User Guide: C++ : Advanced Statistics Gathering

15. Advanced Statistics Gathering

15.1. Example: Instrumenting a Facility

For each facility, CSIM automatically gathers and reports the following statistics:

• mean service time
• mean queue length
• utilization
• mean response time
• throughput
• number of completions

Meters and boxes can easily be used to gather more detailed statistics. The following statements show the declaration of the needed variables:

facility *f;
meter *arrivals;
meter *departures;
box *queue_box;
box *service_box;

The following statements, which would appear in the sim function, show the initialization of the variables:

f = new facility("center");
arrivals = new meter("arrivals");
departures = new meter("completions");
queue_box = new box("queue");
service_box = new box("in service");
The following code shows the instrumentation of the facility:

customer()
{

double timestamp1;
double timestamp2;

create("customer");
arrivals->note_passage();
timestamp1 = queue_box->enter();
f->reserve();
timestamp2 = service_box->enter();
hold (exponential(0.8));
f release();
service_box->exit(timestamp2);
queue_box->exit(timestamp1);
departures->note_passage();

}

The report for box "queue_box" would give statistics on response times (under the heading "statistics on elapsed times") and queue lengths (under the heading "statistics on population"). The report for box "service_box" would give statistics on service times (under the heading "statistics on elapsed times") and utilization (under the heading "statistics on population"). The report for meter "arrivals" would give statistics on the arrival rate and inter-arrival times. The report for meter "departures" would give statistics on the completion rate and inter-completion times. If the arrival and completion rates were sufficiently similar, this quantity would be called the throughput.

Obviously, histograms could be added to any of these meters and boxes to obtain information on the various distributions.

15.2. The Report Function

Although reports can be produced at any time for individual statistics gathering tools, it is most common to generate reports for all tools at the same time, usually when the simulation has converged. This can be done by calling the report function.

Prototype: void report(void)
Example: report();

The report function produces reports for all facilities, storages, and classes, followed by reports for all tables, qtables, meters, and boxes. The sequence of reports begins with a header that includes the model name, the date and time, the current simulation time, and the cpu time used.

15.3. Resetting Statistics

CSIM provides a single function that will clear all accumulated statistics without affecting the state of the system being modeled in any way. This reset function is most often used when warming up a simulation. The simulation is begun with the system in an empty state, simply as a matter of convenience. A small number of customers are allowed to pass through the system, hopefully taking the system closer to its equilibrium state. Then, the statistics are reset and the simulation is run until convergence is achieved.

The reset function has a simple interface.

Prototype: void reset(void)
Example: reset();

Reset clears the statistics that are automatically gathered for facilities, storages, events, and process classes. It also resets the statistics in all non-permanent tables, qtables, meters, and boxes being used in the program. Permanent tables are not affected by calling reset.

In general, resetting statistics returns all the statistical counters and timers maintained by CSIM to their initial values, which are usually zero. But, there are a few subtle and important exceptions to this rule. When a qtable is reset, it remembers the current value for use in computing future values from the relative changes specified by note_entry and note_exit. When a meter is reset, it remembers the time of the last passage for use in computing the next interpassage time. When a box is reset, it remembers the number present for use in computing future populations.

Calling reset in no way changes the state of the system being modeled. It does not change the simulation clock; it does not affect the streams of random numbers being used in the simulation; and it does not affect the states of processes, facilities, storages, events, and mailboxes. The reset function is normally called during a simulation run, whereas the rerun function (see section 21.4.1, "To rerun a CSIM model") is called between successive runs.

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