[PATCH v4 00/15] Changed Paths Bloom Filters

["Garima Singh via GitGitGadget" <gitgitgadget@gmail.com>]

Hey! 

The commit graph feature brought in a lot of performance improvements across
multiple commands. However, file based history continues to be a performance
pain point, especially in large repositories. 

Adopting changed path Bloom filters has been discussed on the list before,
and a prototype version was worked on by SZEDER Gábor, Jonathan Tan and Dr.
Derrick Stolee [1]. This series is based on Dr. Stolee's proof of concept in
[2]

With the changes in this series, git users will be able to choose to write
Bloom filters to the commit-graph using the following command:

'git commit-graph write --changed-paths'

Subsequent 'git log -- path' commands will use these computed Bloom filters
to decided which commits are worth exploring further to produce the history
of the provided path. 

Cost of computing and writing Bloom filters
===========================================

Computing and writing Bloom filters to the commit graph for the first time
implies computing the diffs and the resulting Bloom filters for all the
commits in the repository. This adds a non trivial amount of time to run
time. Every subsequent run is incremental i.e. we reuse the previously
computed Bloom filters. So this is a one time cost. 

Time taken by 'git commit-graph write' with and w/o --changed-paths, speed
up in 'git log -- path' with computed Bloom filters (see a):- 

-------------------------------------------------------------------------
| Repo        | w/o --changed-paths | with --changed-paths | Speed up   |
-------------------------------------------------------------------------
| git [3]     | 0.9 seconds         | 7 seconds            | 2x to 6x   |
| linux [4]   | 16 seconds          | 1 minute 8 seconds   | 2x to 6x   | 
| android [5] | 9 seconds           | 48 seconds           | 2x to 6x   |
| AzDo(see b) | 1 minute            | 5 minutes 2 seconds  | 10x to 30x |
-------------------------------------------------------------------------

a) We tested the performance of git log -- path with randomly chosen paths
of varying depths in each repo. The speed up depends on how deep the files
are in the hierarchy and how often a file has been touched in the commit
history.

b) This internal repository has about 420k commits, 183k files distributed
across 34k folders, the size on disk is about 17 GiB. The most massive gains
on this repository were for files 6-12 levels deep in the tree. 

c) These numbers were collected on a Windows machine, except for the linux
repo which was tested on a Linux machine. 

Future Work (not included in the scope of this series)
======================================================

 1. Supporting multiple path based revision walk
 2. Adopting it in git blame logic. 
 3. Interactions with line log git log -L

Cheers! Garima Singh

[1] https://lore.kernel.org/git/20181009193445.21908-1-szeder.dev@gmail.com/

[2] 
https://lore.kernel.org/git/61559c5b-546e-d61b-d2e1-68de692f5972@gmail.com/

[3] https://github.com/git/git

[4] https://github.com/torvalds/linux

[5] https://android.googlesource.com/platform/frameworks/base/

jeffhost@microsoft.com, me@ttaylorr.com, peff@peff.net, 
garimasigit@gmail.com,jnareb@gmail.com, christian.couder@gmail.com, 
emilyshaffer@gmail.com,gitster@pobox.com

Derrick Stolee (1):
  diff: halt tree-diff early after max_changes

Garima Singh (13):
  commit-graph: define and use MAX_NUM_CHUNKS
  bloom.c: add the murmur3 hash implementation
  bloom.c: introduce core Bloom filter constructs
  bloom.c: core Bloom filter implementation for changed paths.
  commit-graph: compute Bloom filters for changed paths
  commit-graph: examine commits by generation number
  commit-graph: write Bloom filters to commit graph file
  commit-graph: reuse existing Bloom filters during write
  commit-graph: add --changed-paths option to write subcommand
  revision.c: use Bloom filters to speed up path based revision walks
  revision.c: add trace2 stats around Bloom filter usage
  t4216: add end to end tests for git log with Bloom filters
  commit-graph: add GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS test flag

Jeff King (1):
  commit-graph: examine changed-path objects in pack order

 Documentation/git-commit-graph.txt            |   5 +
 .../technical/commit-graph-format.txt         |  30 ++
 Makefile                                      |   2 +
 bloom.c                                       | 275 ++++++++++++++++++
 bloom.h                                       |  90 ++++++
 builtin/commit-graph.c                        |  10 +-
 ci/run-build-and-tests.sh                     |   1 +
 commit-graph.c                                | 213 +++++++++++++-
 commit-graph.h                                |   9 +-
 diff.h                                        |   5 +
 revision.c                                    | 126 +++++++-
 revision.h                                    |  11 +
 t/README                                      |   5 +
 t/helper/test-bloom.c                         |  81 ++++++
 t/helper/test-read-graph.c                    |   4 +
 t/helper/test-tool.c                          |   1 +
 t/helper/test-tool.h                          |   1 +
 t/t0095-bloom.sh                              | 117 ++++++++
 t/t4216-log-bloom.sh                          | 155 ++++++++++
 t/t5318-commit-graph.sh                       |   2 +
 t/t5324-split-commit-graph.sh                 |   1 +
 tree-diff.c                                   |   6 +
 22 files changed, 1139 insertions(+), 11 deletions(-)
 create mode 100644 bloom.c
 create mode 100644 bloom.h
 create mode 100644 t/helper/test-bloom.c
 create mode 100755 t/t0095-bloom.sh
 create mode 100755 t/t4216-log-bloom.sh


base-commit: 3bab5d56259722843359702bc27111475437ad2a
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-497%2Fgarimasi514%2FcoreGit-bloomFilters-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-497/garimasi514/coreGit-bloomFilters-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/497

Range-diff vs v3:

  1:  c3ffd9820d5 =  1:  c3ffd9820d5 commit-graph: define and use MAX_NUM_CHUNKS
  2:  a5aa3415c05 =  2:  a5aa3415c05 bloom.c: add the murmur3 hash implementation
  3:  a7702c1afde =  3:  a7702c1afde bloom.c: introduce core Bloom filter constructs
  4:  8304c297520 =  4:  8304c297520 bloom.c: core Bloom filter implementation for changed paths.
  5:  2d4c0b2da38 =  5:  2d4c0b2da38 diff: halt tree-diff early after max_changes
  6:  c38b9b386ef =  6:  c38b9b386ef commit-graph: compute Bloom filters for changed paths
  7:  d24c85c54ef =  7:  d24c85c54ef commit-graph: examine changed-path objects in pack order
  8:  5ed16f35fed =  8:  5ed16f35fed commit-graph: examine commits by generation number
  9:  55824cda89c <  -:  ----------- diff: skip batch object download when possible
 10:  1e4663523de =  9:  ff6b96aad1e commit-graph: write Bloom filters to commit graph file
 11:  68395d4051b ! 10:  cc8022bdf82 commit-graph: reuse existing Bloom filters during write
     @@ bloom.c: struct bloom_filter *get_bloom_filter(struct repository *r,
      +
       	repo_diff_setup(r, &diffopt);
       	diffopt.flags.recursive = 1;
     - 	diffopt.detect_rename = 0;
     + 	diffopt.max_changes = max_changes;
      
       ## bloom.h ##
      @@ bloom.h: struct bloom_filter_settings {
 12:  7e450e45236 = 11:  c8b86c383ab commit-graph: add --changed-paths option to write subcommand
 13:  b18af58aa3e = 12:  617f549ef25 revision.c: use Bloom filters to speed up path based revision walks
 14:  b5eb280178f = 13:  6beaede7159 revision.c: add trace2 stats around Bloom filter usage
 15:  3019ef72881 = 14:  b899df5c98e t4216: add end to end tests for git log with Bloom filters
 16:  213abb5d895 = 15:  5656e8590e9 commit-graph: add GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS test flag

-- 
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