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From: Derrick Stolee <stolee@gmail.com>
To: "Taylor Blau" <me@ttaylorr.com>, "Jakub Narębski" <jnareb@gmail.com>
Cc: Junio C Hamano <gitster@pobox.com>,
	Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>,
	git <git@vger.kernel.org>, Garima Singh <garimasigit@gmail.com>,
	Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH 1/3] revision: complicated pathspecs disable filters
Date: Thu, 16 Apr 2020 09:26:49 -0400	[thread overview]
Message-ID: <67350f6c-d558-8b9b-4d57-5830f7a74fab@gmail.com> (raw)
In-Reply-To: <20200416005241.GB36156@syl.local>

On 4/15/2020 8:52 PM, Taylor Blau wrote:
> On Thu, Apr 16, 2020 at 12:18:33AM +0200, Jakub Narębski wrote:
>> On Wed, 15 Apr 2020 at 20:37, Derrick Stolee <stolee@gmail.com> wrote:
>> [...]
>>> -->8--
>>> From 89beb9598daabb19e3c896bbceeb0fc1b9ccc6ca Mon Sep 17 00:00:00 2001
>>> From: Derrick Stolee <dstolee@microsoft.com>
>>> Date: Wed, 15 Apr 2020 18:04:25 +0000
>>> Subject: [PATCH] bloom: compute all Bloom hashes from lowercase
>>>
>>> The changed-path Bloom filters currently hash path strings using
>>> the exact string for the path. This makes it difficult* to use the
>>> filters when restricting to case-insensitive pathspecs.
>>>
>>> * I say "difficult" because it is possible to generate all 2^n
>>>   options for the case of a path and test them all, but this is
>>>   a bad idea and should not be done. "Impossible" is an appropriate
>>>   alternative.
>>>
>>> THIS IS A BREAKING CHANGE. Commit-graph files with changed-path
>>> Bloom filters computed by a previous commit will not be compatible
>>> with the filters computed in this commit, nor will we get correct
>>> results when testing across these incompatible versions. Normally,
>>> this would be a completely unacceptable change, but the filters
>>> have not been released and hence are still possible to update
>>> before release.
>>>
>>> TODO: If we decide to move in this direction, then the following
>>> steps should be done (and some of them should be done anyway):
>>>
>>> * We need to document the Bloom filter format to specify exactly
>>>   how we compute the filter data. The details should be careful
>>>   enough that someone can reproduce the exact file format without
>>>   looking at the C code.
>>>
>>> * That document would include the tolower() transformation that is
>>>   being done here.
>>
>> Why not modify the BDAT chunk to include version of
>> case folding transformation or other collation algorithm
>> (other transformation).that is done prior to computing
>> the Bloom filter key? Though that might be unnecessary
>> flexibility...
> 
> If this ends up being something that we want to do, I agree with
> Stolee's reasoning that this should be a breaking change. If we were,
> say, several months into having Bloom filters in a release and decided
> at that point to make the change, then: sure, supporting both by writing
> a bit in the BDAT chunk makes sense.
> 
> But, we're many months away from that state yet, and so I don't think
> the cost of rebuilding what few commit-graphs exist with bloom filters
> in them today to support both ordinary and lower-cased paths in the
> filter.
> 
> Anyway, I'm still not sold on this idea in general (nor do I understand
> it that others are), so I'll respond in more detail in another part of
> the thread...

I agree that this is not a good direction to go. I created the patch
because I was curious how difficult it would be, and it is good to have
a record of the possible direction. However, it complicates the file
format and will have unpredictable effects on the entropy (or on the
performance of history for case-colliding paths).

It is good that we have the capability to extend the filter data in
the future if we really need to.

I'll make a TODO item for myself to try writing that detailed Bloom
filter format documentation as a follow-up. In the meantime, I'll try
to close this out by responding to the feedback we have so far.

Thanks,
-Stolee



  reply	other threads:[~2020-04-16 16:02 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-11  1:02 [PATCH 0/3] Integrate changed-path Bloom filters with 'git blame' Derrick Stolee via GitGitGadget
2020-04-11  1:02 ` [PATCH 1/3] revision: complicated pathspecs disable filters Derrick Stolee via GitGitGadget
2020-04-11 21:40   ` Junio C Hamano
2020-04-13 11:49     ` Derrick Stolee
2020-04-14 18:25       ` Junio C Hamano
2020-04-15 13:27         ` Derrick Stolee
2020-04-15 18:37           ` Derrick Stolee
2020-04-15 19:32             ` Junio C Hamano
2020-04-15 19:39               ` Junio C Hamano
2020-04-15 21:25             ` Junio C Hamano
2020-04-16  0:56               ` Taylor Blau
2020-04-15 22:18             ` Jakub Narębski
2020-04-16  0:52               ` Taylor Blau
2020-04-16 13:26                 ` Derrick Stolee [this message]
2020-04-16 16:33                   ` Taylor Blau
2020-04-16 18:02                     ` Junio C Hamano
2020-04-12 22:22   ` Taylor Blau
2020-04-12 22:30     ` Junio C Hamano
2020-04-13  0:07       ` Taylor Blau
2020-04-13 11:54         ` Derrick Stolee
2020-04-11  1:03 ` [PATCH 2/3] commit: write commit-graph with bloom filters Derrick Stolee via GitGitGadget
2020-04-11 21:57   ` Junio C Hamano
2020-04-12 20:51     ` Taylor Blau
2020-04-13 12:08       ` Derrick Stolee
2020-04-13 22:11         ` Junio C Hamano
2020-04-11  1:03 ` [PATCH 3/3] blame: use changed-path Bloom filters Derrick Stolee via GitGitGadget
2020-04-11 22:03   ` Junio C Hamano
2020-04-12  7:39     ` Eric Sunshine
2020-04-11 21:30 ` [PATCH 0/3] Integrate changed-path Bloom filters with 'git blame' Junio C Hamano
2020-04-13 14:45 ` [PATCH v2 0/4] " Derrick Stolee via GitGitGadget
2020-04-13 14:45   ` [PATCH v2 1/4] revision: complicated pathspecs disable filters Derrick Stolee via GitGitGadget
2020-04-13 16:09     ` Taylor Blau
2020-04-13 22:18       ` Junio C Hamano
2020-04-13 14:45   ` [PATCH v2 2/4] commit: write commit-graph with Bloom filters Derrick Stolee via GitGitGadget
2020-04-13 16:12     ` Taylor Blau
2020-04-13 22:21       ` Junio C Hamano
2020-04-14 15:04         ` Derrick Stolee
2020-04-14 17:26           ` Junio C Hamano
2020-04-14 17:40             ` Derrick Stolee
2020-04-15  0:17               ` Taylor Blau
2020-04-13 14:45   ` [PATCH v2 3/4] commit-graph: write commit-graph in more tests Derrick Stolee via GitGitGadget
2020-04-13 14:45   ` [PATCH v2 4/4] blame: use changed-path Bloom filters Derrick Stolee via GitGitGadget
2020-04-13 16:21   ` [PATCH v2 0/4] Integrate changed-path Bloom filters with 'git blame' Taylor Blau
2020-04-16 20:14   ` [PATCH v3 0/3] " Derrick Stolee via GitGitGadget
2020-04-16 20:14     ` [PATCH v3 1/3] revision: complicated pathspecs disable filters Derrick Stolee via GitGitGadget
2020-06-07 20:33       ` SZEDER Gábor
2020-04-16 20:14     ` [PATCH v3 2/3] tests: write commit-graph with Bloom filters Derrick Stolee via GitGitGadget
2020-04-16 20:14     ` [PATCH v3 3/3] blame: use changed-path " Derrick Stolee via GitGitGadget

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