Язык моделирования GPSS: История и применение

Simulation system language

a programming language
General Purpose Simulation System (GPSS) is a simulation language used for discrete event simulations. It is especially useful in the modelling of queuing systems, with many statistics being collected automatically. The typical simulation consists of Transactions being generated in the system (usually at a certain interval), performing a defined set of rules (like use a resource, wait, transfer), and being removed from the simulation.

GPSS was developed in the 1960s by Geoffrey Gordon, an employee of IBM's Advanced Systems Development Division (ASDD). This division was heavily involved with research into the design of teleprocessing systems, trying to achieve an economic balance of the use of computer resources and shared lines between server terminals. The simulation system, then known as the Gordon Simulator, became very popular in the study of teleprocessing systems within ASDD. It subsequently was fixed and documented on October 25, 1960 in an internal IBM memorandum. Between the winter and summer of 1961, a group of three programmers (including Gordon) rewrote the simulation system with a new algorithm and new block types. It was officially released as a supported IBM label program on September 27, 1961 with 25 block types. At this point, Gordon stopped working on the simulation system. In 1963, GPSS II was released with 32 block types. It introduced system numerical attributes, which allowed tracking the current content of a Storage, the length of a Queue, or the current clock time. In 1965, GPSS III was released. It was made available for IBM Systems 7090/94 and 7040/44. In 1967, GPSS/360 was released to run on the newly released System 360. In 1970, GPSS V was released with 49 block types. In the 1980s, GPSS/VAC and GPSS/PC were released. These appear to be the last official IBM label releases before the language became unlicensed. Subsequently, there were releases for IBM 360, Univac 1108, and CDC. Over time, other implementations were developed for systems including DEC's VAX, a specialized APL version for large scale Univac systems, and Macintosh. In 2001, the Windows program GPSS World was released with new features to GPSS. It includes scripting with PLUS (a Pascal like language), graphical system state displays, graphing, and optimization experiments. In 2009, a graphical Java based tool called JGPSS (Java General Purpose Simulation System) was developed to teach the GPSS simulation language.

A GPSS simulation is written in an Assembly style "block diagram language" with many different single purpose commands ("Blocks") to control the flow of a Transaction, statistics collection, and variables. Simulations have Transaction entry points through the GENERATE command, such as a customer walking into a store. Then, actions are performed through claiming Facilities/Storage (like a cashier), waiting, saving statistics, etc. Finally, the simulated transaction exits the simulation through the TERMINATE command. Each command verb is referred to as a "control". Blocks can be facility oriented (such as machines in a job shop) or transaction oriented (such parts of work in process, signals in electronic components or documents in a bureaucratic procedure). GPSS automatically keep track of statistics for display on a report. Entities can be broadly classified in Resources, Computational entities and Statistical entities. Resources, like Facilities and Storages represent limited capacity resources. Computational entities, like Ampervariables (variables), Functions and random generators are used to represent the state of Transactions or elements of their environment. Statistical entities, like Queues or Tables (histograms) collect statistical information of interest.

PrefixMeaningPjA parameter of the transaction current being processed by the program. M1The transit time of the current transaction. MPjIntermediate transit time of current transaction. PRPriority of current transaction (0 127).

PrefixMeaningCHjThe current count, which is the number of the transaction on a specified user chain. CAjThe average number of transactions on user chain j. CCjTotal number of entries on user chain j. CTjAverage time per transaction on user chain j.

PrefixMeaningNjThe entry count of the total number of transactions which have entered a specified block in the block diagram. This count is automatically maintained by the program. Example: N$SAM for the entry count at block SAM. This count does not include the transaction currently in process at its current block. WjThe wait count, which is the number of transactions currently waiting at a specified block of the block diagram. This count is also maintained automatically by the program. Example: WSHOLD for the current wait count at block HOLD. This count is also exclusive of the transactions currently in process at its current block.

PrefixMeaningKjAn indication that the integer is a constant. Example K3276 for the integer 3276 or KO for the integer zero. RN(x)A computed random number (1<=x<=8). The value of the number is an integer between 0 and 999, inclusive, unless the quantity is to be used as the independent variable of a function. In that case, the number is a fraction greater than or equal to zero, but less than one. In either case, all values within the specified range may be considered equally probable. C1The current value of the simulator clock. This quantity is automatically maintained by the program.

PrefixMeaningSjThe contents of a specified storage in the block diagram. The quantity may be modified by ENTER and LEAVE blocks. Example: S2 for the contents of storage (number) 2. RjThe number of available units of space in the specified storage. This quantity may be modified by ENTER and LEAVE blocks. Example: R195 for the space remaining in storage 195. SRjUtilization of storage j in parts per thousand, i. e., if the utilization was .65 the computed value would be 650. SAjAverage contents of storage j (truncated). SMjMaximum contents of storage j. This quantity is automatically maintained by the program. SCjNumber of entries for storage J. This quantity is automatically maintained by the program. STjAverage time each transaction used storage j (truncated).

PrefixMeaningFjThe status of the specified facility in the block diagram. This value is zero if the facility is available; otherwise, it is one. This quantity may be modified by SEIZE, RELEASE, PREEMPT, and RETURN blocks. Example: F20 for the status of facility 20. FRjUtilization of facility j in parts per thousand, i. e., if the utilization was .88 the value of FRj would be 880. FCjNumber of entries for facility j. FTjAverage time each transaction used facility j (truncated).

PrefixMeaningGjThe current number of members of group j.

PrefixMeaningQjThe length of a specified queue in the block diagram. This quantity may be modified by the QUEUE and DEPART blocks. Example: Q50 for the contents of queue 50. QAjAverage contents of queue j (truncated). QMjMaximum contents of queue j. This quantity is automatically maintained by the programQCjNumber of entries in queue j. Automatically maintained. QZjNumber of entries in queue j. Automatically maintained. QTjAverage time each transaction was on queue j (including zero entries). When referenced the value will be truncated to an
integer. QXjAverage time each transaction was on queue j (excluding zero entries). Truncated.

PrefixMeaningTBjThe computed mean value of a specified histogram type table which is defined by the user. The TABULATE block is used to enter values in one of these tables. Although the computed average can possess a fractional part, it is not retained unless the computed average is to be used as the independent variable of a function. Example: TB42 for the computed mean value of table 42. TCjNumber of entries in table j. TDjComputed standard deviation of table j.

PrefixMeaningXjThe contents of fullword savevalue j. XHjThe contents of halfword save value j. MXj(a,b)The contents of fullword matrix savevalue j, row a, column b. (a and b can be any other SNA)MHj(a,b)The contents of halfword matrix savevalue j, row a, column b.

PrefixMeaningFNjA computed function value. Only the integer portion is retained except when used as a function modifier in GENERATE, ADVANCE or ASSIGN blocks. VjAn arithmetic combination of Standard Numerical Attributes which is called a variable statement and is defined by the user. Only the integer portion is retained. (See Chapter 4. )BVjThe computed value (1 or 0) of Boolean variable j.