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

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 21, 2012, 19:16 UTC
Message-ID: <7v8vjwgfoq.fsf@alter.siamese.dyndns.org>
In-Reply-To: <CACsJy8DE86qzA1=GiKZFRCt5aH8X4iMyDvfrhnqwmbq52szhHg@mail.gmail.com>

Nguyen Thai Ngoc Duy <pclouds@gmail.com> writes:

Show 6 quoted lines

> I'm aware that Jeff's tackling at lower level, which retains
> cache-tree for many more cases.
>
> But this patch seems simple and safe
> to me, and in my experience this case happens quite often (or maybe I
> tend to keep my index clean). Junio, any chance this patch may get in?

I do not think we are talking about a duplicated effort here.

By definition, the change to hook into unpack_trees() and making sure we invalidate all the necessary subtrees in the cache cannot give you a cache tree that is more populated than what you started with. And the train of thought in Peff's message is to improve this invalidation---we currently invalidate everything ;-)

Somebody has to populate the cache tree fully when we _know_ the index matches a certain tree, and adding a call to prime_cache_tree() in strategic places is a way to do so. The most obvious is write-tree, but there are a few other existing codepaths that do so.

Because prime_cache_tree() by itself is a fairly expensive operation that reads all the trees recursively, its benefits need to be evaluated. It should to happen only in an operation that is already heavy-weight, is likely to have read all the trees and have many of them in-core cache, and also relatively rarely happens compared to "git add" so that the cost can be amortised over time, such as "reset --(hard|mixed)".

Switching branches is likely to fall into that category, but that is just my gut feeling. I would feel better at night if somebody did a benchmark ;-)

One thing we do not currently do anywhere that _might_ be of merit is to make a call to cache_tree_update() instead of prime_cache_tree() when we already know that only a very small subpart of the cache-tree is invalid and it is cheaper to repair it by rehashing only a small portion of the index than to re-prime the entire cache tree with prime_cache_tree().