Re: [RFC] Metadata vs Generation Data Chunk

[Abhishek Kumar <abhishekkumar8222@gmail.com>]

Hello everyone,

I have finished all tests that I can think of and instead of throwing
performance numbers at you, I am going to try and tell you a story.

WRITING COMMIT GRAPH
====================

The run time of writing a commit-graph is affected by time taken to
compute generation number(s) and write them to the file.

We had the following possible ideas for writing commit-graph:

MO: Calculate generation number V5 and write the offset into CDAT
    (along with a metadata chunk).

G5O: Calculate generation number V5 and write the corrected date into
    Generation Data chunk.

G5D: Calculate generation number V5 and write the offset into Generation
    Data chunk.

G3IO: Calculate generation number V3 and write GENERATION_NUMBER_MAX into
    CDAT and the offset into Generation Data chunk.

G3ID: Calculate generation number V3 and write GENERATION_NUMBER_MAX into
    CDAT and the corrected date into Generation Data chunk.

G3TO: Calculate generation number V3 and generation number V0, write
    topological levels into CDAT and the offset into Generation Data
    chunk.

G3TD: Calculate generation number V3 and generation number V0, write
    topological levels into CDAT and the offset into Generation Data
    chunk.

Key:
- M -> Metadata, G -> Generation
- 5 -> Generation Number V5, 3 -> Generation Number V3,
- T -> Topological level, I (for infinity) -> Generation Number Max
- O -> Offset, D -> Date

On the Linux repo with HEAD at 08bf1a27, time is taken by different
versions:

| Version | time Taken      |
|---------|-----------------|
| master  | 14.200s         |
| MO      | 14.094s         |
| G5D     | 14.406s         |
| G5O     | 14.323s         |
| G3IO    | 14.175s         |
| G3ID    | 14.258s         |
| G3TO    | 14.331s         |
| G3TD    | 14.419s         |

Observations:

- Writing offset (i.e., 32-bits) is faster than writing corrected date 
  (which are 64-bit long). However, we would have to deal with overflow
  issues.

  The largest offset value observed was of the order 2 ** 29. Assuming
  no further "skips" in offset, we can store around ~635 million
  commits without overflows. This, however, might not be true for other
  repositories.

  A repository with a commit with Unix epoch '0' will overflow.
  If we choose some sane default like commit date must be higher than
  the release date of git, we can store around 110 million commits even
  with '0' epoch.

- Calculating topological levels with generation number V3 takes nearly
  the same time as calculating generation number V5.

- The commit-graph file is larger by 4 Mb (+8%) and 8 Mb (+16%) when
  storing offsets and dates.

Reading Commit Graph
====================

The time taken in using commit-graph to answer queries depends on:
- The number of commits walked.
- The time taken to load commit data from commit-graph.

Number of commits walked for different commands:

| Command                         | Master |   V3   |   V5   |
|---------------------------------|--------|--------|--------|
| log --topo-order -10000         |  48011 |  49954 |  49933 |
| log --topo-order -100 v5.4 v5.5 |   3794 |   3314 |   3312 |
| log --topo-order -100 v4.8 v4.9 |   5736 |   3887 |   3625 |
| merge-base v5.4 v5.5            |  55053 |  57097 |  55109 |
| merge-base v4.8 v4.9            | 635579 | 167468 | 506577 |

- For log --topo-order, V3, and V5 walk 35% fewer commits than
  the topological level when comparing v4.8 and v4.9.

- V3 walks far fewer commits than V5 when comparing by generation
  numbers and then date for paint_down_to_common(). This is unusual
  and is probably due to my implementation. Based on log --topo-order,
  we might expect V5 to perform better than V3.
 
- V3 walks the same number of commits when compared using commit
  date for merge-base.

The time taken for each command rough corresponds to the number of
commits walked, with no difference between Metadata chunk and Generation
Data chunk approaches.

| Command                         | Master  |    V3   |   V5   |
|---------------------------------|---------|---------|--------|
| log --topo-order -10000         |  0.210s |  0.209s | 0.207s |
| log --topo-order -100 v5.4 v5.5 |  0.013s |  0.013s | 0.013s |
| log --topo-order -100 v4.8 v4.9 |  0.015s |  0.013s | 0.013s |
| merge-base v5.4 v5.5            |  0.048s |  0.047s | 0.047s |
| merge-base v4.8 v4.9            |  0.135s |  0.133s | 0.134s |

CONCLUSION
==========

With all this, my inital recommendation becomes more specific as:

- If overflows are unlikely to happen and not accounted for, implement
  generation number V5 using metadata chunk. It has the lowest write
  time and walks just as few commits as V3.

- If overflows are a concern, implement generation number V5 with
  generation data chunk, storing offsets in generation data chunk.

Thanks
- Abhishek