Re: [RFC] Possible idea for GSoC 2020

[Jakub Narebski <jnareb@gmail.com>]

Cc: Stolee, Heba, Jonathan T., Emily Shaffer. 

Junio C Hamano <gitster@pobox.com> writes:
> Jakub Narebski <jnareb@gmail.com> writes:
>
>> A few questions:
>> - is it too late to propose a new project idea for GSoC 2020?
>> - is it too difficult of a project for GSoC?
>> ...
>> ### Graph labelling for speeding up git commands
>>
>>  - Language: C
>>  - Difficulty: hard / difficult
>>  - Possible mentors: Jakub Narębski
>
> I am not running the GSoC or participating in it in any way other
> than just being a reviewer-maintainer of the project, but I would
> appreciate a well-thought-out write-up very much.

I have prepared slides for "Graph operations in Git version control
system" (PDF), mainly describing what was already done to improve their
performance, but they also include a few thoughts about the future (like
additional graph reachability labelings)... unfortunately the slides are
in Polish, not in English.

If there is interest, I could translate them, and put the result
somewhere accessible.

Or I could try to make this information into blog post -- this topic
would really gain from using images (like Derrick Stolee series of
articles on commit-graph).

>> Git uses various clever methods for making operations on very large
>> repositories faster, from bitmap indices for git-fetch[1], to generation
>> numbers (also known as topological levels) in the commit-graph file for
>> commit graph traversal operations like `git log --graph`[2].
>>
>> One possible improvement that can make Git even faster is using min-post
>> intervals labelling.  The basis of this labelling is post-visit order of
>> a depth-first search traversal tree of a commit graph, let's call it
>> 'post(v)'.
>>
>> If for each commit 'v' we would compute and store in the commit-graph
>> file two numbers: 'post(v)' and the minimum of 'post(u)' for all commits
>> reachable from 'v', let's call the latter 'min_graph(v)', then the
>> following condition is true:
>>
>>   if 'v' can reach 'u', then min_graph(v) <= post(u) <= post(v)
>>
>> If for each commit 'v' we would compute and store in the commit-graph
>> file two numbers: 'post(v)' and the minimum of 'post(u)' for commits
>> that were visited during the part of depth-first search that started
>> from 'v' (which is the minimum of post-order number for subtree of a
>> spanning tree that starts at 'v').  Let's call the later 'min_tree(v)'.
>> Then the following condition is true:
>>
>>   if min_tree(v) <= post(u) <= post(v), then 'v' can reach 'u'
>>
>> The task would be to implement computing such labelling (or a more
>> involved variant of it[3][4]), storing it in commit-graph file, and
>> using it for speeding up git commands (starting from a single chosen
>> command) such as:
>>
>>  - git merge-base --is-ancestor A B
>>  - git branch --contains A
>>  - git tag --contains A
>>  - git branch --merged A
>>  - git tag --merged A
>>  - git merge-base --all A B
>>  - git log --topo-sort
>>
>> References:
>>
>> 1. <http://githubengineering.com/counting-objects/>
>> 2. <https://devblogs.microsoft.com/devops/supercharging-the-git-commit-graph-iii-generations/>
>> 3. <https://arxiv.org/abs/1404.4465>
>> 4. <https://github.com/steps/Ferrari>
>>
>> See also discussion in:
>>
>> <https://public-inbox.org/git/86tvl0zhos.fsf@gmail.com/t/>

P.S. A bit more expanded writeup now available
at https://git.github.io/SoC-2020-Ideas/

Best,
-- 
Jakub Narębski