On Thu, Sep 27, 2012 at 7:14 PM, Nguyen Thai Ngoc Duy <pclo...@gmail.com> wrote:
> On Thu, Sep 27, 2012 at 10:51 PM, Shawn Pearce <spea...@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%.

Some projects merge quite often. Android's frameworks/base repository
has a very large number of merges. Out of 79905 commits reachable from
the master branch, 65.3% are merges. So actually there are more merge
commits in the Android history than there are code commits. A cache of
only non-merges may be worthless on such a history.
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