Re: [PATCH v3 0/8] Fix GIT_CEILING_DIRECTORIES that contain symlinks

[Michael Haggerty <mhagger@alum.mit.edu>]

On 10/29/2012 01:15 AM, David Aguilar wrote:
> On Sat, Oct 20, 2012 at 11:51 PM, Junio C Hamano <gitster@pobox.com> wrote:
>> Michael Haggerty <mhagger@alum.mit.edu> writes:
>>
>>> This patch series has the side effect that all of the directories
>>> listed in GIT_CEILING_DIRECTORIES are accessed *unconditionally* to
>>> resolve any symlinks that are present in their paths.  It is
>>> admittedly odd that a feature intended to avoid accessing expensive
>>> directories would now *intentionally* access directories near the
>>> expensive ones.  In the above scenario this shouldn't be a problem,
>>> because /home would be the directory listed in
>>> GIT_CEILING_DIRECTORIES, and accessing /home itself shouldn't be
>>> expensive.
>>
>> Interesting observation.  In the last sentence, "accessing /home"
>> does not exactly mean accessing /home, but accessing / to learn
>> about "home" in it, no?
>>
>>> But there might be other scenarios for which this patch
>>> series causes a performance regression.
>>
>> Yeah, after merging this to 'next', we should ask people who care
>> about CEILING to test it sufficiently.
>>
>> Thanks for rerolling.
> 
> GIT_CEILING_DIRECTORIES was always about trying to avoid
> hitting them at all; they can be (busy) NFS volumes there.
> 
> Here's the description from the 1.6.0 release notes:
> 
> * A new environment variable GIT_CEILING_DIRECTORIES can be used to stop
>   the discovery process of the toplevel of working tree; this may be useful
>   when you are working in a slow network disk and are outside any working tree,
>   as bash-completion and "git help" may still need to run in these places.
> 
> In 8030e44215fe8f34edd57d711a35f2f0f97a0423 Lars added
> GIT_ONE_FILESYSTEM to fix a related issue.
> Do you guys have GIT_CEILING_DIRECTORIES set too?
> 
> We use GIT_CEILING_DIRECTORIES and I'm pretty sure
> we don't want every git command hitting them, so this would
> be a regression when seen from the POV of our current usage
> of this variable, which would be a bummer.

I would certainly withdraw the patch series if it causes a performance hit.

The log message of the original commit (0454dd93bf) described the
following scenario: a /home partition under which user home directories
are automounted, and setting GIT_CEILING_DIRECTORIES=/home to avoid
hitting /home/.git, /home/.git/objects, and /home/objects (which would
attempt to automount those directories).  I believe that this scenario
would not be slowed down by my patches.

How do you use GIT_CEILING_DIRECTORIES that the proposed changes cause a
slowdown?

> Is there another way to accomplish this without the performance hit?
> Maybe something that can be solved with configuration?

Without doing the symlink expansion there is no way for git to detect
that GIT_CEILING_DIRECTORIES contains symlinks and is therefore
ineffective.  So the user has no warning about the misconfiguration
(except that git runs slowly).

On 10/29/2012 02:42 AM, Junio C Hamano wrote:
> Perhaps not canonicalize elements on the CEILING list ourselves? If
> we make it a user error to put symlinked alias in the variable, and
> document it clearly, wouldn't it suffice?

There may be no other choice.  (That, and fix the test suite in another
way to tolerate a $PWD that involves symlinks.)

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/