Re: Changing the default for "core.abbrev"?

From: Kyle J. McKay <mackyle@gmail.com>
Date: Sep 29, 2016, 13:01 UTC
Message-ID: <80D70A8B-0EB6-4118-8193-8D113621C57D@gmail.com>
In-Reply-To: <xmqq37knwcf4.fsf@gitster.mtv.corp.google.com>

On Sep 25, 2016, at 18:39, Linus Torvalds wrote:

Show 14 quoted lines

> The kernel, these days, is at roughly 5 million objects, and while the
> seven hex digits are still often enough for uniqueness (and git will
> always add digits *until* it is unique), it's long been at the point
> where I tell people to do
>
>    git config --global core.abbrev 12
>
> because even though git will extend the seven hex digits until the
> object name is unique, that only reflects the *current* situation in
> the repository. With 5 million objects and a very healthy growth rate,
> a 7-8 hex digit number that is unique today is not necessarily unique
> a month or two from now, and then it gets annoying when a commit
> message has a short git ID that is no longer unique when you go back
> and try to figure out what went wrong in that commit.

On Sep 25, 2016, at 20:46, Junio C Hamano wrote:

Show 11 quoted lines

> Linus Torvalds <torvalds@linux-foundation.org> writes:
>
>> I can just keep reminding kernel maintainers and developers to update
>> their git config, but maybe it would be a good idea to just admit  
>> that
>> the defaults picked in 2005 weren't necessarily the best ones
>> possible, and those could be bumped up a bit?
>
> I am not quite sure how good any new default would be, though.  Just
> like any timeout is not long enough for somebody, growing projects
> will eventually hit whatever abbreviation length they start with.

This made me curious what the situation is really like. So I crunched some data.

Using a recent clone of $korg/torvalds/linux:

$ git rev-parse --verify d597639e203 error: short SHA1 d597639e203 is ambiguous. fatal: Needed a single revision

So the kernel already has 11-character "short" SHA1s that are ambiguous. Is a core.abbrev setting of 12 really good enough?

Here are the stats on the kernel's repository:

Ambiguous length 11 (but not at length 12) info:
   prefixes:       2
                   0 (with 1 or more commit disambiguations)

Ambiguous length 10 (but not at length 11) info:
   prefixes:      12
                   3 (with 1 or more commit disambiguations)
                   0 (with 2 or more commit disambiguations)

Ambiguous length 9 (but not at length 10) info:
   prefixes:     186
                  43 (with 1 or more commit disambiguations)
                   1 (with 2 or more commit disambiguations)
                   0 (with 3 or more disambiguations)

Ambiguous length 8 (but not at length 9) info:
   prefixes:    2723
                 651 (with 1 or more commit disambiguations)
                  40 (with 2 or more commit disambiguations)
                   1 (with 3 or more disambiguations)
   maxambig:       3 (there is 1 of them)

Ambiguous length 7 (but not at length 8) info:
   prefixes:   41864
                9842 (with 1 or more commit disambiguations)
                 680 (with 2 or more commit disambiguations)
                 299 (with 3 or more disambiguations)
   maxambig:       3 (there are 299 of them)

The "maxambig" value is the maximum number of disambiguations for any single prefix at that prefix length. So for prefixes of length 7 there are 299 that disambiguate into 3 objects.

Just out of curiosity, generating stats on the Git repository gives:

Ambiguous length 8 (but not at length 9) info:
   prefixes:       7
                   3 (with 1 or more commit disambiguations)
                   2 (with 2 or more commit disambiguations)
                   0 (with 3 or more disambiguations)

Ambiguous length 7 (but not at length 8) info:
   prefixes:      87
                  36 (with 1 or more commit disambiguations)
                   3 (with 2 or more commit disambiguations)
                   0 (with 3 or more disambiguations)

Running the stats on $github/gitster/git produces some ambiguous length 9 prefixes (one of which contains a commit disambiguation).

--Kyle