Here's a simple version using a single process and two resources. ;;; Noels' e-mail: ;;; I am trying to define the following producer/consumer process using ;;; the simulation package: ;;; ;;; There is a queue of resources of type A. ;;; ;;; Process X takes an A and produces a B (which contains the A).  The B ;;; is placed in a queue of fixed size ;;; ;;; Process Y takes a B, returns the A its queue and throws away the B. ;;; ;;; If any queue is empty a process that needs a resource of that type ;;; must wait.  If any queue is full a process that creates a resource of ;;; that type must wait. ;;; ;;; I can't work out how to model this.  If I model the As and Bs as ;;; resources, I have the problem that "It is an error to attempt to ;;; release more units than the process has allocated." ;;; (<a href="http://planet.plt-scheme.org/package-source/williams/simulation.plt/">http://planet.plt-scheme.org/package-source/williams/simulation.plt/</a> ;;; 2/2/html/simulation-Z-H-8.html#node_sec_7.2.2). ;;; Since X takes A and never releases them, and Y releases As but never ;;; takes them, this is a problem. ;;; ;;; If I schedule As and Bs as events how do I enforce the resource limits? ;;; ;;; This kind of simulation is simple in a message passing thread system ;;; but I don't see how it fits into the simulation package's model. ;;; ;;; Also: the documentation for the simulation package would benefit from ;;; an introductory section that lays out the model used by the package. ;;; ;;; Doug's Response: ;;; I'm not sure what A and B are in you example, but I assume A is something ;;; that is required by both processes - like a work cell or jig - and that ;;; B is something only required by Y.  Also, I'm assuming that X and Y can ;;; be done at independent speeds. ;;; ;;; Let's assume a 'widget' represents the end product/result of X and Y using ;;; an A and a B. (require (planet "simulation-with-graphics.ss"                  ("williams" "simulation.plt"))) (require (planet "random-distributions.ss"                  ("williams" "science.plt"))) (define n 0) (define A #f) (define B #f) (define (scheduler)   (do ((i 1 (+ i 1)))       ((> i n) ())     (wait (random-exponential 1.5))     (schedule now (widget i)))) (define-process (widget i)   ;; Allocate an A for X and Y   (with-resource (A)     (printf "~a: Widget ~a acquires an A~n" (current-simulation-time) i)     (work (random-flat 4.0 8.0)) ; X     ;; Allocate a B for Y     (with-resource (B)       (printf "~a: Widget ~a acquires a B~n" (current-simulation-time) i)       (work (random-flat 8.0 20.0)) ; Y       (printf "~a: Widget ~a releases its B~n" (current-simulation-time) i))     (printf "~a: Widget ~a releases its A~n" (current-simulation-time) i))) (define (run-simulation n-widgets n-As n-Bs)   (with-new-simulation-environment     (set! n n-widgets)     (set! A (make-resource n-As))     (set! B (make-resource n-Bs))     (schedule (at 0.0) (scheduler))     (start-simulation))) (run-simulation 20 4 2) <div class="gmail_quote">On Mon, Jun 23, 2008 at 11:52 AM, Noel Welsh <<a href="mailto:noelwelsh@gmail.com">noelwelsh@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <div class="Ih2E3d">On Mon, Jun 23, 2008 at 4:53 PM, Doug Williams <<a href="mailto:m.douglas.williams@gmail.com">m.douglas.williams@gmail.com</a>> wrote: > I'll reload 3.72 on one of my machines and send you a quick solution.  I'll > still having problems with V4.0 (and V4.0.1) and haven't gotten the > simulation and inference collections ported yet. </div>Don't worry about it too much.  My problem is so simple that it might be easier to just implement it directly. N. </blockquote></div>