On Mon, Oct 1, 2012 at 8:49 AM, Shawn Pearce <spea...@spearce.org> wrote:
> 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.

The good thing about these cache is it's configurable. Merge-preferred
projects can choose to cache the first two parents. Non-merge projects
can choose to cache just the first parent. We don't need a fixed
format for both.
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