction 2.

Transaction 2 is ready to enter the ADVANCE Block, now that it has engaged the salesman. But first, Transaction 1 will leave the simulation by entering the TERMINATE Block.

PRESS      [F5] 

and

CHOOSE     Window / Simulation Snapshot / FEC Snapshot

 

Figure 7—4. FEC Snapshot.

and

CHOOSE     Window / Simulation Snapshot / CEC Snapshot

CLICK ON     The + Sign

 

Figure 7—5. CEC Snapshot, Expanded View.

to view the Expanded View of the CEC and

PRESS      [F8] 

That gets rid of Transaction 1. Notice that the time is 61 and that Transaction 2 still has not started its 41 second interval. Had we not inserted the SEIZE Block, Transaction 2 would not have been delayed before entering the ADVANCE Block, and both it, and Transaction 1, would have been on the FEC. Both Transactions would have been using up their 41 second interval at the same time. In other words, we would have had two customers talking to the salesman at the same time. The use of the SEIZE Block prevented this.

As before, we again leave you on your own. Your assignment is to continue to use the STEP, and SHOW commands and the various windows until you are quite comfortable with the action of the SEIZE Block to serialize (make one-at-a-time) the movement of Transactions into the ADVANCE Block.

Notice that when a Transaction is on a Delay Chain it cannot be on the FEC or the CEC. That makes sense. The Transaction cannot be on the FEC because it wants to gain ownership of the Facility as soon as possible. There’s no point in putting the Transaction back on the CEC until it can enter the SEIZE Block. All these considerations are handled automatically by GPSS World.

Go ahead and switch to the Blocks Window and the Facilities Window to view the simulation from different viewpoints. Verify that the SEIZE Block allows no more than one Transaction at a time in the ADVANCE Block.

When you feel comfortable with the use of Facility entities to force Transactions to proceed one at a time, go ahead and END the Session.

CLICK ON     The X-Upper Right of the Main Window

And answer "No" to all the "Save"queries.

If you got this far fairly, you are doing just fine. If you are having difficulty with the Future Events Chain and the Current Events Chain, you should go back to Lesson 5. If you are not at ease with how a Transaction must wait on a Facility Delay Chain until it gains ownership, please begin Lesson 7 again.

In the next lesson we will look at how easy it is to get dynamic histograms out of GPSS World simulations.

 

Lesson 8 - QUEUE and DEPART Blocks: Queue and Qtable Entities

Unlike the GPSS Blocks studied so far, the QUEUE and DEPART Blocks do nothing other than gather statistics. The statistics are kept in a GPSS Entity called a Queue Entity. Just as with a Facility Entity, a Queue Entity is created automatically when needed. You do not have to define a Queue Entity explicitly.

The most important attribute of a Queue Entity is a number called the "Queue Content". When a Transaction enters a QUEUE Block, the Queue Content of the associated Queue Entity is increased. When a Transaction enters a DEPART Block the Queue Content of the Queue Entity is reduced. The advantage of using Queue Entities is that GPSS World will keep statistics automatically for Queue entities. These statistics are written in the Standard Report, and are available during the simulation as several System Numeric Attributes. In addition you can define Qtable entities, which are visible as histograms in a Table Window. Also, you can open a QUEUE Window on the current Queue entities in your simulation and see all the statistics displayed dynamically.

Start a GPSS World Session reading in the SAMPQUE.GPS Program File.

CLICK ON     The GPSS World Icon in the GPSS World Folder

CHOOSE     File / Open

in the dialog box as shown on the next page

SELECT     Sampque

 

Figure 8—1. The File Open Dialog.

and

SELECT     Open

 

; GPSS World Sample File - SAMPQUE.GPS
**********************************************************************
*
* Barber Shop Simulation
* Time Is In Minutes 
**********************************************************************
        GENERATE  (Exponential(1,0,6.5)) ;Create next customer.
        QUEUE     Barber ;Begin queue time.
        QUEUE     Total_time ;Total haircut time
        TRANSFER  Both,Barb1,Barb2 ;Choose not busy barber.
Barb1   SEIZE     Barber1 ;Get service by Barber1.
        DEPART    Barber ;End queue time.
        ADVANCE   10,2.5 ;Haircut takes a few minutes.
        DEPART    Total_time ;Leave total time queue
        RELEASE   Barber1 ;Give up the barber.
        TRANSFER  ,Next ;Used Barber1-Done with cut.
Barb2   SEIZE     Barber2 ;Get service by Barber2
        DEPART    Barber ;End queue time.
        ADVANCE   13,4 ;Haircut takes a few minutes.
        DEPART    Total_time ;Leave total time queue
        RELEASE   Barber2 ;Give up the barber.
Next    SAVEVALUE Ave_Queue,QT$Barber ;Save resulting average.
        TERMINATE 1 ;Customer leaves.

Recall that this is a simple simulation of a barber shop with one barber. Notice how the QUEUE Barber and DEPART Barber Blocks are used. They define the duration for the automatic queuing statistics associated with the Queue Entity named Barber. In this case, we use QUEUE and DEPART Blocks to "sandwich" the SEIZE Block. This will automatically register a queuing time for each Transaction to enter the DEPART Block. Transactions that are able to own the Facility Entity named Barber immediately, will register a 0 waiting time. The average queue length and the average queuing time will be printed automatically in the GPSS World Standard Report.

We have also placed QUEUE and DEPART Blocks to encompass the entire haircut time, including waiting. We will have statistics for waiting time and for total time to get a haircut, including the wait.

Let’s look at the queue statistics as the simulation runs. First we’ll have to create the simulation.

CHOOSE     Command / Create Simulation

Then open the Queue Window to view the Queue Entity statistics as the simulation runs.

CHOOSE     Window / Simulation Window / Queues Window

Arrange the windows so you can see both Model and Queues Windows. For the present, it is not necessary to see the text in the Model Window. The Queues will start to be displayed once the simulation starts to run and the Queues are entered.

CHOOSE     Command / START

and in the dialog box , replace the 1,

TYPE     11111

and

SELECT     OK

 

Figure 8—2. The Queues Window. Detailed View.

 

Remember that you can view this window in two ways. The detailed view comes up first. It will give you all the information that you would find in the report as well as an icon that show that state of the queue. The non-detailed view will just give you an overview of the size of the queues in your simulation and is very good for a quick scan of these entities. First, let’s look at the various statistics that are available to us in the detailed view of Queue Entities.

Now for an overview of all the queues. This simulation is set up with two barbers, but the queue for the barbers is a single one.

CHOOSE     View / Entity Details

to view the non-detailed view of the Queues. When you have viewed this version of the window and are ready to move on, please either minimize the window by a

CLICK ON     The Smallest Button in the Upper Right of the Window

or close the window by a

CLICK ON     The X-Upper Right of the Window

Now, please halt the simulation.

PRESS     [F4]

GPSS World makes it extremely easy to create histograms that can even be viewed dynamically as the simulation runs. This is done by defining a TABLE or QTABLE Entity for each histogram. Let’s begin by putting all the queuing times into a histogram. Instead of an interactive definition in the simulation, here we’ll add the QTABLE definition to the model. Give the Model Window the focus. Place the cursor at the end of the last line of the model and

PRESS     [Enter]

then

TYPE     Waittime QTABLE Barber,2,3,20 ;Wait times Table

 

Figure 8—3. Addition of a QTABLE Command.

 

This defines our histogram, or in GPSS terms, our QTABLE Entity. Unlike Facility entities and Queue entities, Qtable Entities must be specifically declared in a QTABLE command to be created. As with all Model Statements, a QTABLE Command can be sent to an existing simulation to define a Qtable Entity interactively.

Now, retranslate the model.

CHOOSE     Command / Retranslate

and start the simulation running

CHOOSE     Command / START

and in the dialog box, replace the 1,

TYPE     1111111

and

SELECT     OK

PRESS  [F4] 

Now the easy part, GPSS World has a window for viewing histograms. Open the Table Window.

CHOOSE     Window / Simulation Window / Table Window

In the dialog box you'll see Waittime is already selected.

SELECT     OK

 

Make sure to expand the window to a comfortable size to be able to see the bars in the Table.

Your screen should look something like this.

 

Figure 8—4. The Table Window.

PRESS    [F2] 

to watch the Histogram develop.

The vertical axis is a count of Transactions, and the horizontal axis has a bucket for each waiting time in a range. For example, a waiting time of up to 2 minutes will add a count of 1 to the bucket used for times 0 to 2. Let’s watch how the waiting times are changing in our original barber shop example. Do you see that the mean waiting time is around 15 minutes with some customers waiting 56 or more minutes. They must really need a haircut. When you have seen enough, close this window and switch over to the Blocks Window.

CLICK ON     The X-Upper Right of the Table Window

then

CHOOSE     Window / Simulation Window / Blocks Window

 

Figure 8—5. The Blocks Window, Detailed View.

 

By observing the current Block contents of the first TRANSFER Block (Block 4), you will see that the number of Transactions varies as the customers (Transactions) wait for the barbers. They wait in the Transfer Block until they can enter one of the two SEIZE Blocks that represent ownership of the barber’s services.

Close the Blocks Window.

CLICK ON    The X-Upper Right of the Blocks Window

Now look at the Facility entities representing the barbers.

 

CHOOSE     Window / Simulation Window / Facilities Window

 

 

Figure 8—6. The Facilities Window.

 

Utilization of both barbers is quite high. Perhaps, there will be a high turnover rate of employees if we work them this hard.

All the Transactions waiting to get into the SEIZE Block are on Retry Chains of the Facility entities named Barber1 and Barber2. Each time a barber changes state, from busy to not busy, GPSS World checks all the chains associated with the Facilities to see who should be the next owner. In this case, the Transactions are never reaching the Delay Chain for each barber, but are being placed on the Retry Chains associated with the test at the TRANSFER Block. When both barbers are busy they are placed on Retry Chains for both barbers to try again when one becomes free.

Now, look back at the Model Window. You may have to expand it to see the text of the model. Notice, that the labels for the SEIZE Blocks and the Facility Entity names are slightly different. Block locations, i.e., names used in the label field of a Block statement, are special. You should not use Block location names to name other entity types. However, you can give Queues, Storages, Facilities, etc. the same names and it is perfectly acceptable. You could have a Queue named Barber and a Facility with the same name.

Let’s take a look at another histogram. Interrupt the simulation.

PRESS     [F5] 

CLICK ON     The Model Window

Notice that in our barber shop example, the first barber does not take a constant amount of time to do haircuts. Although he averages 10 minutes to do a haircut, the actual durations range from 7.5 to 12.5 minutes. Barber 2 takes from 9 to 17 minutes. The B operand of the ADVANCE Block is the half range of the Uniform distribution. If you are not familiar with operands A and B of the ADVANCE Block, please refer to Chapter 7 of the GPSS World Reference Manual.

The distribution of haircut times generated by the ADVANCE Block is called a uniform distribution. Its histogram will eventually converge to the shape of a rectangular fence. Let’s check this.

First, we want to insert QUEUE and DEPART Blocks to gather the haircut times. For this purpose we will use the Block Creation Dialog of GPSS World, which provides for "fill-in-the-blank" Block definitions. In the Model Window, just after the Barb1 label

CLICK ON     Anywhere in the DEPART Statement

We do this so that when the Block Statement is created it will be inserted right before the first ADVANCE Block Statement. Now let's fill in the blanks.

CHOOSE     Edit / Insert GPSS Blocks ...

The Block Menu appears.

 

Figure 8—7. The Block Menu.

Now for the QUEUE Block Statement. In the left column of the Block Menu

CLICK ON     QUEUE

The Block Input Dialog appears, ready to take information on your new QUEUE Block. In Operand A

TYPE     Haircut

In Comment

TYPE     Length of cut -- Barber1

The Block Input Dialog should look like this.

 

Figure 8—8. The Block Input Dialog for QUEUE Block.

If you would like to review the details of the QUEUE Block, just click on the Help button. When you are ready to insert the new statement

SELECT     OK

Now look at the Model Window. The newly created QUEUE Statement has been placed just after the line where we put the insertion point. This new statement actually contains tabs, and you can change tabstops in the Settings of the Model Object.

Great. Now let's do the same thing to create the DEPART Statement. First, set the insertion point in the Statement just before where the new Block Statement is to go. In the Model Window just after the new QUEUE Statement

CLICK ON     Anywhere in the next ADVANCE Statement

 

Now go to the Block Menu. In the left column of the Block Menu

CLICK ON     DEPART

The Block Input Dialog appears again, this time ready to take information on your new DEPART Block. In Operand A

TYPE     Haircut

In Comment

TYPE    Length of cut -- Barber1

The Block Input Dialog should look like this.

 

Figure 8—9. The Block Input Dialog for DEPART Block.

SELECT     OK

As before, the new statement has been inserted into the Model.

Next we must define the histogram. For this we must insert the statement ourselves. Move the cursor to the end of the last line of the model,

PRESS     [Enter] 

and

TYPE     Cuttimes QTABLE Haircut,8,0.5,10

This defines our histogram, or in GPSS terms, our Qtable Entity.

Retranslate the model and start it running and open the Tables Window.

CHOOSE     Command / Retranslate

and

CHOOSE     Command / START

in the dialog box, replace the 1,

TYPE     1111111

and

SELECT     OK

PRESS     [F4] 

to halt the simulation while you open the new Table Window, then

CHOOSE     Window / Simulation Window / Table Window

in the dialog box you will see Cuttimes is already selected.

SELECT     OK

then

PRESS     [F2] 

to continue the simulation run.

Your screen should something like this look like this after the simulation has run for a few seconds.

 

Figure 8—10. Haircut Duration Histogram.

 

If you wish, you can print the window to a printer at any time. Just Choose File / Print in the Main Window. You can do this at any time, but it is usually wiser to print after you have halted the simulation so you know exactly what you are printing.

Notice that although the histogram is irregular, over a long period of time, it approaches the rectangular shape of the uniform distribution. Also notice that the average haircut time converges to the 10 minutes we desired. By the way, when a tabulated duration falls exactly on a boundary, such as 11, the frequency class to the left is incremented.

Clearly our results will not be good if we do not simulate enough haircuts. One of the objectives of your study of simulation methods, is to know how long to run your simulations. This topic is covered in more detail in textbooks on simulation techniques. Also, we will discuss the GPSS World ANOVA command in a later lesson.

When you are ready, go ahead and end the GPSS World Session as we have before.

The TABLE and TABULATE statements are also used to create histograms. They are used with GPSS Table entities, and they allow you to use any expressions, not just simulated durations, in your histograms.

Let’s review. We have used the QUEUE and DEPART Blocks in association with Queue and Qtable entities. We saw how easy it is to view histograms, and we took a brief look at how probability distributions converge.

The insertion of QUEUE and DEPART Blocks does not affect the simulation because no simulated time is used, and the flow of Transactions is not affected. The only difference is that additional statistics are collected automatically.

There are many other GPSS Blocks that we will not consider in this tutorial. However, the examples in Chapter 2 of this manual do use some of them. You must, therefore, make use of Chapter 7 of the GPSS World Reference Manual to understand the use of these Blocks.

Most people generally use only a subset of the GPSS Blocks available. However, you should have a general idea of what the remaining Blocks can do for you. Before concluding this lesson, your assignment is to read the list of Block descriptions at the beginning of Chapter 7 in the GPSS World Reference Manual. Don’t worry about details, this exercise is just to acquaint you with the simulation power available.

In the next lesson, we will look at the GPSS World Commands. They let you manage the operation of your simulation environment. See you then.

 

Lesson 9 - GPSS World Commands

The GPSS World Commands are used to manage the construction and execution of your simulations as well as to define certain GPSS entities. In this lesson we will review each of the commands. Let’s start with a list of the commands and a short description of each.

The commands are:

· BVARIABLE - Define a Boolean Variable Entity.

· CLEAR - Reset statistics and delete Transactions.

· CONDUCT - Conduct an Experiment.

· CONTINUE - Resume the Simulation.

· EQU - Assign a value to a User Variable.

· EXIT - End the GPSS World Session.

· FUNCTION - Define a Function Entity.

· FVARIABLE - Define an Fvariable Entity.

· HALT - Stop the simulation and delete all Queued Commands.

· INCLUDE - Read and Translate a secondary Model File.

· INITIAL - Initialize or modify a Logicswitch, Savevalue, or Matrix Entity.

· INTEGRATE - Automatically integrate a time differential in a User Variable.

· MATRIX - Define a Matrix Entity.

· QTABLE - Define a Qtable Entity.

· REPORT - Request an immediate report.

· RESET - Reset the statistics of the Simulation.

· RMULT - Set the seeds of the first 7 Random Number Generators

· SHOW - Evaluate and display Expression.

· START - Set the Termination Count and begin a Simulation.

· STEP - Attempt a limited number of Block entries.

· STOP - Set a Stop Condition based on Block entry attempts.

· STORAGE - Define a Storage Entity.

· TABLE - Define a Table Entity.

· VARIABLE - Define a Variable Entity.

 

This is an unstructured lesson. Your assignment is to use each command in the list at least once.

Start a GPSS World Session as we have before.

CHOOSE      File / Open

in the dialog box

SELECT       Sample2

and

SELECT       Open

CHOOSE       Command / Create Simulation

Now you’re on your own. If you run into trouble, consult on-line help, and Chapter 6 of the GPSS World Reference Manual, in that order.

How did you do? Did you notice that some of the commands have many options, some of which you probably have not yet used? We will not pursue the command features here, but if you want to take full advantage of the GPSS World Commands you should read the rest of Chapter 6 of the GPSS World Reference Manual. We will discuss Manual Simulation and the INCLUDE Command in more detail in the next lesson as well as the Debugging and Design lesson.

Now, close the Sample2 Journal and Model Windows as we have done before.

Before we conclude this lesson let us consider two more ideas. The first of the two ideas is that each name is associated with a value.

Open a new model.

CHOOSE       File / New

SELECT       OK

In the new Model Window

TYPE       Var1 Variable 3#3/3

Save the model as Tmp.gps.

CHOOSE       File / Save As

and in the dialog box

TYPE       Tmp

SELECT     Save

You have defined a GPSS Arithmetic Variable Entity, named Var1, which returns the value of the expression as given in the variable definition. Variables can consist of constants or contain variables that change during the simulation. Now translate this one line model.

CHOOSE       Command / Create Simulation

then

CHOOSE       Command / SHOW

and in the dialog box

TYPE       V$Var1

SELECT       OK

In the Status Line of the Main Window, you should see the evaluated result of the Variable Entity named Var1. The V class SNA V$Var1 returns the value of the Variable Entity named Var1.

Here’s a common mistake.

CHOOSE       Command / SHOW

and in the dialog box

TYPE       Var1

SELECT       OK

What happened? Why wasn’t the variable evaluated? The answer is that a name alone is resolved as its numerical value. If you want to use the name itself, you can enclose it in double quotes to form a string constant. If you have not assigned a value to the name using an EQU statement or a PLUS assignment statement, GPSS World assigns it an integer value of 10000 or above. Further, you must assign values to names using an EQU statement before you can refer to their values in an expression. If you wish a named VARIABLE to be evaluated, don’t forget the V$ before the name of the VARIABLE.

When you use a name, GPSS World assigns a distinct system number to it. If the name is used as the name of an entity, you do not have to do anything else. However, if you want the name to have a specific value, you must use an EQU statement to assign the value before the entity is created.

In the last case, GPSS World assigned its own number to the name Var. and that’s what you see in the Status Line. Had you used Var. in an EQU statement before defining the Variable Entity, the SHOW Var. would return your number.

WARNING! If you change the value of a name after you define a named entity, you will no longer be able to use the name to refer to the old entity. Changing the value of a name does not redefine entities. Any old entity now has an entity number that is different from the new value of the name. Also, remember that Block labels are numbered by GPSS World. You should not use Block labels to name other entities. When you start your simulation, you will get an error message if you have done so.

This concludes our lesson. In the next lesson, we return to the more structured approach. We will now take a closer look at Manual Simulation, a very powerful feature of GPSS World.

 

Lesson 10 - Manual Simulation

In this lesson we will look at one of the most powerful interactive features of GPSS World.

Any Model Statement can be sent interactively to an existing simulation. This includes GPSS Block Statements. The GENERATE Block is the only exception. When a Block is sent to an existing Simulation Object, it is executed bit not inserted.

We can enter a Block Statement by using the Custom Command Dialog in the Command menu of the Main Window. When the Simulation Object receives the Block Statement, it causes the Active Transaction to attempt to enter a temporary Block described by the Block statement. This method is called Manual Simulation. Such a Block statement does not go into the program that has been translated in the Model Window, and the Block Entity does not become a permanent member of the simulation.

Let’s try it. Start up a GPSS World Session. See Lesson 1 if you have questions about doing this.

We must establish an Active Transaction before we can use manual simulation. There are several ways to do this. We could use a STOP command or we could interrupt a simulation by pressing the o function key. We could also wait until a simulation completes. Normally, simulations end with an Active Transaction in place.

For the purpose of this lesson we will use the STOP command. This causes a Stop Condition to be registered, which will stop when any Transaction attempts to enter any Block. First we must open a model and then establish a Stop Condition

CHOOSE       File / Open

in the dialog box

SELECT       Sample2

and

SELECT       Open

 

; GPSS World Sample File - SAMPLE2.GPS
*************************************************************
*
* Barber Shop Simulation
*
*************************************************************
        GENERATE    300,100     ;Create next customer.
        QUEUE       Barber      ;Begin queue time.
        SEIZE       Barber      ;Own or wait for barber.
        DEPART      Barber      ;End queue time.
        ADVANCE     400,200     ;Haircut takes a few minutes.
        RELEASE     Barber      ;Haircut done. Give up the barber.
        TERMINATE   1           ;Customer leaves.

 

The Model Window should contain the model shown above. Now, let’s translate the model and then open the Blocks Window.

CHOOSE       Command / Create Simulation

and

CHOOSE       Window / Simulation Window / Blocks Window

Select the first GENERATE Block by positioning the mouse pointer over the Block and clicking mouse button 1 once.

CLICK ON       The GENERATE Block Icon

Then

CLICK ON       The Place Icon in the Debug Toolbar at the Top of the Window

Now, we can START a simulation that will stop immediately.

CHOOSE       Command / START

and in the dialog box, replace the 1,

TYPE       1000

SELECT       OK

Notice that a stop message appears in the Journal Window. Before we do anything else, let’s remove all Stop Conditions, but first return to the Blocks Window by clicking anywhere on the window. Again, in the Blocks Window,

CLICK ON       The GENERATE Block Icon

Then

CLICK ON       The Remove Icon in the Debug Toolbar Top of the Window

Since we have set up an Active Transaction, we can type any Block statement we please. In the Model Window,

CHOOSE       Command / Custom

then in the dialog box

TYPE       Assign Price,19.95

SELECT       OK

The Active Transaction now has a Transaction parameter named Price containing the value 19.95. Let’s see.

CHOOSE       Window / Simulation Snapshot / CEC Snapshot

 

and in the Snapshot Window

CLICK ON       The + Sign

in the CEC Window to see the Transactions on the chain. Transaction 1 is the only Transaction. One more click will reveal any parameters of this Transaction.

CLICK ON       The + Sign

 

See, there’s the PRICE parameter. Your screen should look like this:

 

Figure 10—1. Manual Simulation ASSIGN.

 

Now, close the CEC Snapshot Window. Another way to view a parameter value of the Active Transaction is to use the SHOW command.

CHOOSE       Command / SHOW

and

TYPE       P$Price

SELECT       OK

That should confirm the value of 19.95. Now, let’s check out some additional power that manual simulation gives you.

CHOOSE       Command / Custom

and in the dialog box

TYPE       Trace

SELECT       OK

This has the same effect as if the Active Transaction had passed through a TRACE Block. The Trace Indicator for the Active Transaction is now on. Watch the Journal Window to see the trace.

PRESS     [F2]

to continue running the simulation.

Notice, that in the Journal Window, each Block entry results in a trace because the Trace Indicator of the Transaction is on. If the time changes, or a new Transaction is traced, a general message giving the time and Transaction number is written. In this case, we will only see a small number of trace messages since only one Transaction is being traced. This is how the Journal / Simulation Window will look.

 

Figure 10—2. Trace Messages in the Journal Window.

 

If you had wanted traces of all Transactions, you could insert a TRACE Block in the model and retranslate it.

As with any statement, a Manual Simulation statement can be loaded into a function key, for later use. Function keys can be preset by changing the Function Keys Page of the Model Settings Notebook as we did earlier when we loaded AC1 into [F8].

Take a moment now to consider what you can do with Manual Simulation. You can send any Block Statement, except GENERATE, to a running simulation. You can create Transactions with a SPLIT Block, reroute Transactions with a TRANSFER Block, or modify parameters with an ASSIGN Block. You can test the membership of groups with EXAMINE and SCAN Blocks. You can manually take resources out of action with FUNAVAIL and FAVAIL Blocks. You can even activate any Block in the Model by entering an EXECUTE Block in manual simulation mode. Remember though, it is always the Active Transaction that attempts to enter the Block you describe in a manual simulation statement.

Manual simulation is an extremely powerful and flexible feature. You will find that it is an excellent tool for correcting error conditions and for experimentation. It is a simple matter to introduce a sequence of Blocks to the Active Transaction by only a few keystrokes.

Next, we start to explore the interactive graphical windows of the GPSS World simulation environment. We will take a look at each of the most important windows you can use to observe your simulations. See you then.

 

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