[plt-scheme] Parallel execution

On Sat, 14 Jan 2006, Gregory Woodhouse wrote:

> Okay, one more :-)
>
> Consider the following code
>
> (define t1 '())
> (define t2 '())
> (let loop ((i 0))
>    (begin
>      (if (< i 25)
>          (begin
>            (set! x 10)
>            (set! t1 (thread
>                      (lambda () (set! x (* x x)))))
>            (set! t2 (thread
>                      (lambda () (set! x (+ x 1)))))
>            (begin
>              (sleep 2)
>              (display "x = ")
>              (display x)
>              (newline))
>            (loop (+ i 1))))))


Hi Gregory,

Let's try something:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Sleeps for a random period of time (up to a second), then returns x
(define (identity-with-sleep x)
  (sleep (random))
  x)

(define (test-threading)
  (let ((x 10))
    (thread (lambda () (set! x (* (identity-with-sleep x)
                                  (identity-with-sleep x)))))
    (thread (lambda () (set! x (+ (identity-with-sleep x)
                                  (identity-with-sleep 1)))))
    (sleep 2)
    (printf "x = ~s~%" x)))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


Here's what I see with a few runs:

;;;;;;
> (test-threading)
x = 11
> (test-threading)
x = 11
> (test-threading)
x = 11
> (test-threading)
x = 11
> (test-threading)
x = 110
> (test-threading)
x = 100
> (test-threading)
x = 11
;;;;;;

So this does show that threads can be preempted.  I think your earlier
experiments just need the threads to do enough work to trigger preemption.


I do remember seeing something out of PLT Redex that might be appropriate
to your question; check "A Visual Environment for Developing
Context-Sensitive Term Rewriting Systems":

    http://www.ccs.neu.edu/scheme/pubs/rta2004-mfff.pdf

The part in section 3.3 in that paper talks about getting PLT Redex to
show all the possibilities from concurrency.



> What does PLT stand for anyway?

PLT -- Programming Language Theory.  See:

    http://www.plt-scheme.org/who/


Best of wishes!