Transactions are temporarily bound to other entities by occupying linked lists called chains. Some entities, such as Facilities, have several chains. Other entities have just a single Retry Chain. Each Transaction may be on any number of chains. However, occupying one kind of chain sometimes precludes occupancy by the same Transaction on another. For example, a Transaction on one or more Interrupt Chains cannot be on the Future Events Chain.
A Transaction can be on no more than one of the following chains:
·Future Events Chain
·Current Events Chain
·Facility or Storage Delay Chain
·Facility Pending Chain
A Transaction may be waiting for any number of conditions to occur, can be in any number of Transaction Groups, and may be preempted from any number of Facilities at any one time. This means that any single Transaction can be on any number of Interrupt Chains and any number of Group Chains and any number of Retry Chains at the same time.
The Current Events Chain (CEC) is a linked list of ready Transactions which have Blocks yet to be entered before simulated time advances. Although the CEC is kept in priority order, the Active Transaction is usually returned to the CEC ahead of its peers. For this reason, once a Transaction starts to move in the simulation, it tends to keep moving, unless a higher priority Transaction is reactivated.
When the Active Transaction comes to rest on some Transaction Chain, the highest priority Transaction remaining on the CEC becomes the Active Transaction. If the CEC is empty, the most imminent Transaction on the Future Events Chain is moved to the CEC.
The Future Events Chain (FEC) is a time-ordered chain which holds Transactions which must wait for a later simulated time. When all simulation activity for the current clock time is complete, the next Transaction is taken from the FEC. It is this action which causes the system clock to be updated. The value of the system clock is always equal to the scheduled time of the last Transaction to be taken from the FEC.
ADVANCE Blocks and GENERATE Blocks are the only way to place a Transaction on the FEC. These blocks take a time increment as an operand and calculate the absolute time before placing the Transaction on the FEC. When the system clock reaches this absolute time, the Transaction is moved to the CEC so that it may resume its movement in the simulation. In this manner, a duration or inter arrival time can be simulated.
PREEMPT Blocks, and the new DISPLACE Block, can be used to remove Transactions from the FEC. Such Transactions can be rescheduled by entering another ADVANCE Block.
When the scheduler must select a new Active Transaction, if it cannot find one on the CEC, it must take a Transaction from the FEC. This removal of one or more Transactions always causes the system clock to advance. When more than one Transaction resides on the FEC at the next imminent event time, a Time Tie is said to exist. If the Model Settings so dictate, when Time Tied Transactions are moved from the FEC to the CEC, their order is randomized within priorities. This is done to prevent unintentional processing cycles from developing.
A time tie is the occurrence of more than one Transaction with the same time value at the front of the FEC. When a time tie is detected, time tied Transactions are removed from the FEC in random order. For this purpose GPSS World draws pseudo random numbers from the random number generator specified in the Simulate page of the Model Settings Notebook. The removed Transactions are all placed in priority order on the Current Events Chain (CEC). The highest priority Transaction on the CEC then becomes the active Transaction. You can suppress the randomization of time ties by specifying 0 as the associated random number generator.
Transactions which fail all tests required for Block entry are placed on a Retry Chain. These tests occur when a Transaction attempts to enter a GATE, TEST, TRANSFER ALL, or TRANSFER BOTH Block. Each entity has a chain of blocked Transactions, called a Retry Chain. Any Transaction on a Retry Chain is waiting for the value of an SNA to change. When the value of the SNA changes, any Transaction on the Retry Chain of the entity is reactivated. This results in replacement on the CEC. When the Transaction becomes the Active Transaction, the specific condition test is repeated. Since this process often uses computer time without advancing Transactions in the model, an injudicious choice of conditions can lead to an inefficient simulation. The power of GATE and TEST blocks must be exercised with caution.
Transactions on Retry Chains are replaced on the CEC by the process of reactivation. This is discussed below. If, on retry, any test is successful, the Transaction enters its next Block. When a Transaction enters a Block it is removed from all Retry Chains automatically.
Each Facility Entity has four Transaction chains. They are:
·PENDING CHAIN - A list of Transactions waiting to PREEMPT the Facility in "Interrupt Mode".
·INTERRUPT CHAIN - A list of Transactions which have been preempted from ownership of this Facility.
·DELAY CHAIN - A priority chain of Transactions waiting for ownership of the Facility.
·RETRY CHAIN - A list of Transactions which are waiting for the status of the Facility to change.
The Pending Chain holds Transactions waiting to enter an Interrupt Mode PREEMPT Block. A Transaction which attempts to enter an Interrupt Mode PREEMPT Block on behalf of a Facility is refused entry to the PREEMPT Block if any preempted Transactions are on the Interrupt Chain of the Facility. A Transaction which is refused entry is placed on the Pending Chain of the Facility. This causes the Transaction to come to rest in the simulation. When a Transaction gives up ownership of the Facility, the first Transaction on the Pending Chain is given ownership and allowed to enter the PREEMPT Block.
The Interrupt Chain is a list of preempted Transactions. When a Transaction enters a PREEMPT Block, and the Facility is currently owned by another Transaction, ownership is given to the new Transaction. The old Transaction is placed on the Interrupt Chain so that its ownership may be restored later. Transactions on one or more Interrupt Chains can still move in the simulation, however, their movement is restricted. Such a Transaction cannot exist on the FEC and cannot leave an ASSEMBLE, GATHER, or MATCH Block where it has been put in a Match Condition. When a Transaction gives up ownership of the Facility, if the Pending Chain is empty, the first Transaction on the Interrupt Chain is given ownership of the Facility.
The Delay Chain holds Transactions waiting for ownership. A Transaction which attempts to enter a SEIZE Block on behalf of a Facility in use is refused entry to the SEIZE Block and is placed on the Delay Chain of the Facility in priority order. Similarly, a Transaction which attempts to enter a Priority Mode PREEMPT Block on behalf of a Facility in use (by a Transaction of equal or higher priority) is refused entry to the PREEMPT Block and is placed on the Delay Chain of the Facility in priority order. This causes the Transaction to come to rest in the active model and a new Active Transaction to be chosen. Then, when a Transaction gives up ownership of the Facility, if the Pending Chain and the Interrupt Chain are empty, the highest priority Transaction on the Delay Chain is given ownership of the Facility.
The Retry Chain is a list of Transactions waiting for a Facility state change. These Transactions are reactivated when the Facility changes from one state to another.
Transactions waiting on a Delay Chain, a Pending Chain, or an Interrupt Chain, or owning a Facility are said to be "in contention" for the Facility. Since a contending Transaction will eventually become the owner of the Facility, contention for a Facility carries the obligation of releasing the Facility. If a Transaction which owns a Facility attempts to leave the simulation by entering a TERMINATE Block or an ASSEMBLE Block, an Error Stop occurs. However, a preempted Transaction is permitted to leave the simulation. Normally, each Transaction remains in contention until it voluntarily enters a RELEASE or RETURN Block on behalf of that Facility. However, PREEMPT and FUNAVAIL blocks have options which can remove other Transactions from contention for a Facility. This removes the obligation to return ownership as well. In fact, a non-contending Transaction which attempts to enter a RETURN or RELEASE Block will cause an Error Stop.
To summarize, when a Facility is freed by an owning Transaction, pending Interrupt Mode PREEMPTs are first to be given Facility ownership, followed by previously preempted Transactions on the Interrupt Chain, followed by Transactions waiting normally in priority order on the Delay Chain. When a new owner is chosen from the Delay Chain or the Pending Chain, it enters the SEIZE or PREEMPT Block immediately, and then is scheduled by being placed on the CEC behind its priority peers.
Each Storage Entity has two Transaction chains. These chains are linked lists of Transactions:
·DELAY CHAIN - A priority chain of Transactions waiting for storage units.
·RETRY CHAIN - A list of Transactions which are waiting for the status of the Storage Entity to change.
The Delay Chain holds Transactions waiting for storage units. When a Transaction attempts to enter an ENTER Block on behalf of a Storage Entity, its storage demand is compared to the number of storage units available. The maximum available is defined in a STORAGE Command. If the storage demand cannot be satisfied, the Transaction is refused entry to the ENTER Block and is placed on the Delay Chain of the Storage in priority order. This causes the Transaction to come to rest in the simulation. A new Active Transaction is chosen. Then, when a Transaction gives up storage units, the Delay Chain is scanned in priority order, reactivating Transactions whose storage demands can be satisfied. A "first fit with skip" discipline is used. Each Transaction, in turn, is tested. If its demand can be satisfied it is allowed to enter the ENTER Block and is placed on the CEC behind its priority peers. If its demand cannot be satisfied, it remains on the Storage Entity’s Delay Chain.
The Retry Chain is a list of Transactions waiting for a Storage Entity state change. These Transactions are reactivated when the Storage Entity changes from one state to another.
Each Userchain Entity contains a Transaction chain called a User Chain. For a more detailed explanation of the entity type, Userchain, please refer to Chapter 4. Here we discuss the Transaction chain, called the User Chain, which is contained in each Userchain entity.
User chains are linked lists of Transactions which have been removed from the Current Events Chain by a LINK Block. Traditionally, there have been two uses for User Chains.
First, it is possible to implement extremely complex Transaction scheduling disciplines with User Chains. This can be done by assigning a numerical order value to a Transaction Parameter before LINKing the Transaction on a User Chain.
Second, older implementations of GPSS suggest that User Chains be used to avoid scheduling inefficiencies in the GPSS processor. This is less true in GPSS World because blocked Transactions do not remain on the CEC in GPSS World. However, it is still more efficient to avoid testing conditions which cannot possibly result in a successful test. In this case, you can place the blocked Transaction(s) on a User Chain until there is a possibility of success.
It is convenient to think of a GPSS simulation as a set of Transactions which occupy Blocks in a Block Diagram. Both the Block Input Window and the Blocks Window are essentially Block Diagrams. At any one time, every Transaction is in exactly one Block, but most Blocks may contain any number of Transactions. Each Transaction, in turn, gets an opportunity to move according to a prescribed path through the Block Diagram. When a Transaction is refused entry to a Block, it must wait in its current Block until conditions become favorable for its movement. The part of GPSS World that is responsible for this movement is called the Transaction Scheduler. Each Block type has its own routine which is executed when a Transaction attempts to enter that Block type. It is the job of the Transaction Scheduler to call the appropriate routine.
The first thing the Transaction Scheduler does is to identify the "Active Transaction". If the CEC is not empty, the highest priority, head-of-line, Transaction on the CEC becomes the Active Transaction. If the CEC is empty, the Transaction Scheduler replenishes the CEC with the Transaction(s) from the FEC with the lowest time value. This action also updates the system clock.
The Transaction Scheduler then tries to move the Active Transaction as far as it can in the simulation. In effect, the Transaction Scheduler removes the Active Transaction from the CEC, calls the routine for the next sequential Block (NSB), and unless something extraordinary occurs, replaces the Transaction in front of its peers (i.e. same priority) on the CEC. This gives higher priority Transactions a chance to move in the simulation. The CEC replacement can be modified by PRIORITY and BUFFER blocks. After a Transaction enters a BUFFER Block, it is replaced behind its peers on the CEC. BUFFER blocks are useful if a reactivated Transaction must get ahead of the Transaction which reactivated it. Other blocks can interfere with the replacement of a Transaction on the CEC. For example, ADVANCE(+) (i.e. positive time increment) calculates a scheduled time and places the Transaction on the FEC. Other blocks such as LINK, ENTER, SEIZE, and PREEMPT can cause the Active Transaction to come to rest on a Transaction Chain.
Removal from or replacement to the CEC has no effect on the system clock. The simulated time remains the same until there are no Transactions left on the CEC. Continual replacement of the Active Transaction on the CEC gives newly reactivated higher priority Transactions on the CEC a chance to become the Active Transaction. When the Active Transaction comes to rest on a Delay Chain or cannot move because of some other condition, the Transaction Scheduler chooses another Active Transaction and attempts to move it in the simulation.
Transactions must be on the CEC in order to move. Even PREEMPTed or DISPLACEd Transaction must become the Active Transaction before they can attempt entry into their new destination Block.
Since a Transaction may be refused entry into a Block, a Transaction scheduling may not lead to a Block entry. For this reason, most simulations have fewer Block entries than Transaction schedulings. On the other hand, EXECUTE blocks can cause additional Block entries.
When the Active Transaction attempts to enter a Block, the Transaction Scheduler calls the Block routine associated with the next Block type. It is the Block routine which decides whether or not the Transaction can enter the Block. Several Block types can refuse to allow the Transaction to enter. These are: ENTER, SEIZE, PREEMPT, GATE, TEST. In addition, if the Transaction has not cleared all its preemptions, it will be refused by ADVANCE(+) Blocks and will not be allowed to leave ASSEMBLE, GATHER, or MATCH Blocks.
When the Active Transaction cannot enter any Block it is said to "come to rest" within the simulation. It is then removed from the CEC and placed on one of the Transaction chains discussed above. Then, a different Transaction is chosen to be the Active Transaction.
The Active Transaction is "blocked" when it must wait for one or more entities to change state. GATE, TEST, TRANSFER BOTH, and TRANSFER ALL Blocks can require that specific conditions be met at one or more entities before the Active Transaction is allowed to proceed in the model. Each entity has a Retry Chain for Transactions which were blocked while trying to enter one of the above GPSS Blocks. When the state of the entity is changed by some other Transaction, all Transactions on the associated Retry Chain are replaced on the Current Events Chain behind their priority peers.
Reactivation is the movement of blocked Transactions to the CEC. If the Active Transaction changes the state of an entity, it is possible that one or more Transactions will be reactivated before the Active Transaction attempts to enter its next Block. If a higher priority Transaction is reactivated, it will become the Active Transaction. If you wish a newly reactivated Transaction to progress immediately, you must either place the active Transaction on the CEC behind its priority peers (BUFFER or PRIORITY Block, BU option), or you must cause the reactivated Transaction to have a higher priority than the old active Transaction. When a reactivated Transaction becomes the Active Transaction, the original blocking test is retried.
A Transaction is not permitted to be blocked on a test which will never be retried. This will lead to an Error Stop.
If an entity state changes more than once before the system clock is updated, some states may not be detected. This can happen if the entity state is changed twice before the suspended blocked Transaction tests the condition. Usually, this possibility can be excluded by careful use of the BUFFER Block.
Do not use TEST or GATE Blocks in Refuse Mode, or TRANSFER (BOTH or ALL) to Block on User Variables. Transactions cannot be Blocked on Named Values because the latter do not have a Retry Chain. If you need to react to values achieved by an integrated variable, you should associate one or two Transaction generation thresholds with the User Variable. You can do this in the INTEGRATE Command. Otherwise, use a Savevalue Entity instead of a User Variable.
An Assembly Set is a collection of Transactions. Transactions in the same Assembly Set are said to be related. When each Transaction is created, it is given an integer denoting its Assembly Set. Transactions created by GENERATE Blocks are given distinct integers starting with 1. Transactions created by SPLIT Blocks are given the Assembly Set of their parent.
A Transaction can change its Assembly Set by entering an ADOPT Block.
Assembly Sets are useful for causing synchronization among Transactions. It is easy to create, wait for, and destroy related Transactions in a simulation. This makes it easy to represent processes which at some point must wait for certain events to occur. The following GPSS blocks are used for that purpose:
·ADOPT - Set Transaction’s Assembly Set.
·ASSEMBLE - Wait for and destroy related Transactions.
·GATHER - Wait for related Transactions.
·MATCH - Wait for related Transaction to reach conjugate MATCH Block.
·SPLIT - Create related Transactions.
Preemption is the replacement of one Transaction which owns a Facility by another Transaction. The old Transaction is removed from ownership of the Facility and is placed on the Interrupt Chain of the Facility. The new Transaction becomes the owner of the Facility. Preemption occurs when a Transaction enters a PREEMPT Block, and differs depending on the mode of the PREEMPT Block.
PREEMPT Blocks operate in either "Priority Mode" or "Interrupt Mode". In either case, if a Transaction is preempted, it is placed on the Interrupt Chain of the Facility and ownership is given to the Active Transaction. However, the behavior of the two modes differs when the Active Transaction cannot gain ownership of the Facility. If the Active Transaction attempts to enter a Priority Mode PREEMPT Block, and the Facility is owned by another Transaction of equal or higher priority, the Active Transaction comes to rest on the Facility’s Delay Chain FIFO (first in, first out) within priority. If the Active Transaction attempts to enter an Interrupt Mode PREEMPT Block, and there already is a Transaction preempted at the Facility, the Active Transaction comes to rest on the Pending Chain of the Facility.
PREEMPT and DISPLACE Blocks are provided to disrupt service periods. It is common to simulate a service time by having a Transaction SEIZE a Facility and then enter an ADVANCE Block with a positive time argument. When a Transaction is PREEMPTed at any Facility, or if it is DISPLACEd, if it is on the FEC it must be removed. This is done without changing the system clock. Since you may choose to continue a service period where you left off, a residual time is saved when a Transaction is removed from the FEC due to a DISPLACE, PREEMPT or FUNAVAIL Block entry. The residual time is the scheduled Transaction time (BDT) minus the current system clock time. This time is saved automatically and, in addition, it may be stored in a Transaction parameter. When the Transaction regains ownership of all Facilities for which it contends, it may be automatically rescheduled on the FEC using the residual time. If you choose, you may control this process explicitly by the options available in PREEMPT and FUNAVAIL statements.
A preempted Transaction cannot exist on the FEC. If a Transaction on the FEC is preempted, it is removed from the FEC and placed on a Facility’s Interrupt Chain. If a preempted Transaction attempts to enter an ADVANCE Block with a positive time increment, it is refused entry. When all preemptions have been cleared for a Transaction, it may then enter an ADVANCE(+) Block. If the PREEMPTed Transaction has been removed from contention for the Facility with the RE option of the PREEMPT Block, the preempted Transaction is not restricted from the FEC.
Preempted Transactions can move through the simulation and can be preempted from any number of Facilities. A Transaction is represented on the Interrupt Chain of each Facility where it has been PREEMPTed. Since a PREEMPTed Transaction may still move through the simulation, a Transaction may be PREEMPTed from any number of Facilities at any one time. However, a Transaction cannot SEIZE or PREEMPT a Facility at which it is currently PREEMPTed.
A Transaction may be displaced from one Block and moved to another. If a Transaction is on the FEC, CEC, a Delay Chain, a Pending Chain, or a User Chain and is PREEMPTed by a PREEMPT Block with a C operand, or is displaced by a DISPLACE Block, it is removed from the original chain, scheduled for a new Block, and placed on the CEC.
A preempted Transaction, which is still in contention for a Facility, cannot enter a TERMINATE Block. Such Transactions must enter a RELEASE or RETURN Block before they are permitted to TERMINATE. Alternately, if you intend to TERMINATE a preempted Transaction, you could remove the Transaction from contention for the Facility using the RE option in the PREEMPT Block.
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