Help needed to debug (very) slow markdown-mode

Question

I come back to emacs after an eternity without using it. I used it before but now that I'm a grown up I'd like to understand what I do and how to do stuff.

I use emacs mainly for markdown edit at the moment. I have absolutely no problem with the markdown mode I use... until I create a link [foo](url). After that emacs becomes so slow that it's almost impossible to use. I think it may come from my .emacs configuration but I don't know what to do there. I tried to change the markdown mode I used and the result is the same (switching from poly-markdown to markdown-mode+).

Could please help me to learn how to investigate and (if possible) solve that kind of issue when it happens?

## Edit:

Thanks to @Stefan I learnt how to run the profiler and check what's taking all the resources. Obviously it's somehow due to markdown--first-displayable ... char-displayable-p. What can I do now? What are the options?

- ...                                                            1294  99%
 - jit-lock-fontify-now                                          1256  96%
  - jit-lock--run-functions                                      1256  96%
   - run-hook-wrapped                                            1256  96%
    - #<compiled 0x294f1c9>                                      1256  96%
     - font-lock-fontify-region                                  1256  96%
      - font-lock-default-fontify-region                         1256  96%
       - font-lock-fontify-keywords-region                       1256  96%
        - markdown-fontify-inline-links                          1256  96%
         - markdown--first-displayable                           1256  96%
          - cl-find-if                                           1256  96%
           - apply                                               1256  96%
            - cl-find                                            1256  96%
             - apply                                             1256  96%
              - cl-position                                      1256  96%
               - cl--position                                    1256  96%
                - char-displayable-p                             1256  96%

Answer 1

To debug performance problems, one approach is to use the built-in profiler:

M-x profiler-start RET RET
<reproduce the slowdown, ideally for a good 10s or so>
M-x profiler-report RET

Then browse the report (C-u RET is an important command there) to see where all that time is spent.

Seeing your profiler report, you'll probably want to file a bug report with the markdown-mode maintainer to see how to change the code of markdown-fontify-inline-links to try and make it faster.

In the mean time you can try a quick memoizing hack:

(defvar markdown--first-displayable-cache (make-hash-table :test #'equal))

(defun markdown--first-displayable (seq)
  "Return the first displayable character or string in SEQ.
SEQ may be an atom or a sequence."
  (let ((c (gethash seq markdown--first-displayable-cache t)))
    (if (not (eq c t))
        c
      (puthash seq
               (let ((seq (if (listp seq) seq (list seq))))
                 (cond ((stringp (car seq))
                        (cl-find-if
                         (lambda (str)
                           (and (mapcar #'char-displayable-p (string-to-list str))))
                         seq))
                       ((characterp (car seq))
                        (cl-find-if #'char-displayable-p seq))))
               markdown--first-displayable-cache))))