Re: NS port: How to debug excessive garbage collection?

[Alex Gramiak <agrambot@gmail.com>]

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Keith David Bershatsky <esq@lawlist.com> writes:

> The attched patch (debug_mwe_001.diff) is a minimal working example that demonstrates the issue on all three platforms (X11, W32, NS) using the GUI version of Emacs built from the master branch as of 04/08/2019 (a038df77de7b1aa2d73a6478493b8838b59e4982).
>
> The snippet that I am pasting and evaluating in the minibuffer is as follows:
>
> (progn
>   (find-library "simple")
>   (fundamental-mode)
>   (blink-cursor-mode -1)
>   (global-eldoc-mode -1)
>   (setq timer-list nil
>         timer-idle-list nil
>         crosshairs t))
>
> Here are Youtube links to screen recordings of the minimal working example on all three platforms:
>
> NS (screen recording):  https://youtu.be/4IzXfP2j2GY
>
> X11 (screen recording):  https://youtu.be/zrRH72qdmx0
>
> W32 (screen recording):  https://youtu.be/cfIG4fbkesY
>
> MINIMAL WORKING EXAMPLE:
>
> 1.  Imaginary / Pretend:  We imagine that all fake cursors are erased at the outset of update_window while w->current_matrix is still valid; i.e., before scrolling_window does its thing.
>
> 2.  The master cache of fake cursors (w->ch_cache) is set to Qnil.
>
> 3.  Imaginary / Pretend:  We imagine .... As to the rows in the w->desired_matrix that must be updated with update_text_area, we draw fake cursors immediately after draw_glyphs finishes updating the row.  As we are laying fake cursors, we use a temporary cache (w->mc_temp_cache) to store the relevant data so that we can redraw any fake cursors that get erased because they are left/right_overwritten as determined by draw_glyphs.  Once the row has been updated with fake cursors, we set the termporary cache (w->mc_temp_cache) to Qnil and we set the master cache with the new data -- appending new data if the cache is non-nil.
>
> 4.  As to all remaining rows that are not updated with update_text_area (which uses w->desired_matrix), we use the w->current_matrix and draw/cache the fake cursors using the same approach as mentioned in the preceding step; i.e., a temporary cache (w->mc_temp_cache) so that we can fix any fake cursors that got left/right_overwritten and then that cache is set to Qnil and the master cache (w->mc_cache) is updated.
>
> In the above minimal working example, the window Lisp_Object caches are rather simple:
>
> '((make_fixnum (1))
>   (make_fixnum (2))
>   (make_fixnum (3))
>   ...
>   (make_fixnum (99)))

If I'm looking at this right, the float issue is now resolved, but
there's still a cons cell issue. A way to fix this in the MWE is to
reuse the cons cells rather than making new cells each time. I've
included a diff below (that just includes the parts I touched on your
diff) that should solve, or at least mitigate, your problem. Hopefully
it's useful.

Looking at your actual implementation though, I don't see a reason for
your cache to be a Lisp_Object at all; am I missing something? It seems
that you could just use a C array of length 14. Not making any cons
cells at all is the best approach if you can do so.


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diff --git a/src/dispnew.c b/src/dispnew.c
index ccb08ec1b9..068296212f 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -3497,6 +3497,40 @@ update_window (struct window *w, bool force_p)
 
     set_cursor:
 
+
+/* *************************************************************************** */
+/* MULTIPLE-CURSORS */
+
+   for (struct glyph_row *desired_row = MATRIX_ROW (desired_matrix, 0);
+        desired_row < end
+        && (force_p || !input_pending)
+        && BUFFERP (w->contents)
+        && (!NILP (BVAR (XBUFFER (w->contents), crosshairs)));
+        ++desired_row)
+    {
+      if (desired_row->enabled_p)
+        continue;
+      wset_mc_temp_cache (w, Qnil);
+      int vpos = MATRIX_ROW_VPOS (desired_row, desired_matrix);
+      struct glyph_matrix *current_matrix = w->current_matrix;
+      struct glyph_row *current_row = MATRIX_ROW (current_matrix, vpos);
+      if (current_row->enabled_p)
+        {
+          Lisp_Object head = w->mc_temp_cache;
+          // Replace each element with the Lisp integer position
+          for (int i = 0; CONSP (head); ++i, head = XCDR (head))
+            XSETCAR (head, make_fixnum (i));
+        }
+    }
+
+   Lisp_Object head = w->ch_cache;
+   // Replace each element with the Lisp integer position
+   for (int i = 0; CONSP (head); ++i, head = XCDR (head))
+     XSETCAR (head, make_fixnum (i));
+
+/* *************************************************************************** */
+
+
       /* Update the header line after scrolling because a new header
 	 line would otherwise overwrite lines at the top of the window
 	 that can be scrolled.  */
diff --git a/src/window.c b/src/window.c
index ef2ed63850..6ee1f65d9f 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4252,6 +4252,9 @@ make_window (void)
   w->scroll_bar_width = -1;
   w->scroll_bar_height = -1;
   w->column_number_displayed = -1;
+  /* Your MC caches -- note there's only 2 lists ever created for them */
+  w->ch_cache = Fmake_list (make_fixnum (100), Qnil);
+  w->mc_temp_cache = Fmake_list (make_fixnum (100), Qnil);
   /* Reset window_list.  */
   Vwindow_list = Qnil;
   /* Return window.  */