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Re: [PATCH v3 12/14] commit-graph: add large-filters bitmap chunk

From
Taylor Blau <me@ttaylorr.com>
Date
Sep 2, 2020, 20:23 UTC
Message-ID
<20200902202323.GA5281@nand.local>
In-Reply-To
<20200819133526.GE29528@szeder.dev>

(Finally getting back to this review after working on some other topics for a while..., sorry for the late response).

On Wed, Aug 19, 2020 at 03:35:26PM +0200, SZEDER Gábor wrote:
Show 11 quoted lines
> On Tue, Aug 11, 2020 at 04:52:07PM -0400, Taylor Blau wrote:
> > When a commit has more than a certain number of changed paths (commonly
> > 512), the commit-graph machinery represents it as a zero-length filter.
> > This is done since having many entries in the Bloom filter has
> > undesirable effects on the false positivity rate.
>
> This is not the case, the false positive probability depends on the
> ratio of the Bloom filter's size and the number of elements it
> contains, and we size the filters proportional to the number of
> elements they contain, so the number of elements shouldn't affect the
> false positive rate.

I'm not sure that I understand. I agree that the FPR depends on the ratio between the number of elements in the filter and the filter's "size". But, consider a Bloom filter that is too small to faithfully represent all its elements. Such a filter would likely have all its bits set high, in which case every query would return "maybe", and the FPR would go up.

> On the contrary, it's the small filters, up to around 30-35 bytes,
> that tend to have larger than expected false positive rate when using
> double hashing.

I agree that small filters suffer from the same, but I think this is an "in addition" not an "on the contrary".

In either case, I don't think that this is an important detail for the commit message. What matters is the representation (that we truncate >= 512 elements to a length-zero filter), not why (that can be found in another commit). I'd have expected to find the rationale in ed591febb4 (bloom.c: core Bloom filter implementation for changed paths., 2020-03-30), but I couldn't find anything there.

So, I'll drop this sentence entirely to avoid an unimportant detail.

Thanks, Taylor

Previous: SZEDER GáborNext: SZEDER Gábor
Message 67 of 117 in “more miscellaneous Bloom filter improvements”
  1. 00/10 more miscellaneous Bloom filter improvementsTaylor Blau, Aug 3, 2020
  2. 01/10 commit-graph: introduce 'get_bloom_filter_settings()'Taylor Blau, Aug 3, 2020
  3. Jeff KingAug 4, 2020
  4. Taylor BlauAug 4, 2020
  5. 02/10 commit-graph: pass a 'struct repository *' in more placesTaylor Blau, Aug 3, 2020
  6. 03/10 t4216: use an '&&'-chainTaylor Blau, Aug 3, 2020
  7. 04/10 t/helper/test-read-graph.c: prepare repo settingsTaylor Blau, Aug 3, 2020
  8. 05/10 commit-graph: respect 'commitgraph.readChangedPaths'Taylor Blau, Aug 3, 2020
  9. 06/10 commit-graph.c: sort index into commits listTaylor Blau, Aug 3, 2020
  10. Derrick StoleeAug 4, 2020
  11. Taylor BlauAug 4, 2020
  12. 07/10 commit-graph: add large-filters bitmap chunkTaylor Blau, Aug 3, 2020
  13. Taylor BlauAug 3, 2020
  14. Derrick StoleeAug 4, 2020
  15. 08/10 bloom: split 'get_bloom_filter()' in twoTaylor Blau, Aug 3, 2020
  16. Derrick StoleeAug 4, 2020
  17. Taylor BlauAug 4, 2020
  18. 09/10 commit-graph: rename 'split_commit_graph_opts'Taylor Blau, Aug 3, 2020
  19. 10/10 builtin/commit-graph.c: introduce '--max-new-filters=<n>'Taylor Blau, Aug 3, 2020
  20. Derrick StoleeAug 4, 2020
  21. Taylor BlauAug 4, 2020
  22. 00/14 more miscellaneous Bloom filter improvementsTaylor Blau, Aug 5, 2020
  23. 01/14 commit-graph: introduce 'get_bloom_filter_settings()'Taylor Blau, Aug 5, 2020
  24. 02/14 t4216: use an '&&'-chainTaylor Blau, Aug 5, 2020
  25. 09/14 bloom/diff: properly short-circuit on max_changesTaylor Blau, Aug 5, 2020
  26. 10/14 commit-graph.c: sort index into commits listTaylor Blau, Aug 5, 2020
  27. 08/14 bloom: use provided 'struct bloom_filter_settings'Taylor Blau, Aug 5, 2020
  28. 07/14 bloom: split 'get_bloom_filter()' in twoTaylor Blau, Aug 5, 2020
  29. 12/14 commit-graph: add large-filters bitmap chunkTaylor Blau, Aug 5, 2020
  30. Junio C HamanoAug 5, 2020
  31. Taylor BlauAug 5, 2020
  32. Junio C HamanoAug 5, 2020
  33. Taylor BlauAug 5, 2020
  34. Taylor BlauAug 11, 2020
  35. Junio C HamanoAug 11, 2020
  36. 11/14 csum-file.h: introduce 'hashwrite_be64()'Taylor Blau, Aug 5, 2020
  37. 13/14 commit-graph: rename 'split_commit_graph_opts'Taylor Blau, Aug 5, 2020
  38. 03/14 commit-graph: pass a 'struct repository *' in more placesTaylor Blau, Aug 5, 2020
  39. 06/14 commit-graph.c: store maximum changed pathsTaylor Blau, Aug 5, 2020
  40. 04/14 t/helper/test-read-graph.c: prepare repo settingsTaylor Blau, Aug 5, 2020
  41. 05/14 commit-graph: respect 'commitGraph.readChangedPaths'Taylor Blau, Aug 5, 2020
  42. 14/14 builtin/commit-graph.c: introduce '--max-new-filters=<n>'Taylor Blau, Aug 5, 2020
  43. 00/14 more miscellaneous Bloom filter improvementsTaylor Blau, Aug 11, 2020
  44. 01/14 commit-graph: introduce 'get_bloom_filter_settings()'Taylor Blau, Aug 11, 2020
  45. SZEDER GáborAug 11, 2020
  46. Taylor BlauAug 11, 2020
  47. SZEDER GáborAug 11, 2020
  48. Taylor BlauAug 11, 2020
  49. SZEDER GáborAug 11, 2020
  50. Derrick StoleeAug 12, 2020
  51. Taylor BlauAug 14, 2020
  52. 02/14 t4216: use an '&&'-chainTaylor Blau, Aug 11, 2020
  53. 03/14 commit-graph: pass a 'struct repository *' in more placesTaylor Blau, Aug 11, 2020
  54. 04/14 t/helper/test-read-graph.c: prepare repo settingsTaylor Blau, Aug 11, 2020
  55. 05/14 commit-graph: respect 'commitGraph.readChangedPaths'Taylor Blau, Aug 11, 2020
  56. 06/14 commit-graph.c: store maximum changed pathsTaylor Blau, Aug 11, 2020
  57. 07/14 bloom: split 'get_bloom_filter()' in twoTaylor Blau, Aug 11, 2020
  58. 11/14 csum-file.h: introduce 'hashwrite_be64()'Taylor Blau, Aug 11, 2020
  59. 08/14 bloom: use provided 'struct bloom_filter_settings'Taylor Blau, Aug 11, 2020
  60. 09/14 bloom/diff: properly short-circuit on max_changesTaylor Blau, Aug 11, 2020
  61. 10/14 commit-graph.c: sort index into commits listTaylor Blau, Aug 11, 2020
  62. 12/14 commit-graph: add large-filters bitmap chunkTaylor Blau, Aug 11, 2020
  63. Derrick StoleeAug 11, 2020
  64. Taylor BlauAug 11, 2020
  65. Taylor BlauAug 11, 2020
  66. SZEDER GáborAug 19, 2020
  67. Taylor BlauSep 2, 2020
  68. SZEDER GáborSep 1, 2020
  69. Taylor BlauSep 2, 2020
  70. 13/14 commit-graph: rename 'split_commit_graph_opts'Taylor Blau, Aug 11, 2020
  71. SZEDER GáborAug 19, 2020
  72. Taylor BlauSep 2, 2020
  73. 14/14 builtin/commit-graph.c: introduce '--max-new-filters=<n>'Taylor Blau, Aug 11, 2020
  74. SZEDER GáborAug 12, 2020
  75. Taylor BlauAug 14, 2020
  76. SZEDER GáborAug 17, 2020
  77. Taylor BlauSep 2, 2020
  78. Derrick StoleeAug 12, 2020
  79. Taylor BlauAug 14, 2020
  80. SZEDER GáborAug 18, 2020
  81. Taylor BlauSep 3, 2020
  82. SZEDER GáborAug 19, 2020
  83. Taylor BlauSep 3, 2020
  84. SZEDER GáborSep 4, 2020
  85. SZEDER GáborSep 1, 2020
  86. Taylor BlauSep 3, 2020
  87. Junio C HamanoSep 3, 2020
  88. Taylor BlauSep 3, 2020
  89. 00/14 more miscellaneous Bloom filter improvementsTaylor Blau, Sep 3, 2020
  90. 01/14 commit-graph: introduce 'get_bloom_filter_settings()'Taylor Blau, Sep 3, 2020
  91. 02/14 t4216: use an '&&'-chainTaylor Blau, Sep 3, 2020
  92. 04/14 t/helper/test-read-graph.c: prepare repo settingsTaylor Blau, Sep 3, 2020
  93. 03/14 commit-graph: pass a 'struct repository *' in more placesTaylor Blau, Sep 3, 2020
  94. 05/14 commit-graph: respect 'commitGraph.readChangedPaths'Taylor Blau, Sep 3, 2020
  95. 06/14 commit-graph.c: store maximum changed pathsTaylor Blau, Sep 3, 2020
  96. 07/14 bloom: split 'get_bloom_filter()' in twoTaylor Blau, Sep 3, 2020
  97. Jakub NarębskiSep 5, 2020
  98. Taylor BlauSep 5, 2020
  99. Jakub NarębskiSep 5, 2020
  100. Taylor BlauSep 5, 2020
  101. Jakub NarębskiSep 5, 2020
  102. Taylor BlauSep 5, 2020
  103. Taylor BlauSep 5, 2020
  104. SZEDER GáborSep 5, 2020
  105. Taylor BlauSep 5, 2020
  106. Junio C HamanoSep 6, 2020
  107. 08/14 bloom: use provided 'struct bloom_filter_settings'Taylor Blau, Sep 3, 2020
  108. 09/14 bloom/diff: properly short-circuit on max_changesTaylor Blau, Sep 3, 2020
  109. 10/14 commit-graph.c: sort index into commits listTaylor Blau, Sep 3, 2020
  110. 11/14 csum-file.h: introduce 'hashwrite_be64()'Taylor Blau, Sep 3, 2020
  111. René ScharfeSep 4, 2020
  112. Taylor BlauSep 4, 2020
  113. 12/14 commit-graph: add large-filters bitmap chunkTaylor Blau, Sep 3, 2020
  114. 13/14 commit-graph: rename 'split_commit_graph_opts'Taylor Blau, Sep 3, 2020
  115. Taylor BlauSep 4, 2020
  116. 14/14 builtin/commit-graph.c: introduce '--max-new-filters=<n>'Taylor Blau, Sep 3, 2020
  117. Derrick StoleeSep 4, 2020

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