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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>
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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>
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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>
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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>
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Docfix.
* ds/commit-graph:
commit-graph: fix documentation inconsistencies
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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>
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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>
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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>
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Derrick Stolee
Garima Singh
SZEDER Gábor
Junio C Hamano
Ville Skyttä