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From: Mike Hommey <mh@glandium.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Junio C Hamano <gitster@pobox.com>, Johannes Sixt <j6t@kdbg.org>,
	Git Mailing List <git@vger.kernel.org>, Jeff King <peff@peff.net>
Subject: Re: [PATCH 4/4] core.abbrev: raise the default abbreviation to 12 hexdigits
Date: Fri, 30 Sep 2016 09:56:38 +0900	[thread overview]
Message-ID: <20160930005638.almd66ralshknoxa@glandium.org> (raw)
In-Reply-To: <CA+55aFx9Utm9yDZceks+5q9c8ydc2QMYshWwJ0G0GHWWLwSsXQ@mail.gmail.com>

On Thu, Sep 29, 2016 at 12:06:23PM -0700, Linus Torvalds wrote:
> On Thu, Sep 29, 2016 at 11:55 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > For the kernel, just the *math* right now actually gives 12
> > characters. For current git it actually seems to say that 8 is the
> > correct number. For small projects, you'll still see 7.
> 
> Sorry, the git number is 9, not 8. The reason is that git has roughly
> 212k objects, and 9 hex digits gets expected collisions at about 256k
> objects.
> 
> So the logic means that we'll see 7 hex digits for projects with less
> than 16k objects, 8 hex digits if there are less than 64k objects, and
> 9 hex digits for projects like git that currently have fewer than 256k
> objects.
> 
> But git itself might not be *that* far from going to 10 hex digits
> with my patch.
> 
> The kernel uses 12 he digits because the collision math says that's
> the right thing for a project with between 4M and 16M objects (with
> the kernel being at 5M).

OTOH, how often does one refer to trees or blobs with abbreviated sha1s?
Most of the time, you'd use abbreviated sha1s for commits. And the number
of commits in git and the kernel repositories are much lower than the
number of overall objects.

rev-list --all --count on the git repo gives me 46790. On the kernel, it
gives 618078.

Now, the interesting thing is looking at the *actual* collisions in
those spaces.

At 9 digits, there's only one commit collision in the kernel repo:
  45f014c5264f5e68ef0e51b36f4ef5ede3d18397
  45f014c52eef022873b19d6a20eb0ec9668f2b09

And two commit collisions at 8 digits in the git repo:
  1536dd9c1df0b7167b139f6666080cc4774ef63f
  1536dd9c61b5582cf079999057cb715dd6dc6620

  2e6e3e82ee36b3e1bec1db8db24817270080424e
  2e6e3e829f3759823d70e7af511bc04cd05ad0af

At 7 digits, there are 5 actual commit collisions in the git repo and
718 in the kernel repo only one of those collisions involve more than 2
commits.

Mike

  parent reply	other threads:[~2016-09-30  1:29 UTC|newest]

Thread overview: 111+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-26  1:39 Changing the default for "core.abbrev"? Linus Torvalds
2016-09-26  3:46 ` Junio C Hamano
2016-09-26  4:34   ` Jeff King
2016-09-26  4:45     ` Junio C Hamano
2016-09-26 11:57       ` [PATCH 0/10] helping people resolve ambiguous sha1s Jeff King
2016-09-26 11:59         ` [PATCH 01/10] get_sha1: detect buggy calls with multiple disambiguators Jeff King
2016-09-26 16:37           ` Junio C Hamano
2016-09-26 17:21             ` Jeff King
2016-09-26 17:50               ` Junio C Hamano
2016-09-26 11:59         ` [PATCH 02/10] get_sha1: avoid repeating ourselves via ONLY_TO_DIE Jeff King
2016-09-26 11:59         ` [PATCH 03/10] get_sha1: propagate flags to child functions Jeff King
2016-09-26 11:59         ` [PATCH 04/10] get_short_sha1: peel tags when looking for treeish Jeff King
2016-09-26 12:11           ` Jeff King
2016-09-26 16:55           ` Junio C Hamano
2016-09-26 17:23             ` Jeff King
2016-09-26 12:00         ` [PATCH 05/10] get_short_sha1: refactor init of disambiguation code Jeff King
2016-09-26 12:00         ` [PATCH 06/10] get_short_sha1: NUL-terminate hex prefix Jeff King
2016-09-26 17:10           ` Junio C Hamano
2016-09-26 17:25             ` Jeff King
2016-09-26 17:36               ` Junio C Hamano
2016-09-26 12:00         ` [PATCH 07/10] get_short_sha1: mark ambiguity error for translation Jeff King
2016-09-26 12:00         ` [PATCH 08/10] sha1_array: let callbacks interrupt iteration Jeff King
2016-09-26 12:00         ` [PATCH 09/10] for_each_abbrev: drop duplicate objects Jeff King
2016-09-26 12:00         ` [PATCH 10/10] get_short_sha1: list ambiguous objects on error Jeff King
2016-09-26 16:36           ` Linus Torvalds
2016-09-27  5:42             ` Jacob Keller
2016-09-27 12:38             ` Jeff King
2016-09-29 13:01             ` Kyle J. McKay
2016-09-29 13:24               ` Jeff King
2016-09-29 14:36                 ` Kyle J. McKay
2016-09-29 14:55                   ` Jeff King
2016-09-26 17:30           ` Junio C Hamano
2016-09-26 17:34             ` Jeff King
2016-09-26 17:39               ` Junio C Hamano
2016-09-29 11:46           ` Kyle J. McKay
2016-09-29 13:03             ` Jeff King
2016-09-29 17:19               ` Junio C Hamano
2016-09-30  5:51                 ` Jacob Keller
2019-02-04 16:12     ` [RFC/PATCH] core.abbrev doc: document and test the abbreviation length Ævar Arnfjörð Bjarmason
2019-02-04 19:13       ` Junio C Hamano
2019-02-04 20:04       ` Junio C Hamano
2019-02-04 21:36         ` Ævar Arnfjörð Bjarmason
2019-02-04 23:32         ` Jeff King
2019-02-04 23:50           ` Ævar Arnfjörð Bjarmason
2019-02-06 18:29             ` Jeff King
2019-02-06 18:36               ` Ævar Arnfjörð Bjarmason
2016-09-26  6:33   ` Changing the default for "core.abbrev"? Matthieu Moy
2016-09-26 12:09     ` Jeff King
2016-09-29 13:01   ` Kyle J. McKay
2016-09-26  7:13 ` Christian Couder
2016-09-28 23:30 ` [PATCH 0/4] raising core.abbrev default to 12 hexdigits Junio C Hamano
2016-09-28 23:30   ` [PATCH 1/4] config: allow customizing /etc/gitconfig location Junio C Hamano
2016-09-29  9:53     ` Jakub Narębski
2016-09-29 17:20       ` Junio C Hamano
2016-09-29 17:45         ` Matthieu Moy
2016-09-28 23:30   ` [PATCH 2/4] t13xx: do not assume system config is empty Junio C Hamano
2016-09-29  9:01     ` Jeff King
2016-09-29 18:13       ` Junio C Hamano
2016-09-29 18:26         ` Jeff King
2016-09-29 18:57           ` Junio C Hamano
2016-09-29 19:18             ` Jeff King
2016-09-29 19:57               ` Junio C Hamano
2016-09-29 19:06           ` Junio C Hamano
2016-09-29 19:26             ` Jeff King
2016-09-29 21:03               ` Junio C Hamano
2016-09-29 21:08                 ` Jeff King
2016-09-28 23:30   ` [PATCH 3/4] worktree: honor configuration variables Junio C Hamano
2016-09-28 23:30   ` [PATCH 4/4] core.abbrev: raise the default abbreviation to 12 hexdigits Junio C Hamano
2016-09-29  2:44     ` SZEDER Gábor
2016-09-29  5:27       ` Lukas Fleischer
2016-09-29  9:22         ` Jeff King
2016-09-29  9:15       ` Jeff King
2016-09-29 10:03         ` Matthieu Moy
2016-09-29 12:52         ` SZEDER Gábor
2016-09-29  5:58     ` Johannes Sixt
2016-09-29 18:05       ` Junio C Hamano
2016-09-29 18:37         ` Linus Torvalds
2016-09-29 18:55           ` Linus Torvalds
2016-09-29 19:06             ` Linus Torvalds
2016-09-29 19:42               ` Junio C Hamano
2016-09-30  0:56               ` Mike Hommey [this message]
2016-09-30  1:01                 ` Linus Torvalds
2016-09-30 19:41                   ` Ævar Arnfjörð Bjarmason
2016-09-29 19:16             ` Jeff King
2016-09-29 19:40               ` Linus Torvalds
2016-09-29 19:45                 ` Junio C Hamano
2016-09-29 21:53                   ` Linus Torvalds
2016-09-29 23:13                     ` Junio C Hamano
2016-09-29 23:20                       ` Junio C Hamano
2016-09-30  0:20                       ` Linus Torvalds
2016-09-30  0:28                         ` Linus Torvalds
2016-09-30  0:57                           ` Linus Torvalds
2016-09-30  1:18                             ` Linus Torvalds
2016-09-30  3:54                               ` Junio C Hamano
2016-09-30  4:10                                 ` Junio C Hamano
2016-09-30  4:18                                   ` Linus Torvalds
2016-09-30  4:29                                     ` Linus Torvalds
2016-09-30  4:27                                   ` Junio C Hamano
2016-09-30  4:35                                     ` Junio C Hamano
2016-09-30 18:40                                     ` Junio C Hamano
2016-09-30 18:51                                       ` Linus Torvalds
2016-09-30 19:00                                         ` Junio C Hamano
2016-09-30  4:11                                 ` Linus Torvalds
2016-09-30  8:06                               ` Jeff King
2016-09-30 17:54                                 ` Linus Torvalds
2016-09-30 18:05                                   ` Jeff King
2016-09-30 18:21                                   ` Linus Torvalds
2016-09-30 20:01                                     ` Junio C Hamano
2016-09-30 17:56                                 ` Junio C Hamano
2016-09-30  7:47                       ` Jeff King
2016-09-29  9:25     ` Jeff King

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