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