[gambit-list] is this a problem in my code?

Álvaro Castro-Castilla alvaro.castro.castilla at gmail.com
Thu Oct 4 08:24:47 EDT 2012


I'm trying to figure out a parallel implementation of for-each (I'm going
to use it for spawning system processes mostly).
Anyway, the problem is like this: If I use any kind of recursion inside the
thread created in the (new-thread) proc, I get this error:
*** ERROR IN for-each -- Uncaught exception: #<type-exception #31>
(thread-join! '#<thread #32>)

As soon as I remove the call to (recur) (it would happen also with letrec
for instance), it works as expected. I really don't understand how internal
recursion in the thread could yield this error.

Thanks for your help

(define (parallel-for-each f l #!key (max-thread-number 2))
  (let* ((pending-jobs l)
         (jobs-mutex (make-mutex))
         (available-jobs? (lambda ()
                            (mutex-lock! jobs-mutex)
                            (let ((more? (not (null? pending-jobs))))
                              (mutex-unlock! jobs-mutex)
         (next-element (lambda ()
                         (mutex-lock! jobs-mutex)
                         (let ((next (car pending-jobs)))
                           (set! pending-jobs (cdr pending-jobs))
                           (mutex-unlock! jobs-mutex)
         (results '())
         (results-mutex (make-mutex))
         (add-to-results! (lambda (r)
                            (mutex-lock! results-mutex)
                            (set! results (cons r results))
                            (mutex-unlock! results-mutex))))
    (let ((new-thread
           (lambda ()
             (thread-start! (make-thread
                             (lambda ()
                               (let recur ()
                                 (add-to-results! (f (next-element)))
                                 (if (available-jobs?)
      (let do-times ((n 0)
                     (thread-pool '()))
        (if (< n max-thread-number)
            (do-times (++ n)
                      (cons (new-thread) thread-pool))
            (for-each thread-join! thread-pool))))
    (reverse results)))
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