Re: Emacs 23.0 is much slower starting than Emacs 22.3

[David De La Harpe Golden <david@harpegolden.net>]

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Stefan Monnier wrote:
> 
> Setting vc-handled-backends to nil should indeed stop VC from doing
> those directory walks.  So now I don't understand why the walks done by
> VC are so much cheaper than the ones done by locate-dominating-file.
> Could it be that all the files you opened were under CVS control (so
> the VC search stops before having to walk up the directories)?
> 
> 

Eventually found vc-find-root. So - it versus locate-dominating-file ?

While they're doing a fairly similar job, they do look to me like 
they're implemented importantly differently -locate-dominating-file 
takes a regex and passes it to directory-files for each directory. 
AFAICS directory-files scans the whole directory each time through for 
the match. OTOH, vc-find-root just tests if /path/to/directory/witness 
exists.  The former is rather slower and (I think) consier. I'm not sure 
   that there's any real performance problem with directory-files itself 
for what it actually does, though maybe it could be made faster. Maybe 
not worthwhile though - easier to rewrite locate-dominating-file to not 
use it as below...

Profiling says locate-dominating-file is spending most all its time  in 
directory-files trying that doomed match again and again...

find-file-noselect         1108        48.279532999  0.0435735857
find-file-noselect-1       1108        29.139766999  0.0262994287
after-find-file            1108        26.323623999  0.0237577833
normal-mode                1108        21.492128000  0.0193972274
hack-local-variables       1108        19.570003     0.0176624575
hack-project-variables     1108        19.266695999  0.0173887148
project-find-settings-file 1108        19.254854999  0.0173780279
locate-dominating-file     1108        19.237415000  0.0173622879
directory-files            4152        18.688168999  0.0045010040


So is locate-dominating-file's regex-matching really necessary in 
project-find-settings-file? The only call to locate-dominating-file I 
can see is it just searching for a (hardcoded name) ".dir.settings.el" 
in project-find-settings-file, so vc-find-root's rather simpler witness 
technique would work just fine?

+++ And indeed, attached patch makes emacs23 1108-file-startup rather 
more competitive with emacs22 on my system. Though I didn't do much 
testing to see if project settings functionality was 100% a-okay -
though it seemed to pick up a /usr/local/src/emacs-22.3/.dir-settings.el
I made okay...

emacs   patch 	withsettings 	realtime
22	n/a	n/a		22.853
23	no	no 		38.518
23	no	yes		38.729
23	yes  	no		23.295
23	yes	yes		23.340










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Index: lisp/files.el
===================================================================
RCS file: /sources/emacs/emacs/lisp/files.el,v
retrieving revision 1.1005
diff -U 8 -r1.1005 files.el
--- lisp/files.el	18 Oct 2008 18:40:25 -0000	1.1005
+++ lisp/files.el	24 Oct 2008 23:37:19 -0000
@@ -711,18 +711,59 @@
 (defun locate-file-completion (string path-and-suffixes action)
   "Do completion for file names passed to `locate-file'.
 PATH-AND-SUFFIXES is a pair of lists, (DIRECTORIES . SUFFIXES)."
   (locate-file-completion-table (car path-and-suffixes)
                                 (cdr path-and-suffixes)
                                 string nil action))
 (make-obsolete 'locate-file-completion 'locate-file-completion-table "23.1")
 
-(defun locate-dominating-file (file regexp)
-  "Look up the directory hierarchy from FILE for a file matching REGEXP."
+(defun locate-dominating-file (file name)
+   "Look up the directory hierarchy from FILE for a file named NAME.
+If found, return the file named NAME's full name and path, otherwise
+return nil. See locate-dominating-file-by-regexp if you have more
+complex requirements than a simple whole-string match on the name,
+though beware it is slower."
+   ;; copied from vc-find-root, then changed to return file
+   ;; rather than containing dir.
+   ;; Represent /home/luser/foo as ~/foo so that we don't try to look for
+   ;; witnesses in /home or in /.
+   (setq file (abbreviate-file-name file))
+   (let ((root nil)
+	 (prev-file file)
+	 ;; `user' is not initialized outside the loop because
+	 ;; `file' may not exist, so we may have to walk up part of the
+	 ;; hierarchy before we find the "initial UID".
+	 (user nil)
+	 try)
+     (while (not (or root
+		     (null file)
+		     ;; As a heuristic, we stop looking up the hierarchy of
+		     ;; directories as soon as we find a directory belonging
+		     ;; to another user.  This should save us from looking in
+		     ;; things like /net and /afs.  This assumes that all the
+		     ;; files inside a project belong to the same user.
+		     (let ((prev-user user))
+		       (setq user (nth 2 (file-attributes file)))
+		       (and prev-user (not (equal user prev-user))))
+		     (string-match
+		      ;; from vc-ignore-dir-regexp !
+		      "\\`\\(?:[\\/][\\/]\\|/\\(?:net\\|afs\\|\\.\\.\\.\\)/\\)\\'"
+		      file)))
+       (setq try (file-exists-p (expand-file-name name file)))
+       (cond (try (setq root file))
+	     ((equal file (setq prev-file file
+				file (file-name-directory
+				      (directory-file-name file))))
+	      (setq file nil))))
+     (when root (expand-file-name name root))))
+
+(defun locate-dominating-file-by-regexp (file regexp)
+  "Look up the directory hierarchy from FILE for a file matching REGEXP.
+This function is slower than `locate-dominating-file', only use if necessary."
   (catch 'found
     ;; `user' is not initialized yet because `file' may not exist, so we may
     ;; have to walk up part of the hierarchy before we find the "initial UID".
     (let ((user nil)
           ;; Abbreviate, so as to stop when we cross ~/.
           (dir (abbreviate-file-name (file-name-as-directory file)))
           files)
       (while (and dir
@@ -3154,17 +3195,17 @@
 (defun project-find-settings-file (file)
   "Find the settings file for FILE.
 This searches upward in the directory tree.
 If a settings file is found, the file name is returned.
 If the file is in a registered project, a cons from
 `project-directory-alist' is returned.
 Otherwise this returns nil."
   (setq file (expand-file-name file))
-  (let* ((settings (locate-dominating-file file "\\`\\.dir-settings\\.el\\'"))
+  (let* ((settings (locate-dominating-file file ".dir-settings.el"))
          (pda nil))
     ;; `locate-dominating-file' may have abbreviated the name.
     (if settings (setq settings (expand-file-name settings)))
     (dolist (x project-directory-alist)
       (when (and (eq t (compare-strings file nil (length (car x))
                                         (car x) nil nil))
                  (> (length (car x)) (length (car pda))))
         (setq pda x)))