Re: Scrolling huge buffers and cc-mode

[Eli Zaretskii]

> Date: Tue, 18 Dec 2012 13:56:13 +0400
> From: Dmitry Antipov <address@hidden>
> 
> Attached is a C++ source file to reproduce the painfully slow scrolling
> under cc-mode. Test is simple: just visit it and press <page-down>, or <next>,
> or whatever else bind to scroll-up-command, until buffer end is reached.
> The file is ~900K; when scrolling reaches 50%, everything is stopped, may
> be up to a few minutes (!), there is no response to any keyboard input
> including C-g, and CPU consumption is ~80-90%. After that, scrolling is
> resumed, but it's still _very_ slow. This looks like a C++ issue only
> since xdisp.c of nearly the same size may be scrolled much faster.

(Added Alan to the discussion.)

It looks like this is due to font-lock (invoked by JIT Lock each time
a new screenful is about to be displayed) that is specific to C++.  To
see this, fontify the whole buffer, or turn on JIT Stealth and wait
for it to finish, and then try scrolling -- you will see a normal
scrolling speed.

While JIT Stealth was working, I noticed that one of my cores was busy
100%.  So I turned on the profiler, and the result of running it for
about 1 minute is below.  If I understand correctly, it blames
scan-sexps for most of the CPU load.

  - apply                                                         25518  84%
    - jit-lock-stealth-fontify                                    25517  84%
      - jit-lock-fontify-now                                      25517  84%
        - byte-code                                               25517  84%
          - run-hook-with-args                                    25517  84%
            - font-lock-fontify-region                            25517  84%
              - c-font-lock-fontify-region                        25517  84%
                - font-lock-default-fontify-region                25495  84%
                  - font-lock-fontify-keywords-regio              25495  84%
                    - c-font-lock-enclosing-decls                 25389  83%
                      - c-beginning-of-decl-1                     25388  83%
                        - byte-code                               25388  83%
                          - c-beginning-of-statement              25388  83%
                            - byte-code                           25387  83%
                              - byte-code                         25373  83%
                                  scan-sexps                      25372  83%
                                  scan-lists                          1   0%
                              - c-backward-sws                        5   0%
                                  looking-at                          4   0%
                                  previous-single-pr                  1   0%
                              - c-looking-at-inexpr-                  5   0%
                                  looking-at                          3   0%
                                  c-backward-sws                      2   0%
                              - c-beginning-of-macro                  2   0%
                                  beginning-of-line                   2   0%
                              - c-crosses-statement-                  2   0%
                                  append                              2   0%
                            - c-at-macro-vsemi-p                      1   0%
                              - c-backward-sws                        1   0%
                                  looking-at                          1   0%
                      + c-parse-state                                 1   0%
                    + c-font-lock-declarations                       97   0%
                    + #<compiled 0xea75d7>                            5   0%
                    + cwarn-font-lock-match-referenc                  2   0%
                    + c-font-lock-<>-arglists                         2   0%
                + mapc                                               22   0%
    + auto-revert-buffers                                             1   0%
  + timer-event-handler                                            2496   8%
  + byte-code                                                      1211   3%
  + redisplay_internal (C function)                                 392   1%
    Automatic GC                                                    294   0%
  + call-interactively                                              226   0%
  + jit-lock-stealth-fontify                                         95   0%
  + jit-lock-fontify-now                                             40   0%
  + run-hook-with-args                                                8   0%
  + font-lock-fontify-region                                          3   0%
  + profiler-calltree-walk                                            1   0%
    tooltip-hide                                                      1   0%
  + list                                                              1   0%
  + input-pending-p                                                   1   0%