Re: Some git performance measurements..

Linus Torvalds [off-list ref] writes:

Less than a hundredth of a second may not sound much, but when we have 
1700+ directories in the kernel trees, doing that for each possible 
.gitignore file is really really expensive!

The only thing that wants to use excluded() in the unpack_trees()
codepath is the code to allow overwriting an existing, untracked file in
verify_absent().  When we find an untracked file at the same path as we
are just trying to check out a file, we allow overwriting it only if
that file is "ignored".

But the way the unpack_trees_rec() and the gitignore handling in dir.c
are structured currently means we do push/pop exclude-per-directory
stack as we enter and leave a new subdirectory.  We do not do this
lazily on demand.

The newer gitattributes subsystem maintains a similar per-directory data
structure but this is purely done on-demand; until somebody asks "what
are the attrs for this path", we do not read .gitattributes file.  We
should be able to restructure exclude-per-directory code in a similar
way.