Re: Iteration over frame->face_alist is a huge performance suck

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 01 Jul 2021 20:58:07 -0700
> From: "John Coughlin" <jack@johnbcoughlin.com>
> 
> Recently I have been investigating slowdowns in overall responsiveness
> and snappiness in my emacs setup, which arise during the course of
> normal work. I attached a sampling profiler to the process
> (Instruments on MacOS), and recorded ten or so seconds of
> mashing the movement cursors in my org-agenda window. The result is
> that 93.4% of the total samples are inside of the function
> lface_from_face_name_no_resolve, in xfaces.c. The culprit seems to be
> a large association list, frame->face_list, which in my current
> session contains over 1000 faces.

This is a known problem.  The current implementation of face lookup
doesn't scale well enough to such usage patterns.

> - This may be less of a problem in vanilla emacs, but some packages create faces
>   that result in quite deep recursive calls to merge_named_face. Each such frame
>   in the stack (I observed upwards of 50 such frames with my org-agenda button
>   mashing) is doing its own face lookups.

Yes, and watch out for faces that inherit from other faces, which
themselves inherit from other faces.

> So, what should be done about this?

We have a solution designed and almost implemented: see bug#41200.
Unfortunately, its development stalled.  It would be good to finalize
the code, resolve the issues that were found with it (as discussed in
the bug thread), and install it.