From: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
---
Documentation/technical/commit-graph-format.txt | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/Documentation/technical/commit-graph-format.txt b/Documentation/technical/commit-graph-format.txt
index de56f9f1efd..1beef171822 100644
--- a/Documentation/technical/commit-graph-format.txt
+++ b/Documentation/technical/commit-graph-format.txt
@@ -97,10 +97,10 @@ CHUNK DATA:
bit on. The other bits correspond to the position of the last parent.
Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
- * The ith entry, BIDX[i], stores the number of 8-byte word blocks in all
- Bloom filters from commit 0 to commit i (inclusive) in lexicographic
- order. The Bloom filter for the i-th commit spans from BIDX[i-1] to
- BIDX[i] (plus header length), where BIDX[-1] is 0.
+ * The ith entry, BIDX[i], stores the number of bytes in all Bloom filters
+ from commit 0 to commit i (inclusive) in lexicographic order. The Bloom
+ filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header
+ length), where BIDX[-1] is 0.
* The BIDX chunk is ignored if the BDAT chunk is not present.
Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
--
gitgitgadget
