Re: git status: small difference between stating whole repository and small subdirectory

> On Sat, Feb 18, 2012 at 5:25 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> Jeff King <peff@peff.net> writes:
>>
>>> That being said, we do have an index extension to store the tree sha1 of
>>> whole directories (i.e., we populate it when we write a whole tree or
>>> subtree into the index from the object db, and it becomes invalidated
>>> when a file becomes modified). This optimization is used by things like
>>> "git commit" to avoid having to recreate the same sub-trees over and
>>> over when creating tree objects from the index. But we could also use it
>>> here to avoid having to even read the sub-tree objects from the object
>>> db.
>>
>> Like b65982b (Optimize "diff-index --cached" using cache-tree, 2009-05-20)
>> perhaps?
>
> This optimizes the case when a cached tree matches entirely.I wonder
> whether it's faster if we switch to tree-tree diff whenever we find
> valid cached trees. If cache-tree is fully valid, "git diff --cached
> foo" would be equivalent to "git diff HEAD foo".

> I tried "git diff --raw HEAD HEAD~100" (where HEAD was
> v3.1-rc1-272-g73e0881 on linux-2.6) and "git diff --cached --raw
> HEAD~100" with no cache-tree. The former is a little bit faster than
> the latter (177ms vs 275ms). On gentoo-x86, 70k worktree files, it's
> 4.33s vs 4.45s. But in tree-tree diff we pay high in cold cache case
> for loading trees from "HEAD". So no, probably not worth more code
> changes. Your optimization is good enough.