Re: Performance asymmetry with overlays

[Luc Teirlinck]

Apparently your particular test runs a lot faster with overlays
centered at point-max.  I do not know whether that is really a general
rule, however.  Somebody who knows more about the internal
implementation of overlays than I do should comment on that.

The folllowing variation on your test contains only one single added
line:

(overlay-recenter (point-max)) 

With this change, execution speed seems to be essentially independent
of the starting position.

(defun overlay-test ()
  (interactive)
  (let ((start (point-min))
        (end (point-max))
        ov)
    (goto-char start)
    (overlay-recenter (point-max))
    (beginning-of-line)
    (while (and (not (eobp)) (< (point) end))
      (setq ov (make-overlay (point) (1+ (point))))
      (overlay-put ov 'display overlay-test-string)
      (forward-line))))