bug#46494: 28.0.50; [native-comp] Problems with async background compile

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 46494@debbugs.gnu.org
> Date: Tue, 16 Mar 2021 16:53:21 +0000
> From:  Andrea Corallo via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Hi Andy,
> 
> is there anything left to be done for this bug?

I'm not Andy (so let's wait for him to speak up), but here's my take.

This bug had 3 parts:

  a) Bug #46256 describes problems with AOT compiled native-comp emacs not
     finding prebuilt .eln files when built for mingw64 64bit on Windows.

     As a result, emacs complains with an echo area warning for every .eln
     file that it cannot find in the expected location.

     The stream of frequent warnings that causes make emacs mostly
     unresponsive to user input.

  b) The "background" async compilation of .eln files is CPU intensive and
     somewhat slow. The default settings run a compile on every available
     core, which is unfriendly for other workloads running on the same
     machine.

     It would be helpful to users to have a command to show the state of
     the async background compilation, including the running compile
     processes and the queue of pending compilation requests.

  c) Quitting emacs when async compilation processes are running sometimes
     causes crashes in the compile processes, which show the emacs abort
     dialog (once for each async process). The dialogs disappear after a
     short delay (presumably due to the parent emacs having exited).

c) Has been solved.
b) doesn't seem to be a problem IME, we use half the cores, and
   there's a way to customize that number
a) I didn't see at all, so I think it's also solved.