HELP (Программа GPSS), страница 10

3. Translate the model (Ctrl+Alt+S).

4. Press the Function Key.

That's all there is to it. The PLUS Experiment will run, reporting its status to the Journal. When it completes, it will create a full report, as well. In the case of an Optimizing Experiment, the report will include a mathematical description of the Response Surface around the optimum. Then, analyzing the Report is up to you.

Both types of generated PLUS Experiment contain invocation of a PLUS Run Procedure. It is your responsibility to ensure that this procedure sets up the run conditions appropriately. The Run Procedure is called each time the Experiment begins a new simulation run. Optionally, you can have the Experiment Generator create a template Run Procedure for you. Even so, you must still adapt it to your own conditions. The next section tells you how.

13.4.1 The Run Procedure

The Run Procedure is the flexible connection between the generated experiment and the user’s simulation. Both Screening Experiments and Optimizing Experiments use them. Generated Experiments repeatedly call the Run Procedure that the user names in the creation dialog in order to execute each run in the experiment.

Every experiment should have a Run Procedure written by you or generated by GPSS World. A Run Procedure is used to establish the measurement period during the simulation that is to be observed. It sets up each run in the Experiment according to the particular requirements of your simulation and is invoked once for each run in the experiment. Run Procedures are PLUS Procedures that are called by Experiments and therefore may contain DoCommand calls.

Normally the Run Procedure contains setup commands for the simulation such as RMULT Commands to control the Random Number Generators, and START and RESET Commands to set up the measurement period of the run.

Here are the most important things you should take care of in the Run Procedure:

· Use CLEAR OFF to remove old transactions between replication but still preserve the Results Matrix.

· If the Experiment does not set the treatment levels elsewhere, do it here based on arguments of the Run Procedure. This is not necessary for generated experiments. They set the treatment levels for you.

· Use RMULT to set the Random Number Generators according to an argument in the run procedure. If you have not defined a replicate dimension in the Result Matrix, you should “completely randomize” and use distinct random number stream seeds for each run.

· Use PLUS String Procedures to create the DoCommand argument.

· Use START to begin the start-up period.

· Use RESET to reset the statistics accumulators prior to the measurement

period.

· Use START to begin the measurement period.

· Unless the Experiment stores results in the Result Matrix using a PLUS assignment statement, do it here based on arguments of the Run Procedure. This is not necessary for generated experiments.

When an experiment is generated automatically, the generator optionally creates a template Run Procedure which can be modified as desired. As an example, here is the default Run Procedure created by the Experiment Generators:

PROCEDURE RunProc(Run_Number) BEGIN

DoCommand("CLEAR OFF"); /* Must use OFF to preserve results. */

/* EXPAND THIS RMULT IF YOU HAVE MORE RNGs. */

/* All Random Number Streams must have new seeds. */

TEMPORARY CommandString;

/* Evaluate before passing to DoCommand. */

CommandString = Catenate("RMULT ",Run_Number#111);

/* DoCommand compiles the string in Global Context. */

DoCommand(CommandString);

/* SET UP YOUR OWN RUN CONDITIONS. */

DoCommand("START 100,NP"); /* Get past the Startup Period. */

DoCommand("RESET"); /* Begin the Measurement Period. */

DoCommand("START 1000,NP"); /* Run the Simulation. */

END;

Figure 13-10. The Default Run Procedure

When you let GPSS World create a Run Procedure, it is up to you to customize it to fit your own situation. You can edit it before, or after, it is inserted into your Model Object.

13.4.2 Screening Experiments

Screening Experiments are used to look for factors which must be investigated more thoroughly or those which should be considered to be irrelevant. However, the combinatorial nature of measuring main effects and all the possible interactions often makes the complete set of runs, called a full factorial experiment, too lengthy and/or expensive to run. To make the procedure more manageable, we restrict the number of treatment levels to two per factor, and we look for ways to run only a fraction of the possible runs without sacrificing the most important information. The Screening Experiment Generator of GPSS World does most of the work for you.

You must decide which information you can give up in order to reduce the number of runs in the experiment. Normally it would be the higher-level factor interactions that are usually insignificant and can be ignored. Even if you suspect otherwise, remember that the purpose of the screening experiment is to identify the important factors to be studied in more depth later. When you ask GPSS World to reduce a full factorial screening experiment to a fractional set of runs, some effects get mixed with others to the point where you can’t distinguish them in the Analysis of Variance. GPSS World will partition the set of all possible effects subsets called Alias Groups. Only the combined result of all effects in the group is reported in the Analysis of Variance. This means that if factor A and factor B are in an Alias Group, you will have lost any ability to tell which is responsible for the observed effect. Perhaps even worse than that, it is possible for two strong effects in the same Alias Group to cancel each other, thereby concealing the actual effects of each factor.

As you can see, it is extremely undesirable to have more than one main effect in an Alias Group. However, it is usually possible to arrange Alias Groups so that each main effect and the most important 2-way interactions are in Alias Groups of their own, with only higher order interactions as companions. GPSS World provides default Alias Groups which isolate main effects, if possible, and it makes it easy for you to change the Alias Groups if the defaults are not suitable. In any case, when you generate a GPSS World Screening Experiment, you should always examine the Alias Groups before you begin the runs. If you haven’t separated the most important effects into distinct groups, the interpretation of results will be difficult.

Generating A PLUS Screening Experiment

The first step in creating a Screening Experiment automatically is to open the dialog. You access it by clicking "Insert Experiment" in the Edit Menu of the Main Window, and clicking on the "Screening Experiment ... " item. You must have an Open Model Object active in order to be able to do this.

Now just fill in the blanks and click OK when you're done. Figure 13-11 shows the Screening Experiment Dialog.

Figure 13-11. The Screening Experiment Generator Dialog

To specify the PLUS Experiment that is about to be generated, we simply fill in the fields of the Dialog. The “

Experiment Name” and the “‘Run Procedure’ Name” fields are used as the PROCEDURE names in the generated experiments. The EXPERIMENT calls the Run Procedure repeatedly.

The Factors of the Experiment come next. Each factor name is actually the name of a User Variable and must obey the GPSS World naming conventions. It must begin with an alphabetic character and it must not clash with a keyword, SNA, or SNA class. Since screening experiments in GPSS World are full or fractional 2k experiments, there are two treatment levels to be specified for each factor. You must specify the names and two treatment levels for between 1 and 6 factors, inclusively. Factors must be specified consecutively starting with the A factor. The choice of treatment levels is crucial. You may want to do some preliminary experimentation so that the choices you make are good ones. Choose levels that are far apart to elicit a change of behavior and try to avoid levels where the effects are hidden by other factors.

The “Fraction” group is next. It allows us to specify what faction of the full 2k experiment is to be run. The run count that will result is specified to the right. Choose a smaller fraction to decrease the run count.

The “Result Expression” is required. You must specify an expression to be evaluated as the metric of the simulation.

Next we have two checkboxes that allow us to select additional options. First, you can have GPSS World generate a template Run Procedure to go along with the generated experiment. You can edit this Run Procedure to suit your needs. Second, you can have GPSS World set up a setting which loads the appropriate CONDUCT Command into the setting for Function Key 11. If you choose this, after you create the Simulation Object, you only have to press the F11 Function Key to run the experiment.

Before we generate the experiment we should examine the Alias Groups. To do so, click the Alias Groups button to examine the consequences. This brings up the Alias Groups Dialog.

Figure 13-12 shows the Alias Group Dialog for a one-half fraction.

Figure 13-12. The Alias Group Dialog. Improved Experimental Design.

From the Alias Groups Dialog we can see that this experimental design isolates all the main factors, does not confound them with 2-way interactions, and it even isolates the 2-way interactions of the first few main effects.

Take a closer look the effects used as "Generators" in the Alias Group Dialog. We can change the partition of the effects into a different set of Alias Groups if we so desire by simply using a different set high level interactions as generators. To do this, change the generators and click on the Generate Button. Before you do this there are several things to know about the generators. First, you should know that when GPSS World creates Alias Groups it first expands the set of generators by forming all possible modulo 2 products and adding them to the set of generators, now called the "Expanded Set" of generators. For example if ABCD and BCDE are generators, their product, AE, will be in the Expanded Set of generators (ABCDE * BCDE = ABBCCDDE = A (BB) (CC) (DD) E = AE ). Note that any effect in the Expanded Set of generators will not appear in the report of effects. This means that you must not choose generators that result in an important effect occurring in the Expanded Set of generators. You can return to the default generators suggested by GPSS World by clicking on the Use Default Button. Click the Generate Button to create the new list of Alias Groups.

The next step is to insert the PLUS code comprising the experiment into your Model Object. When you have made the best compromise between the number of runs and the aliasing of effects, click the OK Button in the Alias Group Dialog and the Insert Experiment Button in the Screening Experiment Generator Dialog. If you have asked GPSS World to generate a Run Procedure, next we see a Dialog that allows you to personalize it. The details about personalizing the Run Procedure are in the previous section.

In any case, you may want to edit the generated experiment itself in your Model Window. You can change the Run Procedure there, as well. Just click OK and all the new GPSS World Statements comprising the Screening Experiment are placed at the bottom of your Model. A sample of a generated PLUS Experiment follows in the next section.

The Generated Experiment

*******************************************************

*

* ScreenEthernet

* Fractional Factorial Screening Experiment

*

*******************************************************

Global_Delays MATRIX ,2,2,2,2,2

INITIAL Global_Delays,UNSPECIFIED

EXPERIMENT ScreenEthernet() BEGIN

/* Run 1 */

Node_Count = 100;

Min_Msg = 512;

Max_Msg = 12144;

Fraction_Short_Msgs = 600;

Intermessage_Time = 1.0;

IF (StringCompare(DataType(Global_Delays[1,1,1,1,1]),”UNSPECIFIED”)’E’0)

THEN BEGIN

/* Run Procedure Call */

DoTheRun(1);

Global_Delays[1,1,1,1,1] = QT$Global_Delays;

END;

/*********** Runs 2-15 would go here ************/

/* Run 16 */

Node_Count = 200;

Min_Msg = 1024;

Max_Msg = 24288;

Fraction_Short_Msgs = 300;

Intermessage_Time = 1.0;

IF (StringCompare(DataType(Global_Delays[2,2,2,2,1]),”UNSPECIFIED”)’E’0)

THEN BEGIN

/* Run Procedure Call */

DoTheRun(16);