From: "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: peff@peff.net, me@ttaylorr.com, garimasigit@gmail.com,
szeder.dev@gmail.com, jnareb@gmail.com,
Derrick Stolee <dstolee@microsoft.com>,
Derrick Stolee <dstolee@microsoft.com>
Subject: [PATCH 03/12] Documentation: changed-path Bloom filters use byte words
Date: Fri, 01 May 2020 15:30:20 +0000 [thread overview]
Message-ID: <03b2c84db36754b321dd15cd8a9e1a22b7c11c36.1588347029.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.622.git.1588347029.gitgitgadget@gmail.com>
From: Derrick Stolee <dstolee@microsoft.com>
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>
---
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
next prev parent reply other threads:[~2020-05-01 15:31 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-01 15:30 [PATCH 00/12] Integrating line-log and changed-path Bloom filters Derrick Stolee via GitGitGadget
2020-05-01 15:30 ` [PATCH 01/12] bloom: fix whitespace around tab length Derrick Stolee via GitGitGadget
2020-05-01 22:51 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 02/12] test-bloom: fix usage typo Derrick Stolee via GitGitGadget
2020-05-01 22:51 ` Taylor Blau
2020-05-01 15:30 ` Derrick Stolee via GitGitGadget [this message]
2020-05-01 22:55 ` [PATCH 03/12] Documentation: changed-path Bloom filters use byte words Taylor Blau
2020-05-01 15:30 ` [PATCH 04/12] bloom: de-duplicate directory entries Derrick Stolee via GitGitGadget
2020-05-01 23:06 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 05/12] bloom: parse commit before computing filters Derrick Stolee via GitGitGadget
2020-05-01 23:10 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 06/12] bloom: use num_changes not nr for limit detection Derrick Stolee via GitGitGadget
2020-05-01 23:12 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 07/12] completion: offer '--(no-)patch' among 'git log' options SZEDER Gábor via GitGitGadget
2020-05-01 23:44 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 08/12] line-log: remove unused fields from 'struct line_log_data' SZEDER Gábor via GitGitGadget
2020-05-01 23:46 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 09/12] t4211-line-log: add tests for parent oids SZEDER Gábor via GitGitGadget
2020-05-04 17:31 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 10/12] line-log: more responsive, incremental 'git log -L' SZEDER Gábor via GitGitGadget
2020-05-04 17:52 ` Taylor Blau
2020-05-04 17:55 ` Derrick Stolee
2020-05-01 15:30 ` [PATCH 11/12] line-log: try to use generation number-based topo-ordering SZEDER Gábor via GitGitGadget
2020-05-04 21:25 ` Taylor Blau
2020-05-01 15:30 ` [PATCH 12/12] line-log: integrate with changed-path Bloom filters Derrick Stolee via GitGitGadget
2020-05-04 21:50 ` Taylor Blau
2020-05-01 17:34 ` [PATCH 00/12] Integrating line-log and " Junio C Hamano
2020-05-11 11:56 ` [PATCH v2 " Derrick Stolee via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 01/12] bloom: fix whitespace around tab length Derrick Stolee via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 02/12] test-bloom: fix usage typo Derrick Stolee via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 03/12] bloom: parse commit before computing filters Derrick Stolee via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 04/12] Documentation: changed-path Bloom filters use byte words Derrick Stolee via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 05/12] bloom: de-duplicate directory entries Derrick Stolee via GitGitGadget
2020-06-07 21:45 ` SZEDER Gábor
2020-05-11 11:56 ` [PATCH v2 06/12] bloom: use num_changes not nr for limit detection Derrick Stolee via GitGitGadget
2020-08-04 14:51 ` SZEDER Gábor
2020-05-11 11:56 ` [PATCH v2 07/12] completion: offer '--(no-)patch' among 'git log' options SZEDER Gábor via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 08/12] line-log: remove unused fields from 'struct line_log_data' SZEDER Gábor via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 09/12] t4211-line-log: add tests for parent oids SZEDER Gábor via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 10/12] line-log: more responsive, incremental 'git log -L' SZEDER Gábor via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 11/12] line-log: try to use generation number-based topo-ordering SZEDER Gábor via GitGitGadget
2020-05-11 11:56 ` [PATCH v2 12/12] line-log: integrate with changed-path Bloom filters Derrick Stolee via GitGitGadget
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