From: Nguyen Thai Ngoc Duy <pclouds@gmail.com>
Date: Fri, 28 Sep 2012 02:14:54 GMT
Subject: Re: Commit cache to speed up rev-list and merge
Message-ID: <CACsJy8APNsDrTH+hUxgcB=MY0eECCGktm6P1feEEpz2427OQqg@mail.gmail.com>
In-Reply-To: <CAJo=hJtoqYEL5YiKawCt_SsSUqfCeYEQzY8Ntyb91cNfNS1w_Q@mail.gmail.com>

On Thu, Sep 27, 2012 at 10:51 PM, Shawn Pearce <spearce@spearce.org> wrote:
> In Linus' Linux kernel tree there are currently about 323,178 commits.
> If we store just the pre-parsed commit time as an int32 field this is
> an additional 1.2 MiB of data in the pack-*.idx file, assuming we can
> use additional data like pack offset position to correlate commit to
> the parsed int. If we stored parent pointers in a similar way you
> probably need at least 3.6 MiB of additional disk space on the index.
> For example, use 12 bytes for each commit to store enough of the
> parsed commit time to sort commits, and up to 2 parent pointers per
> commit.... with a reserved magic value for octopus merges to mean the
> commit itself has to be parsed to get the graph structure correct.

This is much better than my naive approach (storing sha-1 and
timestamps). We could use less space by storing parent pointer of
non-merge commits only. Merge commits linux-2.6 is 6% the number of
commits. git.git has higher percentage, 21%. I bet many projects do
not merge as much and the number of merge commits is less than 5%.
-- 
Duy