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From: Derrick Stolee <stolee@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: "SZEDER Gábor" <szeder.dev@gmail.com>,
	git@vger.kernel.org, "Jeff King" <peff@peff.net>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Stefan Beller" <sbeller@google.com>,
	"Duy Nguyen" <pclouds@gmail.com>,
	"Jonathan Tan" <jonathantanmy@google.com>
Subject: Re: [PATCH 0/4] Bloom filter experiment
Date: Tue, 16 Oct 2018 07:13:03 -0400
Message-ID: <8c66cbe3-6830-05cb-f3bb-be2e4902e8f5@gmail.com> (raw)
In-Reply-To: <xmqqlg6y1ovh.fsf@gitster-ct.c.googlers.com>

On 10/16/2018 12:45 AM, Junio C Hamano wrote:
> Derrick Stolee <stolee@gmail.com> 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.

Thanks,
-Stolee


  reply index

Thread overview: 76+ 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  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 [this message]
2018-10-16 12:57                               ` Ævar Arnfjörð Bjarmason
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
2018-10-11 12:49 [PATCH 1/2] One filter per commit Derrick Stolee
2018-10-11 19:11 ` [PATCH] Per-commit and per-parent filters for 2 parents Jonathan Tan

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