Re: Parallel Threads

[Taylor R Campbell]

> Date: Sat, 28 Jan 2023 02:36:03 +0000
> From: Glenn Takanishi <glenn@neuralmachines.com>
> 
> I using Mit-scheme 12.1 trying to use the program "thread.scm" in the 
> runtime directory.  This program has the procedure "make-thread" in it.  
> But when I tried to run a test with the following program below, it 
> failed with the error message "Unbound variable: make-thread".
> 
> [...]
> ;; (load "/usr/local/mit-scheme/lib/mit-scheme/runtime/thread.scm")
> [...]
> 
> I wondering why this threads extension is not documented as the 
> thread.scm program looks good.  I'd like to see how this runs.
> 
> Can you tell me what I need to add to the test code above to get it to 
> run.

This file, thread.scm, is not an extension -- it's a built-in part of
MIT Scheme.  The make-thread procedure in it is purely internal, not
for use of the API.

To start a thread, use create-thread, like this:

(create-thread #f
  (lambda ()
    (sleep-current-thread 1000)
    (write-line 'hello-world)))

Longer example with a mutex and condition variable:

(define m (make-thread-mutex))
(define c (make-condition-variable))
(define done? #f)
(define t
  (create-thread #f
    (lambda ()
      (sleep-current-thread 1000)
      (with-thread-mutex-lock m
        (lambda () 
          (set! done? #t)
          (condition-variable-broadcast! c))))))
(with-thread-mutex-lock m
  (lambda ()
    (do () (done?)
      (condition-variable-wait! c m))))

FYI: The threads do not actually run in parallel at the moment.  They
are time-shared.  So they are useful for doing I/O asynchronously, but
not for parallelizing computation.  (This might change some day.)