bug#26402: 25.2; Font rendering extremely slow on any non-default font

[Eli Zaretskii <eliz@gnu.org>]

> From: Gennady Uraltsev <gennady.uraltsev@gmail.com>
> Cc: 26402@debbugs.gnu.org
> Date: Sun, 9 Apr 2017 13:07:03 +0200
> 
> 1) I must admit that I am not very familiar with GDB. I have done some
> programming so I am familiar with the basic concepts but not the tool.
> If you give me an outline of what to do I can try to do it (I do not
> mean step by step instructions, I guess I can figure something out and
> if I really can't I could ask for clarifications).

Thanks.

The function we are interested in is font-info, or Ffont_info on the C
level.  Its code is in the file font.c around line 5100.  It calls
various other functions, like fs_query_fontset and font_open_by_name.
I'd like to know which one of them takes most of the time, then drill
down into that function and find out which of its subroutines takes
most of the time, etc.  One way of doing that is step through the code
and find the line whose execution takes a perceptible time.

Alternatively, you could use the 'perf' utility available on GNU/Linux
systems, to profile the code of Ffont_info and its subroutines and
find out which one takes most of the time.

> I see that on kubuntu 16.10 with emacs 24.5.1 and with emacs25.1.2 the
> problem does not appear: everything works smoothly.

Maybe the fonts available on those systems are different?

> 3) Just a question: is there a way to ask emacs to dump the info about
> the versions of all the libraries it uses?

Yes, the 'ldd' command is your friend.