Re: Suggestion: vc-hooks.el: Make vc-handled-backends more conservative?

[T. V. Raman]

Stephane --
Here is what caused me to chase down the problem.

Consider an environment where your home dir (and all users home
dirs) come from nfs and are visible as /home/user

Also assume that stat calls on nfs dirs  is slow.

I found that everytime I opened a file or saved a buffer I saw
slowness. If the nfs filers were having a bad day then things in
emacs got really really bad.

I also observed by tailing /var/log/messages that the machine was
waiting on nfs responses for stat calls on /home/MCVS and
/home/{arch} --- neither of which existed --- but that still
meant you waited for the nfs system to say so.

After grepping around for {arch} and MCVS in the elisp sources I
saw that they showed up in VC -- I then toggled debug on quit
(which is what I should have done int he first place)--
saw where emacs was blocked, and the rest was easy --- since the
call in vc-find-root was the one that was initiating the stat
call on /home/MCVS and /home{arch} and since the code indicated
that the cleanest way to turn it off (clean == closest to point
of problem) was vc-ignore-dir-regexp that was the variable I
ended up setting.

Hope this helps--

>>>>> "Stefan" == Stefan Monnier <address@hidden> writes:
    >> The last two --arch and MCVS have a particularly bad
    >> performance penalty on systems that have NFS mounts in
    >> places other than /net or /afs --- se variable
    >> vc-ignore-dir-regexp
    Stefan> 
    >> Unless one discovers the above variable
    >> (vc-ignore-dir-regexp is declared as a defvar, and not as
    >> defcustom) nfs based systems take a heavy performance hit
    >> --- since function vc-find-root recursively ascends the
    >> directory tree in attempting to check if files are under
    >> version control.
    Stefan> 
    >> Suggestions:
    Stefan> 
    >> A) Possibly make vc-handled-backends more conservative by
    >> pruning it to have RCS CVS and SVN on it by default.
    Stefan> 
    >> B) Make vc-ignore-dir-regexp a customizable option via
    >> defcustom, and add a pointer to it in the docstring for
    >> vc-handled-backends so that users discover its purpose
    Stefan> 
    Stefan> I think B is a good idea in general.  Especially
    Stefan> since vc-ignore-dir-regexp is useful not just for NFS
    Stefan> mounts but for any other circumstance where the user
    Stefan> wants to disable VC in some part of his file-system.
    Stefan> 
    Stefan> I think A might be a good idea as well.  MetaCVS is a
    Stefan> rather obscure revision control system.  GNU Arch is
    Stefan> less so, but it's not that popular either.  And there
    Stefan> are several more still unsupported but whose support
    Stefan> should hopefully come at some point (Mercury, bzr,
    Stefan> git, DaRCS, monotone, codeville, OpenCM,
    Stefan> you-name-it).  So we most likely will not want *all*
    Stefan> supported systems to be "active".  Maybe now is not
    Stefan> the worst time to start down that road.
    Stefan> 
    >> In my case I chased it down by examining the source code
    >> after observing that emacs was making excessive nfs
    >> accesses during file load and save.
    Stefan> 
    Stefan> Just to make sure that it can only be fixed with
    Stefan> something like vc-ignore-dir-regexp, could you
    Stefan> describe the particular circumstance where this
    Stefan> happens, as well as describe as precisely as possible
    Stefan> what actually happens?
    Stefan> 
    Stefan> 
    Stefan>         Stefan

-- 
Best Regards,
--raman

      
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