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From: SZEDER Gábor <szeder.dev@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Ævar Arnfjörð Bjarmason <avarab@gmail.com>,
	Derrick Stolee <stolee@gmail.com>,
	Git mailing list <git@vger.kernel.org>, Jeff King <peff@peff.net>,
	Stefan Beller <sbeller@google.com>,
	Ramsay Jones <ramsay@ramsayjones.plus.com>,
	git@jeffhostetler.com, Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH v6 00/14] Serialized Git Commit Graph
Date: Fri, 16 Mar 2018 20:48:49 +0100
Message-ID: <CAM0VKjmVgiWsqo8rQWwP9+mEq0tLinc8xoUM=8XdMP3VTBwJxw@mail.gmail.com> (raw)
In-Reply-To: <xmqq370z3m5o.fsf@gitster-ct.c.googlers.com>

On Fri, Mar 16, 2018 at 7:33 PM, Junio C Hamano <gitster@pobox.com> wrote:
> SZEDER Gábor <szeder.dev@gmail.com> writes:
>
>> You should forget '--stdin-packs' and use '--stdin-commits' to generate
>> the initial graph, it's much faster even without '--additive'[1].  See
>>
>>   https://public-inbox.org/git/CAM0VKj=wmkBNH=psCRztXFrC13RiG1EaSw89Q6LJaNsdJDEFHg@mail.gmail.com/
>>
>> I still think that the default behaviour for 'git commit-graph write'
>> should simply walk history from all refs instead of enumerating all
>> objects in all packfiles.
>
> Somehow I missed that one.  Thanks for the link to it.
>
> It is not so surprising that history walking runs rings around
> enumerating objects in packfiles, if packfiles are built well.
>
> A well-built packfile tends to has newer objects in base form and
> has delta that goes in backward direction (older objects are
> represented as delta against newer ones).  This helps warlking from
> the tips of the history quite a bit, because your delta base cache
> will tend to have the base object (i.e. objects in the newer part of
> the history you just walked) that will be required to access the
> "next" older part of the history more often than not.
>
> Trying to read the objects in the pack in their object name order
> would essentially mean reading them in a cryptgraphically random
> order.  Half the time you will end up wanting to access an object
> that is near the tip of a very deep delta chain even before you've
> accessed any of the base objects in the delta chain.

I came up with a different explanation back then: we are only interested
in commit objects when creating the commit graph, and only a small-ish
fraction of all objects are commit objects, so the "enumerate objects in
packfiles" approach has to look at a lot more objects:

  # in my git fork
  $ git rev-list --all --objects |cut -d' ' -f1 |\
    git cat-file --batch-check='%(objecttype) %(objectsize)' >type-size
  $ grep -c ^commit type-size
  53754
  $ wc -l type-size
  244723 type-size

I.e. only about 20% of all objects are commit objects.

Furthermore, in order to look at an object it has to be zlib inflated
first, and since commit objects tend to be much smaller than trees and
especially blobs, there are a lot less bytes to inflate:

  $ grep ^commit type-size |cut -d' ' -f2 |avg
  34395730 / 53754 = 639
  $ cat type-size |cut -d' ' -f2 |avg
  3866685744 / 244723 = 15800

So a simple revision walk inflates less than 1% of the bytes that the
"enumerate objects packfiles" approach has to inflate.


>> [1] - Please excuse the bikeshed: '--additive' is such a strange
>>       sounding option name, at least for me.  '--append', perhaps?
>
> Yeah, I think "fetch --append" is probably a precedence.

  reply index

Thread overview: 109+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-27  2:32 [PATCH v5 00/13] " Derrick Stolee
2018-02-27  2:32 ` [PATCH v5 01/13] commit-graph: add format document Derrick Stolee
2018-02-27  2:32 ` [PATCH v5 02/13] graph: add commit graph design document Derrick Stolee
2018-02-27  2:32 ` [PATCH v5 03/13] commit-graph: create git-commit-graph builtin Derrick Stolee
2018-02-27  2:32 ` [PATCH v5 04/13] csum-file: add CSUM_KEEP_OPEN flag Derrick Stolee
2018-03-12 13:55   ` Derrick Stolee
2018-03-13 21:42     ` Junio C Hamano
2018-03-14  2:26       ` Derrick Stolee
2018-03-14 17:00         ` Junio C Hamano
2018-02-27  2:32 ` [PATCH v5 05/13] commit-graph: implement write_commit_graph() Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 06/13] commit-graph: implement 'git-commit-graph write' Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 07/13] commit-graph: implement git commit-graph read Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 08/13] commit-graph: add core.commitGraph setting Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 09/13] commit-graph: close under reachability Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 10/13] commit: integrate commit graph with commit parsing Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 11/13] commit-graph: read only from specific pack-indexes Derrick Stolee
2018-02-27 20:15   ` Stefan Beller
2018-02-27  2:33 ` [PATCH v5 12/13] commit-graph: build graph from starting commits Derrick Stolee
2018-02-27  2:33 ` [PATCH v5 13/13] commit-graph: implement "--additive" option Derrick Stolee
2018-02-27 18:50 ` [PATCH v5 00/13] Serialized Git Commit Graph Stefan Beller
2018-03-14 19:27 ` [PATCH v6 00/14] " Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 01/14] csum-file: rename hashclose() to finalize_hashfile() Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 02/14] csum-file: refactor finalize_hashfile() method Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 03/14] commit-graph: add format document Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 04/14] graph: add commit graph design document Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 05/14] commit-graph: create git-commit-graph builtin Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 06/14] commit-graph: implement write_commit_graph() Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 07/14] commit-graph: implement 'git-commit-graph write' Derrick Stolee
2018-03-18 13:25     ` Ævar Arnfjörð Bjarmason
2018-03-19 13:12       ` Derrick Stolee
2018-03-19 14:36         ` Ævar Arnfjörð Bjarmason
2018-03-19 18:27           ` Derrick Stolee
2018-03-19 18:48             ` Ævar Arnfjörð Bjarmason
2018-03-14 19:27   ` [PATCH v6 08/14] commit-graph: implement git commit-graph read Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 09/14] commit-graph: add core.commitGraph setting Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 10/14] commit-graph: close under reachability Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 11/14] commit: integrate commit graph with commit parsing Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 12/14] commit-graph: read only from specific pack-indexes Derrick Stolee
2018-03-15 22:50     ` SZEDER Gábor
2018-03-19 13:13       ` Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 13/14] commit-graph: build graph from starting commits Derrick Stolee
2018-03-14 19:27   ` [PATCH v6 14/14] commit-graph: implement "--additive" option Derrick Stolee
2018-03-14 20:10   ` [PATCH v6 00/14] Serialized Git Commit Graph Ramsay Jones
2018-03-14 20:43   ` Junio C Hamano
2018-03-15 17:23     ` Johannes Schindelin
2018-03-15 18:41       ` Junio C Hamano
2018-03-15 21:51         ` Ramsay Jones
2018-03-16 11:50         ` Johannes Schindelin
2018-03-16 17:27           ` Junio C Hamano
2018-03-19 11:41             ` Johannes Schindelin
2018-03-16 16:28     ` Lars Schneider
2018-03-19 13:10       ` Derrick Stolee
2018-03-16 15:06   ` Ævar Arnfjörð Bjarmason
2018-03-16 16:38     ` SZEDER Gábor
2018-03-16 18:33       ` Junio C Hamano
2018-03-16 19:48         ` SZEDER Gábor [this message]
2018-03-16 20:06           ` Jeff King
2018-03-16 20:19             ` Jeff King
2018-03-19 12:55               ` Derrick Stolee
2018-03-20  1:17                 ` Derrick Stolee
2018-03-16 20:49         ` Jeff King
2018-04-02 20:34   ` [PATCH v7 " Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 01/14] csum-file: rename hashclose() to finalize_hashfile() Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 02/14] csum-file: refactor finalize_hashfile() method Derrick Stolee
2018-04-07 22:59       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 03/14] commit-graph: add format document Derrick Stolee
2018-04-07 23:49       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 04/14] graph: add commit graph design document Derrick Stolee
2018-04-08 11:06       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 05/14] commit-graph: create git-commit-graph builtin Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 06/14] commit-graph: implement write_commit_graph() Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 07/14] commit-graph: implement git-commit-graph write Derrick Stolee
2018-04-08 11:59       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 08/14] commit-graph: implement git commit-graph read Derrick Stolee
2018-04-02 21:33       ` Junio C Hamano
2018-04-03 11:49         ` Derrick Stolee
2018-04-08 12:59       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 09/14] commit-graph: add core.commitGraph setting Derrick Stolee
2018-04-08 13:39       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 10/14] commit-graph: close under reachability Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 11/14] commit: integrate commit graph with commit parsing Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 12/14] commit-graph: read only from specific pack-indexes Derrick Stolee
2018-04-02 20:34     ` [PATCH v7 13/14] commit-graph: build graph from starting commits Derrick Stolee
2018-04-08 13:50       ` Jakub Narebski
2018-04-02 20:34     ` [PATCH v7 14/14] commit-graph: implement "--additive" option Derrick Stolee
2018-04-05  8:27       ` SZEDER Gábor
2018-04-10 12:55     ` [PATCH v8 00/14] Serialized Git Commit Graph Derrick Stolee
2018-04-10 12:55       ` [PATCH v8 01/14] csum-file: rename hashclose() to finalize_hashfile() Derrick Stolee
2018-04-10 12:55       ` [PATCH v8 02/14] csum-file: refactor finalize_hashfile() method Derrick Stolee
2018-04-10 12:55       ` [PATCH v8 03/14] commit-graph: add format document Derrick Stolee
2018-04-10 19:10         ` Stefan Beller
2018-04-10 19:18           ` Derrick Stolee
2018-04-11 20:58         ` Jakub Narebski
2018-04-12 11:28           ` Derrick Stolee
2018-04-13 22:07             ` Jakub Narebski
2018-04-10 12:55       ` [PATCH v8 04/14] graph: add commit graph design document Derrick Stolee
2018-04-15 22:48         ` Jakub Narebski
2018-04-10 12:55       ` [PATCH v8 05/14] commit-graph: create git-commit-graph builtin Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 06/14] commit-graph: implement write_commit_graph() Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 07/14] commit-graph: implement git-commit-graph write Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 08/14] commit-graph: implement git commit-graph read Derrick Stolee
2018-04-14 22:15         ` Jakub Narebski
2018-04-15  3:26           ` Eric Sunshine
2018-04-10 12:56       ` [PATCH v8 09/14] commit-graph: add core.commitGraph setting Derrick Stolee
2018-04-14 18:33         ` Jakub Narebski
2018-04-10 12:56       ` [PATCH v8 10/14] commit-graph: close under reachability Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 11/14] commit: integrate commit graph with commit parsing Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 12/14] commit-graph: read only from specific pack-indexes Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 13/14] commit-graph: build graph from starting commits Derrick Stolee
2018-04-10 12:56       ` [PATCH v8 14/14] commit-graph: implement "--append" option Derrick Stolee

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