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#!/bin/sh
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test_description='Testing the various Bloom filter computations in bloom.c'
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. ./test-lib.sh
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test_expect_success 'compute unseeded murmur3 hash for empty string' '
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cat >expect <<-\EOF &&
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Murmur3 Hash with seed=0:0x00000000
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EOF
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test-tool bloom get_murmur3 "" >actual &&
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test_cmp expect actual
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'
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test_expect_success 'compute unseeded murmur3 hash for test string 1' '
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cat >expect <<-\EOF &&
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Murmur3 Hash with seed=0:0x627b0c2c
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EOF
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test-tool bloom get_murmur3 "Hello world!" >actual &&
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test_cmp expect actual
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'
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test_expect_success 'compute unseeded murmur3 hash for test string 2' '
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cat >expect <<-\EOF &&
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Murmur3 Hash with seed=0:0x2e4ff723
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EOF
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test-tool bloom get_murmur3 "The quick brown fox jumps over the lazy dog" >actual &&
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test_cmp expect actual
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'
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bloom.c: introduce core Bloom filter constructs
Introduce the constructs for Bloom filters, Bloom filter keys
and Bloom filter settings.
For details on what Bloom filters are and how they work, refer
to Dr. Derrick Stolee's blog post [1]. It provides a concise
explanation of the adoption of Bloom filters as described in
[2] and [3].
Implementation specifics:
1. We currently use 7 and 10 for the number of hashes and the
size of each entry respectively. They served as great starting
values, the mathematical details behind this choice are
described in [1] and [4]. The implementation, while not
completely open to it at the moment, is flexible enough to allow
for tweaking these settings in the future.
Note: The performance gains we have observed with these values
are significant enough that we did not need to tweak these
settings. The performance numbers are included in the cover letter
of this series and in the commit message of the subsequent commit
where we use Bloom filters to speed up `git log -- path`.
2. As described in [1] and [3], we do not need 7 independent hashing
functions. We use the Murmur3 hashing scheme, seed it twice and
then combine those to procure an arbitrary number of hash values.
3. The filters will be sized according to the number of changes in
each commit, in multiples of 8 bit words.
[1] Derrick Stolee
"Supercharging the Git Commit Graph IV: Bloom Filters"
https://devblogs.microsoft.com/devops/super-charging-the-git-commit-graph-iv-Bloom-filters/
[2] Flavio Bonomi, Michael Mitzenmacher, Rina Panigrahy, Sushil Singh, George Varghese
"An Improved Construction for Counting Bloom Filters"
http://theory.stanford.edu/~rinap/papers/esa2006b.pdf
https://doi.org/10.1007/11841036_61
[3] Peter C. Dillinger and Panagiotis Manolios
"Bloom Filters in Probabilistic Verification"
http://www.ccs.neu.edu/home/pete/pub/Bloom-filters-verification.pdf
https://doi.org/10.1007/978-3-540-30494-4_26
[4] Thomas Mueller Graf, Daniel Lemire
"Xor Filters: Faster and Smaller Than Bloom and Cuckoo Filters"
https://arxiv.org/abs/1912.08258
Helped-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
5 years ago
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test_expect_success 'compute bloom key for empty string' '
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cat >expect <<-\EOF &&
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Hashes:0x5615800c|0x5b966560|0x61174ab4|0x66983008|0x6c19155c|0x7199fab0|0x771ae004|
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Filter_Length:2
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Filter_Data:11|11|
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EOF
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test-tool bloom generate_filter "" >actual &&
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test_cmp expect actual
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'
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test_expect_success 'compute bloom key for whitespace' '
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cat >expect <<-\EOF &&
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Hashes:0xf178874c|0x5f3d6eb6|0xcd025620|0x3ac73d8a|0xa88c24f4|0x16510c5e|0x8415f3c8|
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Filter_Length:2
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Filter_Data:51|55|
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EOF
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test-tool bloom generate_filter " " >actual &&
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test_cmp expect actual
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'
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test_expect_success 'compute bloom key for test string 1' '
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cat >expect <<-\EOF &&
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Hashes:0xb270de9b|0x1bb6f26e|0x84fd0641|0xee431a14|0x57892de7|0xc0cf41ba|0x2a15558d|
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Filter_Length:2
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Filter_Data:92|6c|
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EOF
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test-tool bloom generate_filter "Hello world!" >actual &&
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test_cmp expect actual
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'
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test_expect_success 'compute bloom key for test string 2' '
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cat >expect <<-\EOF &&
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Hashes:0x20ab385b|0xf5237fe2|0xc99bc769|0x9e140ef0|0x728c5677|0x47049dfe|0x1b7ce585|
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Filter_Length:2
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Filter_Data:a5|4a|
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EOF
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test-tool bloom generate_filter "file.txt" >actual &&
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test_cmp expect actual
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'
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test_expect_success 'get bloom filters for commit with no changes' '
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git init &&
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git commit --allow-empty -m "c0" &&
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cat >expect <<-\EOF &&
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Filter_Length:0
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Filter_Data:
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EOF
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test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&
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test_cmp expect actual
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'
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test_expect_success 'get bloom filter for commit with 10 changes' '
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rm actual &&
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rm expect &&
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mkdir smallDir &&
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for i in $(test_seq 0 9)
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do
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echo $i >smallDir/$i
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done &&
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git add smallDir &&
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git commit -m "commit with 10 changes" &&
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cat >expect <<-\EOF &&
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Filter_Length:14
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Filter_Data:02|b3|c4|a0|34|e7|fe|eb|cb|47|fe|a0|e8|72|
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EOF
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test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&
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test_cmp expect actual
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'
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test_expect_success EXPENSIVE 'get bloom filter for commit with 513 changes' '
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rm actual &&
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rm expect &&
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mkdir bigDir &&
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bloom: use num_changes not nr for limit detection
As diff_tree_oid() computes a diff, it will terminate early if the
total number of changed paths is strictly larger than max_changes.
This includes the directories that changed, not just the file paths.
However, only the file paths are reflected in the resulting diff
queue's "nr" value.
Use the "num_changes" from diffopt to check if the diff terminated
early. This is incredibly important, as it can result in incorrect
filters! For example, the first commit in the Linux kernel repo
reports only 471 changes, but since these are nested inside several
directories they expand to 513 "real" changes, and in fact the
total list of changes is not reported. Thus, the computed filter
for this commit is incorrect.
Demonstrate the subtle difference by using one fewer file change
in the 'get bloom filter for commit with 513 changes' test. Before,
this edited 513 files inside "bigDir" which hit this inequality.
However, dropping the file count by one demonstrates how the
previous inequality was incorrect but the new one is correct.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
5 years ago
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for i in $(test_seq 0 511)
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do
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echo $i >bigDir/$i
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done &&
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git add bigDir &&
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git commit -m "commit with 513 changes" &&
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cat >expect <<-\EOF &&
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Filter_Length:0
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Filter_Data:
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EOF
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test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&
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test_cmp expect actual
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'
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test_done
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