Re: [PATCH v2 00/19] Multiparent diff tree-walker + combine-diff speedup

From: Duy Nguyen <pclouds@gmail.com>
Date: Feb 24, 2014, 23:43 UTC
Message-ID: <CACsJy8BXMVNVAyqPEbHTkGxSSEJ6DpYUVwZqthiMQfO7Tj9T8A@mail.gmail.com>
In-Reply-To: <cover.1393257006.git.kirr@mns.spb.ru>

On Mon, Feb 24, 2014 at 11:21 PM, Kirill Smelkov <kirr@mns.spb.ru> wrote:

Show 8 quoted lines

> Hello up there.
>
> Here go combine-diff speedup patches in form of first reworking diff
> tree-walker to work in general case - when a commit have several parents, not
> only one - we are traversing all 1+nparent trees in parallel.
>
> Then we are taking advantage of the new diff tree-walker for speeding up
> combine-diff, which for linux.git results in ~14 times speedup.

I think there is another use case for this n-tree walker (but I'm not entirely sure yet as I haven't really read the series). In git-log (either with pathspec or --patch) we basically do this

diff HEAD^ HEAD diff HEAD^^ HEAD^ diff HEAD^^^ HEAD^^ diff HEAD^^^^ HEAD^^^ ...

so except HEAD (and the last commit), all commits' tree will be read/diff'd twice. With n-tree walker I think we may be able to diff them in batch to reduce extra processing: commit lists are split into 16-commit blocks where 16 trees are fed to the new tree walker at the same time. I hope it would make git-log a bit faster (especially for -S). Maybe not much.

-- 
Duy