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2021-02-18chunk-format: add technical docsLibravatar Derrick Stolee1-0/+3
The chunk-based file format is now an API in the code, but we should also take time to document it as a file format. Specifically, it matches the CHUNK LOOKUP sections of the commit-graph and multi-pack-index files, but there are some commonalities that should be grouped in this document. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-18doc: add corrected commit date infoLibravatar Abhishek Kumar1-6/+22
With generation data chunk and corrected commit dates implemented, let's update the technical documentation for commit-graph. Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com> Reviewed-by: Taylor Blau <me@ttaylorr.com> Reviewed-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-09-29Merge branch 'tb/bloom-improvements'Libravatar Junio C Hamano1-1/+1
"git commit-graph write" learned to limit the number of bloom filters that are computed from scratch with the --max-new-filters option. * tb/bloom-improvements: commit-graph: introduce 'commitGraph.maxNewFilters' builtin/commit-graph.c: introduce '--max-new-filters=<n>' commit-graph: rename 'split_commit_graph_opts' bloom: encode out-of-bounds filters as non-empty bloom/diff: properly short-circuit on max_changes bloom: use provided 'struct bloom_filter_settings' bloom: split 'get_bloom_filter()' in two commit-graph.c: store maximum changed paths commit-graph: respect 'commitGraph.readChangedPaths' t/helper/test-read-graph.c: prepare repo settings commit-graph: pass a 'struct repository *' in more places t4216: use an '&&'-chain commit-graph: introduce 'get_bloom_filter_settings()'
2020-09-22Merge branch 'cd/commit-graph-doc'Libravatar Junio C Hamano1-1/+1
Doc update. * cd/commit-graph-doc: commit-graph-format.txt: fix no-parent value
2020-09-17bloom: encode out-of-bounds filters as non-emptyLibravatar Taylor Blau1-1/+1
When a changed-path Bloom filter has either zero, or more than a certain number (commonly 512) of entries, the commit-graph machinery encodes it as "missing". More specifically, it sets the indices adjacent in the BIDX chunk as equal to each other to indicate a "length 0" filter; that is, that the filter occupies zero bytes on disk. This has heretofore been fine, since the commit-graph machinery has no need to care about these filters with too few or too many changed paths. Both cases act like no filter has been generated at all, and so there is no need to store them. In a subsequent commit, however, the commit-graph machinery will learn to only compute Bloom filters for some commits in the current commit-graph layer. This is a change from the current implementation which computes Bloom filters for all commits that are in the layer being written. Critically for this patch, only computing some of the Bloom filters means adding a third state for length 0 Bloom filters: zero entries, too many entries, or "hasn't been computed". It will be important for that future patch to distinguish between "not representable" (i.e., zero or too-many changed paths), and "hasn't been computed". In particular, we don't want to waste time recomputing filters that have already been computed. To that end, change how we store Bloom filters in the "computed but not representable" category: - Bloom filters with no entries are stored as a single byte with all bits low (i.e., all queries to that Bloom filter will return "definitely not") - Bloom filters with too many entries are stored as a single byte with all bits set high (i.e., all queries to that Bloom filter will return "maybe"). These rules are sufficient to not incur a behavior change by changing the on-disk representation of these two classes. Likewise, no specification changes are necessary for the commit-graph format, either: - Filters that were previously empty will be recomputed and stored according to the new rules, and - old clients reading filters generated by new clients will interpret the filters correctly and be none the wiser to how they were generated. Clients will invoke the Bloom machinery in more cases than before, but this can be addressed by returning a NULL filter when all bits are set high. This can be addressed in a future patch. Note that this does increase the size of on-disk commit-graphs, but far less than other proposals. In particular, this is generally more efficient than storing a bitmap for which commits haven't computed their Bloom filters. Storing a bitmap incurs a penalty of one bit per commit, whereas storing explicit filters as above incurs a penalty of one byte per too-large or empty commit. In practice, these boundary commits likely occupy a small proportion of the overall number of commits, and so the size penalty is likely smaller than storing a bitmap for all commits. See, for example, these relative proportions of such boundary commits (collected by SZEDER Gábor): | Percentage of | commit-graph | | | commits modifying | file size | | ├────────┬──────────────┼───────────────────┤ pct. | | 0 path | >= 512 paths | before | after | change | ┌────────────────┼────────┼──────────────┼─────────┼─────────┼───────────┤ | android-base | 13.20% | 0.13% | 37.468M | 37.534M | +0.1741 % | | cmssw | 0.15% | 0.23% | 17.118M | 17.119M | +0.0091 % | | cpython | 3.07% | 0.01% | 7.967M | 7.971M | +0.0423 % | | elasticsearch | 0.70% | 1.00% | 8.833M | 8.835M | +0.0128 % | | gcc | 0.00% | 0.08% | 16.073M | 16.074M | +0.0030 % | | gecko-dev | 0.14% | 0.64% | 59.868M | 59.874M | +0.0105 % | | git | 0.11% | 0.02% | 3.895M | 3.895M | +0.0020 % | | glibc | 0.02% | 0.10% | 3.555M | 3.555M | +0.0021 % | | go | 0.00% | 0.07% | 3.186M | 3.186M | +0.0018 % | | homebrew-cask | 0.40% | 0.02% | 7.035M | 7.035M | +0.0065 % | | homebrew-core | 0.01% | 0.01% | 11.611M | 11.611M | +0.0002 % | | jdk | 0.26% | 5.64% | 5.537M | 5.540M | +0.0590 % | | linux | 0.01% | 0.51% | 63.735M | 63.740M | +0.0073 % | | llvm-project | 0.12% | 0.03% | 25.515M | 25.516M | +0.0050 % | | rails | 0.10% | 0.10% | 6.252M | 6.252M | +0.0027 % | | rust | 0.07% | 0.17% | 9.364M | 9.364M | +0.0033 % | | tensorflow | 0.09% | 1.02% | 7.009M | 7.010M | +0.0158 % | | webkit | 0.05% | 0.31% | 17.405M | 17.406M | +0.0047 % | (where the above increase is determined by computing a non-split commit-graph before and after this patch). Given that these projects are all "large" by commit count, the storage cost by writing these filters explicitly is negligible. In the most extreme example, android-base (which has 494,848 commits at the time of writing) would have its commit-graph increase by a modest 68.4 KB. Finally, a test to exercise filters which contain too many changed path entries will be introduced in a subsequent patch. Suggested-by: SZEDER Gábor <szeder.dev@gmail.com> Suggested-by: Jakub Narębski <jnareb@gmail.com> Helped-by: Derrick Stolee <dstolee@microsoft.com> Helped-by: SZEDER Gábor <szeder.dev@gmail.com> Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-09-15commit-graph-format.txt: fix no-parent valueLibravatar Conor Davis1-1/+1
The correct value from commit-graph.c: #define GRAPH_PARENT_NONE 0x70000000 Signed-off-by: Conor Davis <git@conor.fastmail.fm> Reviewed-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-08-17commit-graph: use the "hash version" byteLibravatar Derrick Stolee1-2/+7
The commit-graph format reserved a byte among the header of the file to store a "hash version". During the SHA-256 work, this was not modified because file formats are not necessarily intended to work across hash versions. If a repository has SHA-256 as its hash algorithm, it automatically up-shifts the lengths of object names in all necessary formats. However, since we have this byte available for adjusting the version, we can make the file formats more obviously incompatible instead of relying on other context from the repository. Update the oid_version() method in commit-graph.c to add a new value, 2, for sha-256. This automatically writes the new value in a SHA-256 repository _and_ verifies the value is correct. This is a breaking change relative to the current 'master' branch since 092b677 (Merge branch 'bc/sha-256-cvs-svn-updates', 2020-08-13) but it is not breaking relative to any released version of Git. The test impact is relatively minor: the output of 'test-tool read-graph' lists the header information, so those instances of '1' need to be replaced with a variable determined by GIT_TEST_DEFAULT_HASH. A more careful test is added that specifically creates a repository of each type then swaps the commit-graph files. The important value here is that the "git log" command succeeds while writing a message to stderr. Helped-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Reviewed-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-06-08commit-graph-format.txt: all multi-byte numbers are in network byte orderLibravatar SZEDER Gábor1-1/+1
The commit-graph format specifies that "All 4-byte numbers are in network order", but the commit-graph contains 8-byte integers as well (file offsets in the Chunk Lookup table), and their byte order is unspecified. Clarify that all multi-byte integers are in network byte order. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-05-11Documentation: changed-path Bloom filters use byte wordsLibravatar Derrick Stolee1-4/+4
In Documentation/technical/commit-graph-format.txt, the definition of the BIDX chunk specifies the length is a number of 8-byte words. During development we discovered that using 8-byte words in the Murmur3 hash algorithm causes issues with big-endian versus little- endian machines. Thus, the hash algorithm was adapted to work on a byte-by-byte basis. However, this caused a change in the definition of a "word" in bloom.h. Now, a "word" is a single byte, which allows filters to be as small as two bytes. These length-two filters are demonstrated in t0095-bloom.sh, and a larger filter of length 25 is demonstrated as well. The original point of using 8-byte words was for alignment reasons. It also presented opportunities for extremely sparse Bloom filters when there were a small number of changes at a commit, creating a very low false-positive rate. However, modifying the format at this point is unlikely to be a valuable exercise. Also, this use of single-byte granularity does present opportunities to save space. It is unclear if 8-byte alignment of the filters would present any meaningful performance benefits. Modify the format document to reflect reality. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-04-06commit-graph: write Bloom filters to commit graph fileLibravatar Garima Singh1-0/+30
Update the technical documentation for commit-graph-format with the formats for the Bloom filter index (BIDX) and Bloom filter data (BDAT) chunks. Write the computed Bloom filters information to the commit graph file using this format. Helped-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Garima Singh <garima.singh@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2019-06-19commit-graph: add base graphs chunkLibravatar Derrick Stolee1-2/+9
To quickly verify a commit-graph chain is valid on load, we will read from the new "Base Graphs Chunk" of each file in the chain. This will prevent accidentally loading incorrect data from manually editing the commit-graph-chain file or renaming graph-{hash}.graph files. The commit_graph struct already had an object_id struct "oid", but it was never initialized or used. Add a line to read the hash from the end of the commit-graph file and into the oid member. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2019-01-22commit-graph: rename "large edges" to "extra edges"Libravatar SZEDER Gábor1-2/+2
The optional 'Large Edge List' chunk of the commit graph file stores parent information for commits with more than two parents, and the names of most of the macros, variables, struct fields, and functions related to this chunk contain the term "large edges", e.g. write_graph_chunk_large_edges(). However, it's not a really great term, as the edges to the second and subsequent parents stored in this chunk are not any larger than the edges to the first and second parents stored in the "main" 'Commit Data' chunk. It's the number of edges, IOW number of parents, that is larger compared to non-merge and "regular" two-parent merge commits. And indeed, two functions in 'commit-graph.c' have a local variable called 'num_extra_edges' that refer to the same thing, and this "extra edges" term is much better at describing these edges. So let's rename all these references to "large edges" in macro, variable, function, etc. names to "extra edges". There is a GRAPH_OCTOPUS_EDGES_NEEDED macro as well; for the sake of consistency rename it to GRAPH_EXTRA_EDGES_NEEDED. We can do so safely without causing any incompatibility issues, because the term "large edges" doesn't come up in the file format itself in any form (the chunk's magic is {'E', 'D', 'G', 'E'}, there is no 'L' in there), but only in the specification text. The string "large edges", however, does come up in the output of 'git commit-graph read' and in tests looking at its input, but that command is explicitly documented as debugging aid, so we can change its output and the affected tests safely. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-07-18Merge branch 'ds/commit-graph'Libravatar Junio C Hamano1-5/+5
Docfix. * ds/commit-graph: commit-graph: fix documentation inconsistencies
2018-06-28commit-graph: fix documentation inconsistenciesLibravatar Derrick Stolee1-5/+5
The commit-graph feature shipped in Git 2.18 has some inconsistencies in the constants used by the implementation and specified by the format document. The commit data chunk uses the key "CDAT" in the file format, but was previously documented to say "CGET". The commit data chunk stores commit parents using two 32-bit fields that typically store the integer position of the parent in the list of commit ids within the commit-graph file. When a parent does not exist, we had documented the value 0xffffffff, but implemented the value 0x70000000. This swap is easy to correct in the documentation, but unfortunately reduces the number of commits that we can store in the commit-graph. Update that estimate, too. Reported-by: Grant Welch <gwelch925@gmail.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-06-22Documentation: spelling and grammar fixesLibravatar Ville Skyttä1-1/+1
Signed-off-by: Ville Skyttä <ville.skytta@iki.fi> Reviewed-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-04-02commit-graph: add format documentLibravatar Derrick Stolee1-0/+97
Add document specifying the binary format for commit graphs. This format allows for: * New versions. * New hash functions and hash lengths. * Optional extensions. Basic header information is followed by a binary table of contents into "chunks" that include: * An ordered list of commit object IDs. * A 256-entry fanout into that list of OIDs. * A list of metadata for the commits. * A list of "large edges" to enable octopus merges. The format automatically includes two parent positions for every commit. This favors speed over space, since using only one position per commit would cause an extra level of indirection for every merge commit. (Octopus merges suffer from this indirection, but they are very rare.) Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>