From: "Ævar Arnfjörð Bjarmason" <email@example.com> To: Derrick Stolee <firstname.lastname@example.org> Cc: "Junio C Hamano" <email@example.com>, "SZEDER Gábor" <firstname.lastname@example.org>, email@example.com, "Jeff King" <firstname.lastname@example.org>, "Stefan Beller" <email@example.com>, "Duy Nguyen" <firstname.lastname@example.org>, "Jonathan Tan" <email@example.com> Subject: Re: [PATCH 0/4] Bloom filter experiment Date: Tue, 16 Oct 2018 14:57:30 +0200 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> On Tue, Oct 16 2018, Derrick Stolee wrote: > On 10/16/2018 12:45 AM, Junio C Hamano wrote: >> Derrick Stolee <firstname.lastname@example.org> writes: >> >>> 2. The filters are sized according to the number of changes in each >>> commit, with a minimum of one 64-bit word. >>> ... >>> 6. When we compute the Bloom filters, we don't store a filter for >>> commits whose first-parent diff has more than 512 paths. >> Just being curious but was 512 taken out of thin air or is there >> some math behind it, e.g. to limit false positive rate down to >> certain threshold? With a wide-enough bitset, you could store >> arbitrary large number of paths with low enough false positive, I >> guess, but is there a point where there is too many paths in the >> change that gives us diminishing returns and not worth having a >> filter in the first place? > 512 is somewhat arbitrary, but having a maximum size is not. >> In a normal source-code-control context, the set of paths modified >> by any single commit ought to be a small subset of the entire paths, >> and whole-tree changes ought to be fairly rare. In a project for >> which that assumption does not hold, it might help to have a >> negative bloom filter (i.e. "git log -- A" asks "does the commit >> modify A?" and the filter would say "we know it does not, because we >> threw all the paths that are not touched to the bloom filter"), but >> I think that would optimize for a wrong case. > > A commit with many changed paths is very rare. The 512 I picked above > is enough to cover 99% of commits in each of the repos I sampled when > first investigating Bloom filters. > > When a Bloom filter response says "maybe yes" (in our case, "maybe not > TREESAME"), then we need to verify that it is correct. In the extreme > case that every path is changed, then the Bloom filter does nothing > but add extra work. > > These extreme cases are also not unprecedented: in our Azure Repos > codebase, we were using core.autocrlf to smudge CRLFs to LFs, but > when it was time to dogfood VFS for Git, we needed to turn off the > smudge filter. So, there is one commit that converts every LF to a > CRLF in every text file. Storing a Bloom filter for those ~250,000 > entries would take ~256KB for essentially no value. By not storing a > filter for this commit, we go immediately to the regular TREESAME > check, which would happen for most pathspecs. > > This is all to say: having a maximum size is good. 512 is big enough > to cover _most_ commits, but not so big that we may store _really_ big > filters. Makes sense. 512 is good enough to hardcode initially, but I couldn't tell from briefly skimming the patches if it was possible to make this size dynamic per-repo when the graph/filter is written. I.e. we might later add some discovery step where we look at N number of commits at random, until we're satisfied that we've come up with some average/median number of total (recursive) tree entries & how many tend to be changed per-commit. I.e. I can imagine repositories (with some automated changes) where we have 10k files and tend to change 1k per commit, or ones with 10k files where we tend to change just 1-10 per commit, which would mean a larger/smaller filter would be needed / would do.
next prev parent reply other threads:[~2018-10-16 12:57 UTC|newest] Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-03 13:23 We should add a "git gc --auto" after "git clone" due to commit graph Ævar Arnfjörð Bjarmason 2018-10-03 13:36 ` SZEDER Gábor 2018-10-03 13:42 ` Derrick Stolee 2018-10-03 14:18 ` Ævar Arnfjörð Bjarmason 2018-10-03 14:01 ` Ævar Arnfjörð Bjarmason 2018-10-03 14:17 ` SZEDER Gábor 2018-10-03 14:22 ` Ævar Arnfjörð Bjarmason 2018-10-03 14:53 ` SZEDER Gábor 2018-10-03 15:19 ` Ævar Arnfjörð Bjarmason 2018-10-03 16:59 ` SZEDER Gábor 2018-10-05 6:09 ` Junio C Hamano 2018-10-10 22:07 ` SZEDER Gábor 2018-10-10 23:01 ` Ævar Arnfjörð Bjarmason 2018-10-03 19:08 ` Stefan Beller 2018-10-03 19:21 ` Jeff King 2018-10-03 20:35 ` Ævar Arnfjörð Bjarmason 2018-10-03 17:47 ` Stefan Beller 2018-10-03 18:47 ` Ævar Arnfjörð Bjarmason 2018-10-03 18:51 ` Jeff King 2018-10-03 18:59 ` Derrick Stolee 2018-10-03 19:18 ` Jeff King 2018-10-08 16:41 ` SZEDER Gábor 2018-10-08 16:57 ` Derrick Stolee 2018-10-08 18:10 ` SZEDER Gábor 2018-10-08 18:29 ` Derrick Stolee 2018-10-09 3:08 ` Jeff King 2018-10-09 13:48 ` Bloom Filters (was Re: We should add a "git gc --auto" after "git clone" due to commit graph) Derrick Stolee 2018-10-09 18:45 ` Ævar Arnfjörð Bjarmason 2018-10-09 18:46 ` Jeff King 2018-10-09 19:03 ` Derrick Stolee 2018-10-09 21:14 ` Jeff King 2018-10-09 23:12 ` Bloom Filters Jeff King 2018-10-09 23:13 ` [PoC -- do not apply 1/3] initial tree-bitmap proof of concept Jeff King 2018-10-09 23:14 ` [PoC -- do not apply 2/3] test-tree-bitmap: add "dump" mode Jeff King 2018-10-10 0:48 ` Junio C Hamano 2018-10-11 3:13 ` Jeff King 2018-10-09 23:14 ` [PoC -- do not apply 3/3] test-tree-bitmap: replace ewah with custom rle encoding Jeff King 2018-10-10 0:58 ` Junio C Hamano 2018-10-11 3:20 ` Jeff King 2018-10-11 12:33 ` Bloom Filters Derrick Stolee 2018-10-11 13:43 ` Jeff King 2018-10-09 21:30 ` We should add a "git gc --auto" after "git clone" due to commit graph SZEDER Gábor 2018-10-09 19:34 ` [PATCH 0/4] Bloom filter experiment SZEDER Gábor 2018-10-09 19:34 ` [PATCH 1/4] Add a (very) barebones Bloom filter implementation SZEDER Gábor 2018-10-09 19:34 ` [PATCH 2/4] commit-graph: write a Bloom filter containing changed paths for each commit SZEDER Gábor 2018-10-09 21:06 ` Jeff King 2018-10-09 21:37 ` SZEDER Gábor 2018-10-09 19:34 ` [PATCH 3/4] revision.c: use the Bloom filter to speed up path-limited revision walks SZEDER Gábor 2018-10-09 19:34 ` [PATCH 4/4] revision.c: add GIT_TRACE_BLOOM_FILTER for a bit of statistics SZEDER Gábor 2018-10-09 19:47 ` [PATCH 0/4] Bloom filter experiment Derrick Stolee 2018-10-11 1:21 ` [PATCH 0/2] Per-commit filter proof of concept Jonathan Tan 2018-10-11 1:21 ` [PATCH 1/2] One filter per commit Jonathan Tan 2018-10-11 12:49 ` Derrick Stolee 2018-10-11 19:11 ` [PATCH] Per-commit and per-parent filters for 2 parents Jonathan Tan 2018-10-11 1:21 ` [PATCH 2/2] Only make bloom filter for first parent Jonathan Tan 2018-10-11 7:37 ` [PATCH 0/2] Per-commit filter proof of concept Ævar Arnfjörð Bjarmason 2018-10-15 14:39 ` [PATCH 0/4] Bloom filter experiment Derrick Stolee 2018-10-16 4:45 ` Junio C Hamano 2018-10-16 11:13 ` Derrick Stolee 2018-10-16 12:57 ` Ævar Arnfjörð Bjarmason [this message] 2018-10-16 13:03 ` Derrick Stolee 2018-10-18 2:00 ` Junio C Hamano 2018-10-16 23:41 ` Jonathan Tan 2018-10-08 23:02 ` We should add a "git gc --auto" after "git clone" due to commit graph Junio C Hamano 2018-10-03 14:32 ` Duy Nguyen 2018-10-03 16:45 ` Duy Nguyen 2018-10-04 21:42 ` [RFC PATCH] " Ævar Arnfjörð Bjarmason 2018-10-05 12:05 ` Derrick Stolee 2018-10-05 13:05 ` Ævar Arnfjörð Bjarmason 2018-10-05 13:45 ` Derrick Stolee 2018-10-05 14:04 ` Ævar Arnfjörð Bjarmason 2018-10-05 19:21 ` Jeff King 2018-10-05 19:41 ` Derrick Stolee 2018-10-05 19:47 ` Jeff King 2018-10-05 20:00 ` Derrick Stolee 2018-10-05 20:02 ` Jeff King 2018-10-05 20:01 ` Ævar Arnfjörð Bjarmason 2018-10-05 20:09 ` Jeff King
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