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

From: Nguyen Thai Ngoc Duy <pclouds@gmail.com>
Date: Feb 22, 2012, 12:54 UTC
Message-ID: <CACsJy8BZyokLSpbA=C6YEO1JajyemT9vLG=C9W4E4coA5OfszQ@mail.gmail.com>
In-Reply-To: <7v7gzfefw1.fsf@alter.siamese.dyndns.org>

On Wed, Feb 22, 2012 at 9:55 AM, Junio C Hamano <gitster@pobox.com> wrote:

Show 9 quoted lines

> Nguyen Thai Ngoc Duy <pclouds@gmail.com> writes:
>
>> That makes me think if "diff --cached" can take advantage of
>> cache-tree to avoid walking down valid cached trees and do tree-tree
>> diff in those cases instead. Not sure if it gains us anything but code
>> complexity.
>
> Why do I have this funny feeling that we saw that comment in this thread
> already?

Simple. You wrote it and I missed it.

On Sat, Feb 18, 2012 at 5:25 AM, Junio C Hamano <gitster@pobox.com> wrote:

Show 13 quoted lines

> 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.

-- 
Duy