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Re: Example of threads and concurrency?

From: Eric Abrahamsen
Subject: Re: Example of threads and concurrency?
Date: Sat, 18 Feb 2017 10:43:42 -0800
User-agent: Gnus/5.130014 (Ma Gnus v0.14) Emacs/26.0.50 (gnu/linux)

I floated this on emacs.help, but got no takers. I'm hoping someone here
might be able to help!


Eric Abrahamsen <address@hidden> writes:

> I've read the chapter on threads in the manual, and sort of understand
> it theoretically, but not really in practice.
> Can someone look at the below and tell me if it makes sense? I'm trying
> to make a simple system where there's a global variable, and I update
> the variable with a time-consuming/network-bound computation. I want the
> computation to happen in a thread so it doesn't block, and I want no
> other code to touch the variable while the computation is happening.
> The following works when I run it, but I have no idea if this is
> actually how you do it.
> #+begin_src emacs-lisp
> (defvar important-data-variable nil
>   "Important variable holding important data.
> Our goal is to update this data.")
> (defvar important-data-mutex (make-mutex))
> (defun update-important-data ()
>   (with-mutex important-data-mutex
>    (let ((result '(1 2 3))) ; Slow function here.
>      (setq important-data-variable result))))
> (defun update-data-in-thread ()
>   (let* ((thread (make-thread #'update-important-data))
>        (timer (run-at-time
>                10 nil
>                `(lambda ()
>                   (when (thread-alive-p ,thread)
>                     (thread-signal ,thread 'give-it-up))))))
>     (thread-join thread)))
> #+end_src
> Is everything in the right place? It just occurs to me that If I put
> `thread-join' at the bottom of `update-data-in-thread' then it will
> effectively be synchronous after all! So I put that somewhere else, or I
> don't call it at all.
> Other questions:
> 1. Other code just has to know that it can't touch
>    `important-data-variable' without holding `important-data-mutex',
>    right?
> 2. I want the timer to signal the thread to give up after ten seconds --
>    do I need to put anything in `update-important-data' that handles
>    that signal?
> 3. What do condition vars actually do? The manual has an example that
>    looks just like what I'm doing above:
>    (with-mutex mutex
>      (setq global-variable (some-computation))
>      (condition-notify cond-var))
>    What is cond-var doing here?
> It seems like both mutexes and cond-vars are there so other code knows
> when to keep its hands off `important-data-variable', but I don't 
> understand how they're actually used.
> Any light shed would be very welcome!
> Eric

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