From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Derrick Stolee <stolee@gmail.com>,
git@vger.kernel.org, dstolee@microsoft.com,
git@jeffhostetler.com, gitster@pobox.com,
johannes.schindelin@gmx.de, jrnieder@gmail.com,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 00/18] Multi-pack index (MIDX)
Date: Mon, 08 Jan 2018 13:28:31 +0100 [thread overview]
Message-ID: <87efn0bkls.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <20180108102708.GA19461@sigill.intra.peff.net>
On Mon, Jan 08 2018, Jeff King jotted:
> On Mon, Jan 08, 2018 at 05:20:29AM -0500, Jeff King wrote:
>
>> I.e., what if we did something like this:
>>
>> diff --git a/sha1_name.c b/sha1_name.c
>> index 611c7d24dd..04c661ba85 100644
>> --- a/sha1_name.c
>> +++ b/sha1_name.c
>> @@ -600,6 +600,15 @@ int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
>> if (len == GIT_SHA1_HEXSZ || !len)
>> return GIT_SHA1_HEXSZ;
>>
>> + /*
>> + * A default length of 10 implies a repository big enough that it's
>> + * getting expensive to double check the ambiguity of each object,
>> + * and the chance that any particular object of interest has a
>> + * collision is low.
>> + */
>> + if (len >= 10)
>> + return len;
>> +
>
> Oops, this really needs to terminate the string in addition to returning
> the length (so it was always printing 40 characters in most cases). The
> correct patch is below, but it performs the same.
>
> diff --git a/sha1_name.c b/sha1_name.c
> index 611c7d24dd..5921298a80 100644
> --- a/sha1_name.c
> +++ b/sha1_name.c
> @@ -600,6 +600,17 @@ int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
> if (len == GIT_SHA1_HEXSZ || !len)
> return GIT_SHA1_HEXSZ;
>
> + /*
> + * A default length of 10 implies a repository big enough that it's
> + * getting expensive to double check the ambiguity of each object,
> + * and the chance that any particular object of interest has a
> + * collision is low.
> + */
> + if (len >= 10) {
> + hex[len] = 0;
> + return len;
> + }
> +
> mad.init_len = len;
> mad.cur_len = len;
> mad.hex = hex;
That looks much more sensible, leaving aside other potential benefits of
MIDX.
Given the argument Linus made in e6c587c733 ("abbrev: auto size the
default abbreviation", 2016-09-30) maybe we should add a small integer
to the length for good measure, i.e. something like:
if (len >= 10) {
int extra = 2; /* or just 1? or maybe 0 ... */
hex[len + extra] = 0;
return len + extra;
}
I tried running:
git log --pretty=format:%h --abbrev=7 | perl -nE 'chomp; say length'|sort|uniq -c|sort -nr
On several large repos, which forces something like the disambiguation
we had before Linus's patch, on e.g. David Turner's
2015-04-03-1M-git.git test repo it's:
952858 7
44541 8
2861 9
168 10
17 11
2 12
And the default abbreviation picks 12. I haven't yet found a case where
it's wrong, but if we wanted to be extra safe we could just add a byte
or two to the SHA-1.
next prev parent reply other threads:[~2018-01-08 12:28 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-07 18:14 [RFC PATCH 00/18] Multi-pack index (MIDX) Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 01/18] docs: Multi-Pack Index (MIDX) Design Notes Derrick Stolee
2018-01-08 19:32 ` Jonathan Tan
2018-01-08 20:35 ` Derrick Stolee
2018-01-08 22:06 ` Jonathan Tan
2018-01-07 18:14 ` [RFC PATCH 02/18] midx: specify midx file format Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 03/18] midx: create core.midx config setting Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 04/18] midx: write multi-pack indexes for an object list Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 05/18] midx: create midx builtin with --write mode Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 06/18] midx: add t5318-midx.sh test script Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 07/18] midx: teach midx --write to update midx-head Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 08/18] midx: teach git-midx to read midx file details Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 09/18] midx: find details of nth object in midx Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 10/18] midx: use existing midx when writing Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 11/18] midx: teach git-midx to clear midx files Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 12/18] midx: teach git-midx to delete expired files Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 13/18] t5318-midx.h: confirm git actions are stable Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 14/18] midx: load midx files when loading packs Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 15/18] midx: use midx for approximate object count Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 16/18] midx: nth_midxed_object_oid() and bsearch_midx() Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 17/18] sha1_name: use midx for abbreviations Derrick Stolee
2018-01-07 18:14 ` [RFC PATCH 18/18] packfile: use midx for object loads Derrick Stolee
2018-01-07 22:42 ` [RFC PATCH 00/18] Multi-pack index (MIDX) Ævar Arnfjörð Bjarmason
2018-01-08 0:08 ` Derrick Stolee
2018-01-08 10:20 ` Jeff King
2018-01-08 10:27 ` Jeff King
2018-01-08 12:28 ` Ævar Arnfjörð Bjarmason [this message]
2018-01-08 13:43 ` Johannes Schindelin
2018-01-09 6:50 ` Jeff King
2018-01-09 13:05 ` Johannes Schindelin
2018-01-09 19:51 ` Stefan Beller
2018-01-09 20:12 ` Junio C Hamano
2018-01-09 20:16 ` Stefan Beller
2018-01-09 21:31 ` Junio C Hamano
2018-01-10 17:05 ` Johannes Schindelin
2018-01-10 10:57 ` Jeff King
2018-01-08 13:43 ` Derrick Stolee
2018-01-09 7:12 ` Jeff King
2018-01-08 11:43 ` Ævar Arnfjörð Bjarmason
2018-06-06 8:13 ` Ævar Arnfjörð Bjarmason
2018-06-06 10:27 ` [RFC PATCH 0/2] unconditional O(1) SHA-1 abbreviation Ævar Arnfjörð Bjarmason
2018-06-06 10:27 ` [RFC PATCH 1/2] config.c: use braces on multiple conditional arms Ævar Arnfjörð Bjarmason
2018-06-06 10:27 ` [RFC PATCH 2/2] sha1-name: add core.validateAbbrev & relative core.abbrev Ævar Arnfjörð Bjarmason
2018-06-06 12:04 ` Christian Couder
2018-06-06 11:24 ` [RFC PATCH 00/18] Multi-pack index (MIDX) Derrick Stolee
2018-01-10 18:25 ` Martin Fick
2018-01-10 19:39 ` Derrick Stolee
2018-01-10 21:01 ` Martin Fick
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