Re: multi-threaded Emacs

[Stefan Monnier <monnier@iro.umontreal.ca>]

>     Actually, yielding in QUIT will take a lot of work.  It's definitely not
>     "cooperative" seen from Elisp's point of view (where QUIT can be run
>     implicitly all over the place).

> I am surprised.  Perhaps you're right, but why do you think so?
> Anywhere that calls QUIT is a place where it is safe for an error to happen,
> so why not a thread switch?

It will be safe in the sense that it won't cause a crash.  But it will
mess up Elisp's semantics.  Consider:

   (dotimes (i 1000) (toto))

if you run this code twice in separate threads and allow context
switches at QUIT, then you'll basically be doing "preemptive
concurrency" seen from Elisp's point of view.  Among other things, the
two threads will be fighting over the value of `i'.

So before this level of concurrency can be made to work we'll need to
fix dynamic scoping to be "per-thread" rather than global.

Note that the `yield' primitive won't be easy to use for the same kind
of reason (until we fix dynamic-scoping that is).


        Stefan