Re: Make computational threads leave user interface usable

[Paul Pogonyshev]

And, of course, I forgot to attach the patch.

Paul

On 1 November 2017 at 16:06, Paul Pogonyshev <address@hidden> wrote:
> Since several versions Emacs has Lisp threads, but they are not used
> much, because 1) only one thread executes at any given time; 2)
> threads yield control to each other only with explicit (thread-yield)
> call or IO blocks. Which means that it is pointless to start a new
> thread for heavy computation: it will lock UI until finished anyway.
>
> Attached patch tries to solve point 2 only by making threads
> automatically yield control to each other from time to time. The patch
> is mainly for discussion.
>
> To see its effect, evaluate this expression:
>
>     (make-thread (lambda ()
>                    (dotimes (n 10000000)
>                      (when (= (% n 1000000) 0)
>                        (message "%s" n)))
>                    (message "done")))
>
> In normal Emacs, UI is frozen until the thread completes. You see
> messages in the echo area, but that's rather a special case: you
> cannot e.g. navigate or type in the current buffer.
>
> With the patch, however, computation thread periodically (and
> automatically: no alteration of the expression is needed) yields to UI
> thread, leaving Emacs responsive while computation is going on.
>
> There are some problems, though.
>
> * Computation is 3-4 times slower than without auto-yielding. You can
> compare to unpatched Emacs or bind `thread-inhibit-auto-yield' to t in
> the thread function. This is probably due to the fact it auto-yields
> ~50 times per second. But on the other hand, does it really have to be
> that slow? I don't know much about Emacs internals, maybe someone with
> more knowledge can say if it is unavoidable, or yielding is just not
> optimized because it is just not done that frequently currently.
>
> * Message buffer contents seems screwed. But this is probably
> "normal", as non-main threads shouldn't touch UI as I understand. This
> expression is just an example.
>
> * Variable `thread-auto-yield-after' is accessible from Lisp, but
> rebinding doesn't take effect immediately. Which is especially bad if
> you rebind from nil to a non-nil value.
>
> In general, what are the thoughts about the patch? Does it look
> interesting, or is auto-yielding simply out of question?
>
> Paul
>
> P.S. Please CC me on replies, I'm not subscribed to the list.

auto-yielding.diff
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