Re: Is automatic yield support feasible for threads?

[Philipp <p.stephani2@gmail.com>]

> Am 04.11.2021 um 18:10 schrieb ndame via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>:
> 
> AFAIK Emacs has cooperative threading, so a badly behaving thread can prevent UI updates, therefore if some costly operation is moved into a separate thread then the code has to be modified in order to yield periodically if the code doesn't do I/O.
> 
> If that's the case then what if the interpreter itself could support automatic yielding when some flag is set, so it could yield automatically after every several instructions?
> 
> For example, make-thread could have an optional argument specifying after how many instructions a thread-yield should be called automatically and if this argument is set then a global flag is set, so the interpreter knowns it should call yield regularly.
> 
> This automatic yielding would have some cost, but in return any code could safely be moved into a separate thread without the danger of locking up the UI if this optional argument is given to make-thread. So it could makes using cooperative threads easier.
> 
> Could such an automatic yield support be feasible for threads?

I don't think it's feasible.  Emacs Lisp code typically relies heavily on mutating global state (think buffer contents, the buffer list, `point', dynamic variables, ...), and normally doesn't put such state mutations into properly synchronized critical sections.  Yielding from such unprotected critical sections would result in very subtle bugs (`point' randomly moving around, corruption of internal data structures, buffers vanishing surprisingly, ...).