7 Resources
In a simulation model, a resource represents an entity (or entities) that is/are shared among processes. Each resource has a quantity associated with it that represents the number of (indivisible) units for the resource.
7.1 The resource Structure
| (struct resource (units |
| units-available |
| units-allocated |
| satisfied |
| queue)) |
| units : exact-positive-integer? |
| units-available : natural-number/c |
| units-allocated : natural-number/c |
| satisfied : set? |
| queue : set? |
Represents a resource, possibly with multiple allocatable units, in a resource model. When there are insufficient units available to fulfull a resource request, the request is queues until the number of units requested are available. The queued request may include a priority and the process may reneg on the request based on time or the occurance of a specific event.
units – the total number of units of the resource.
units-available – the number of units of the resource that are currently available, that is, are not currently allocated.
units-allocated – the number of units of the resource that are currently allocated.
satisfied – a set of resource allocations that are currently satisfied.
queue – a set of resource allocations yet to be satisfied.
| (make-resource [units]) → resource? |
| units : exact-positive-integer? = 1 |
Returns a newly created resource instance with the specified number of units. If unit is not specified, 1 is used.
7.1.1 Shortcuts to Resource Set Variables
The resource structure includes two sets to implement the queues of satisfied allocations, the satisfied field, and the waiting allocatons, the queue field. The following functions provide access to the n field of the respective sets to facilitate data collection. See Chapter 9 Sets for more details.
| (resource-queue-variable-n resource) → variable? |
| resource : resource? |
Returns the variable from the n field of the set in the queue field of resource.
| (resource-satisfied-variable-n resource) → variable? |
| resource : resource? |
Returns the variable from the n field of the set in the satisfied field of resource.
7.2 Requesting and Relinquishing Resources
7.2.1 Requesting Resources
| |||
| |||
| |||
| |||
| |||
|
Requests the specified number of units of resource. If the requested number of units are not available, the request is places in the queue of waiting allocations, in the queue field, with the specified priority. The process requesting the resource(s) will delay until the resource(s) are available and allocated to the process. It is an error ro request more units of a resource than exist.
The process may reneg on the request by using the #:leave keyword. The options for the #:leave keyword are the same as for the schedule macro. Note that specifying #:leave (in 0.0) will reneg at the current time, but will allow all now events to be executed, which may relinquish the resource(s) required. This is not the same as specifying #:leave now, which will reneg immediately, and may be more appropriate than checking the units-available field.
| ||||||||||||||||||||||||||||
| resource : resource? | ||||||||||||||||||||||||||||
| units : exact-positive-integer? | ||||||||||||||||||||||||||||
| priority : real? | ||||||||||||||||||||||||||||
| reneg : (or/c real? exact?) | ||||||||||||||||||||||||||||
| (resource-request resource units priority) → any | ||||||||||||||||||||||||||||
| resource : resource? | ||||||||||||||||||||||||||||
| units : exact-positive-integer? | ||||||||||||||||||||||||||||
| priority : real? | ||||||||||||||||||||||||||||
| (resource-request resource units) → any | ||||||||||||||||||||||||||||
| resource : resource? | ||||||||||||||||||||||||||||
| units : exact-positive-integer? | ||||||||||||||||||||||||||||
| (resource-request resource) → any | ||||||||||||||||||||||||||||
| resource : resource? |
Functional equivalent to the request macro, which should be used instead. This is used in legacy code to request resources.
7.2.2 Relinquishing Resources
| (relinquish resource #:units units) |
| (relinquish resource) |
Relinquishes the specified number of units of resource. If the number of units to relinquish is not specified, then all of the units allocated to the process are released. It is an error to attempt to relinquish more units that the process has allocated.
| (resource-relinquish resource units) → any |
| resource : resource? |
| units : exact-positive-integer? |
| (resource-relinquish resource) → any |
| resource : resource? |
Functional equivalent to the relinquish macro, which should be used instead. This is used in legacy code to request resources.
7.2.3 The with-resource Macro
| ||
|
Evaluates the body expressions with the specified number of units of resource.
Since the sequence
| (request resource #:units units) |
| (relinquish resource) |
is by far the most frequent usage of request and relinquish, the with-resource macro is provided to simplify this common usage.
7.3 Example – Resources
This example extends the simple simulation model of Chapters 5 and 6 by adding attendents to the system. Each customer must interact with an attendant. The two attendants are modeled using a single resource with two units. This means there is a single queue for the two attendants. Again, the output is a simple trace of customer arrivals, attendant acquisitions, and departures.
| #lang scheme/base |
| ; Example 2 - Resources |
| (require (planet williams/simulation/simulation)) |
| (require (planet williams/science/random-distributions)) |
| (define n-attendants 2) |
| (define attendant #f) |
| (define (generator n) |
| (for ((i (in-range n))) |
| (wait (random-exponential 4.0)) |
| (schedule now (customer i)))) |
| (define-process (customer i) |
| (printf "~a: customer ~a enters~n" |
| (current-simulation-time) i) |
| (request attendant) |
| (printf "~a: customer ~a gets an attendant~n" |
| (current-simulation-time) i) |
| (work (random-flat 2.0 10.0)) |
| (relinquish attendant) |
| (printf "~a: customer ~a leaves~n" |
| (current-simulation-time) i)) |
| (define (run-simulation n) |
| (with-new-simulation-environment |
| (set! attendant (make-resource n-attendants)) |
| (schedule (at 0.0) (generator n)) |
| (start-simulation))) |
| (run-simulation 10) |
Produces the following output.
| 0.6153910608822503: customer 0 enters |
| 0.6153910608822503: customer 0 gets an attendant |
| 5.599485116393393: customer 1 enters |
| 5.599485116393393: customer 1 gets an attendant |
| 6.411843645405005: customer 2 enters |
| 8.48917994426752: customer 0 leaves |
| 8.48917994426752: customer 2 gets an attendant |
| 10.275428842274628: customer 1 leaves |
| 16.82673428503317: customer 2 leaves |
| 23.525886616767437: customer 3 enters |
| 23.525886616767437: customer 3 gets an attendant |
| 27.18604340910279: customer 3 leaves |
| 32.1644631797164: customer 4 enters |
| 32.1644631797164: customer 4 gets an attendant |
| 33.14558760001698: customer 5 enters |
| 33.14558760001698: customer 5 gets an attendant |
| 39.67682614849173: customer 4 leaves |
| 40.486553934113665: customer 6 enters |
| 40.486553934113665: customer 6 gets an attendant |
| 41.168084930967424: customer 5 leaves |
| 45.72670063299798: customer 6 leaves |
| 46.747675912143016: customer 7 enters |
| 46.747675912143016: customer 7 gets an attendant |
| 49.212327970772435: customer 8 enters |
| 49.212327970772435: customer 8 gets an attendant |
| 50.556538752352886: customer 9 enters |
| 51.46738784004611: customer 8 leaves |
| 51.46738784004611: customer 9 gets an attendant |
| 52.514846525674855: customer 7 leaves |
| 57.02720211166597: customer 9 leaves |
The attendant is declared as a global variable. The actual resource is created (and assigned to the global variable) in run-simulation. The actual usage, the request and relinquish calls, are in the customer process.
A few things to note at this point are:
We went back to a simple function for the generator event.
In this example, the only delay waiting for an attendant is for customer 2, which has to wait for one of the attendants to be relinquished before they can interact.
We explicitly called request and relinquish – mainly to show their usage. We will generally use with-resource in subsequent models.