[racket] Concurrent execution

From: Sam Tobin-Hochstadt (samth at ccs.neu.edu)
Date: Mon Oct 3 08:38:24 EDT 2011

On Mon, Oct 3, 2011 at 2:25 AM, Jukka Tuominen
<jukka.tuominen at finndesign.fi> wrote:
> Sorry for thinking aload again, but I'm learning these issues myself in the background, and I wonder if something like the following might work?
> Concurrent List Processing -idea
> Two phase process
> 1) Distribution phase
> Create a recursive procedure combining cons and threads so that each item in the original list will be replaced by a named thread. Sending out new threads won't wait for individual threads to return in this phase. The result of this phase is a list of named threads.
> 2) Collection phase
> This phase is again a recursive cons process, but this time sequential. It starts from the beginning of the named-thread list, and as soon as the first thread returns its value, it moves on. The process goes on until the end of the list is reached. The resulting list is the return value.

Here's your idea, implemented with futures.  I'm assuming that you
want to apply a particular procedure to each element of the list, as
in `map':

(define (par-map f l)
  (define fs (for/list ([elem (in-list l)]) (future (lambda () (f elem))))
  (map touch fs))

> - If the original list to be processed is very long, then the construction of the second phase list shouldn't wait for bottle neck items to cons the list elsewhere where possible. The speed gain wouldn't be substantial though, since it is just building a list, and you cannot finish it until the last item is available, anyway.

You'd want to basically make the result list into a queue in which
elements are inserted.
sam th
samth at ccs.neu.edu

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