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bug#36566: 27.0.50; debug is sometimes horribly slow

From: Michael Heerdegen
Subject: bug#36566: 27.0.50; debug is sometimes horribly slow
Date: Thu, 11 Jul 2019 00:46:00 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Noam Postavsky <address@hidden> writes:

> > sometimes it takes 10 or 20 seconds until the debugger pops up.  I
> > debugged code using widgets (who are complicated long lists).  But both
> > the number of frames and the length of the printed lists were not
> > exorbitant - a quite normal situation.
> Can you give an example recipe that produces the slowness?  It's not
> usually slow, right?  (When I trigger the debugger in master right now,
> I don't notice problem.)

I investigated a bit and found that the main factor of the sluggishness
seems to be my private (global) setting of print-gensym -> t: I can
toggle sluggishness in both the Emacs with my setup and emacs -Q just by
toggling this variable.  My debugger use case involves gensyms (see
later).  Ok, so it's probably mainly a printing issue.

Here is my use case: load the attached file (don't compile, I made it
contain a bug).  M-x find-cmd-widget, and insert a "links" expression
by hitting the INS button and select "links" with the "expr" button.

With print-gemsym -> nil, the debugger needs approx. 2 seconds here to
pop up, and with print-gemsym -> t, approx. 12.  So I wonder why this
setting makes it that slow - I would expect a small time penalty, but
such a big difference?

BTW, extra points if the debugger provided commands/ a menu to tune such
settings (like printing variables) live when using it.

Attachment: find-cmd-widget.el
Description: application/emacs-lisp



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