Re: Example of threads and concurrency?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

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

Thanks,
Eric

Eric Abrahamsen <eric@ericabrahamsen.net> 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