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From: Derrick Stolee <stolee@gmail.com>
To: git@vger.kernel.org
Cc: sbeller@google.com, pclouds@gmail.com, avarab@gmail.com,
	Derrick Stolee <stolee@gmail.com>
Subject: [PATCH v2 00/24] Multi-pack-index (MIDX)
Date: Mon, 25 Jun 2018 10:34:10 -0400
Message-ID: <20180625143434.89044-1-dstolee@microsoft.com> (raw)
In-Reply-To: <20180607140338.32440-1-dstolee@microsoft.com>

From: Derrick Stolee <stolee@gmail.com>

This v2 patch has several significant changes from v1. Thanks for all
the feedback that informed these changes:

* The 'midx' builtin is renamed to 'multi-pack-index'

* The 'core.midx' config setting is renamed to 'core.multiPackIndex'

* Many die() or error() statements are marked for translation

* The packfile name lookup chunk is dropped in favor of dynamic
  calculation on read.

* The 'read' verb in the builtin is moved to a test tool.

And one item that I'm saving for investigation while v2 is under review:

* Consider using a merge sort when constructing/deduplicating the list
  of objects and offsets instead of sorting the objects in batches by
  first byte.

Thanks,
-Stolee

---

The multi-pack-index (MIDX) is explained fully in
the design document 'Documentation/technical/multi-pack-index.txt'.
The short description is that the MIDX stores the
information from all of the IDX files in a pack
directory. The crucial design decision is that the
IDX files still exist, so we can fall back to the IDX
files if there is any issue with the MIDX (or core.midx
is set to false, or a user downgrades Git, etc.)

The MIDX feature has been part of our GVFS releases
for a few months (since the RFC). It has behaved well,
indexing over 31 million commits and trees across up
to 250 packfiles. These MIDX files are nearly 1GB in
size and take ~20 seconds to rewrite when adding new
IDX information. This ~20s mark is something I'd like
to improve, and I mention how to make the file
incremental (similar to split-index) in the design
document. I also want to make the commit-graph file
incremental, so I'd like to do that at the same time
after both the MIDX and commit-graph are stable.


Lookup Speedups
---------------

When looking for an object, Git uses an most-recently-
used (MRU) cache of packfiles. This does pretty well to
minimize the number of misses when searching through
packfiles for an object, especially if there is one
"big" packfile that contains most of the objets (so it
will rarely miss and is usually one of the first two
packfiles in the list). The MIDX does provide a way
to remove these misses, improving lookup time. However,
this lookup time greatly depends on the arrangement of
the packfiles.

For instance, if you take the Linux repository and repack
using `git repack -adfF --max-pack-size=128m` then all
commits will be in one packfile, all trees will be in
a small set of packfiles and organized well so 'git
rev-list --objects HEAD^{tree}' only inspects one or two
packfiles.

GVFS has the notion of a "prefetch packfile". These are
packfiles that are precomputed by cache servers to
contain the commits and trees introduced to the remote
each day. GVFS downloads these packfiles and places them
in an alternate. Since these are organized by "first
time introduced" and the working directory is so large,
the MRU misses are significant when performing a checkout
and updating the .git/index file.

To test the performance in this situation, I created a
script that organizes the Linux repository in a similar
fashion. I split the commit history into 50 parts by
creating branches on every 10,000 commits of the first-
parent history. Then, `git rev-list --objects A ^B`
provides the list of objects reachable from A but not B,
so I could send that to `git pack-objects` to create
these "time-based" packfiles. With these 50 packfiles
(deleting the old one from my fresh clone, and deleting
all tags as they were no longer on-disk) I could then
test 'git rev-list --objects HEAD^{tree}' and see:

        Before: 0.17s
        After:  0.13s
        % Diff: -23.5%

By adding logic to count hits and misses to bsearch_pack,
I was able to see that the command above calls that
method 266,930 times with a hit rate of 33%. The MIDX
has the same number of calls with a 100% hit rate.



Abbreviation Speedups
---------------------

To fully disambiguate an abbreviation, we must iterate
through all packfiles to ensure no collision exists in
any packfile. This requires O(P log N) time. With the
MIDX, this is only O(log N) time. Our standard test [2]
is 'git log --oneline --parents --raw' because it writes
many abbreviations while also doing a lot of other work
(walking commits and trees to compute the raw diff).

For a copy of the Linux repository with 50 packfiles
split by time, we observed the following:

        Before: 100.5 s
        After:   58.2 s
        % Diff: -59.7%


Derrick Stolee (24):
  multi-pack-index: add design document
  multi-pack-index: add format details
  multi-pack-index: add builtin
  multi-pack-index: add 'write' verb
  midx: write header information to lockfile
  multi-pack-index: load into memory
  multi-pack-index: expand test data
  packfile: generalize pack directory list
  multi-pack-index: read packfile list
  multi-pack-index: write pack names in chunk
  midx: read pack names into array
  midx: sort and deduplicate objects from packfiles
  midx: write object ids in a chunk
  midx: write object id fanout chunk
  midx: write object offsets
  config: create core.multiPackIndex setting
  midx: prepare midxed_git struct
  midx: read objects from multi-pack-index
  midx: use midx in abbreviation calculations
  midx: use existing midx when writing new one
  midx: use midx in approximate_object_count
  midx: prevent duplicate packfile loads
  packfile: skip loading index if in multi-pack-index
  midx: clear midx on repack

 .gitignore                                   |   3 +-
 Documentation/config.txt                     |   4 +
 Documentation/git-multi-pack-index.txt       |  56 ++
 Documentation/technical/multi-pack-index.txt | 109 +++
 Documentation/technical/pack-format.txt      |  77 ++
 Makefile                                     |   3 +
 builtin.h                                    |   1 +
 builtin/multi-pack-index.c                   |  46 +
 builtin/repack.c                             |   8 +
 cache.h                                      |   1 +
 command-list.txt                             |   1 +
 config.c                                     |   5 +
 environment.c                                |   1 +
 git.c                                        |   1 +
 midx.c                                       | 900 +++++++++++++++++++
 midx.h                                       |  20 +
 object-store.h                               |  33 +
 packfile.c                                   | 173 +++-
 packfile.h                                   |   9 +
 sha1-name.c                                  |  70 ++
 t/helper/test-read-midx.c                    |  54 ++
 t/helper/test-tool.c                         |   1 +
 t/helper/test-tool.h                         |   1 +
 t/t5319-multi-pack-index.sh                  | 191 ++++
 24 files changed, 1724 insertions(+), 44 deletions(-)
 create mode 100644 Documentation/git-multi-pack-index.txt
 create mode 100644 Documentation/technical/multi-pack-index.txt
 create mode 100644 builtin/multi-pack-index.c
 create mode 100644 midx.c
 create mode 100644 midx.h
 create mode 100644 t/helper/test-read-midx.c
 create mode 100755 t/t5319-multi-pack-index.sh


base-commit: 53f9a3e157dbbc901a02ac2c73346d375e24978c
-- 
2.18.0.rc1


  parent reply index

Thread overview: 192+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-07 14:03 [PATCH 00/23] " Derrick Stolee
2018-06-07 14:03 ` [PATCH 01/23] midx: add design document Derrick Stolee
2018-06-11 19:04   ` Stefan Beller
2018-06-18 18:48     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 02/23] midx: add midx format details to pack-format.txt Derrick Stolee
2018-06-11 19:19   ` Stefan Beller
2018-06-18 19:01     ` Derrick Stolee
2018-06-18 19:41       ` Stefan Beller
2018-06-07 14:03 ` [PATCH 03/23] midx: add midx builtin Derrick Stolee
2018-06-07 17:20   ` Duy Nguyen
2018-06-18 19:23     ` Derrick Stolee
2018-06-11 21:02   ` Stefan Beller
2018-06-18 19:40     ` Derrick Stolee
2018-06-18 19:55       ` Stefan Beller
2018-06-18 19:58         ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 04/23] midx: add 'write' subcommand and basic wiring Derrick Stolee
2018-06-07 17:27   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 05/23] midx: write header information to lockfile Derrick Stolee
2018-06-07 17:35   ` Duy Nguyen
2018-06-12 15:00   ` Duy Nguyen
2018-06-19 12:54     ` Derrick Stolee
2018-06-19 14:59       ` Duy Nguyen
2018-06-19 15:24         ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 06/23] midx: struct midxed_git and 'read' subcommand Derrick Stolee
2018-06-07 17:54   ` Duy Nguyen
2018-06-20 13:13     ` Derrick Stolee
2018-06-07 18:31   ` Duy Nguyen
2018-06-20 13:33     ` Derrick Stolee
2018-06-20 15:07       ` Duy Nguyen
2018-06-20 16:39         ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 07/23] midx: expand test data Derrick Stolee
2018-06-07 14:03 ` [PATCH 08/23] midx: read packfiles from pack directory Derrick Stolee
2018-06-07 18:03   ` Duy Nguyen
2018-06-20 16:33     ` [PATCH] packfile: generalize pack directory list Derrick Stolee
2018-06-07 14:03 ` [PATCH 09/23] midx: write pack names in chunk Derrick Stolee
2018-06-07 18:26   ` Duy Nguyen
2018-06-21 15:25     ` Derrick Stolee
2018-06-21 17:38       ` Junio C Hamano
2018-06-22 18:25         ` Derrick Stolee
2018-06-22 18:31           ` Junio C Hamano
2018-06-22 18:32             ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 10/23] midx: write a lookup into the pack names chunk Derrick Stolee
2018-06-09 16:43   ` Duy Nguyen
2018-06-21 17:23     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 11/23] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-06-09 17:07   ` Duy Nguyen
2018-06-21 17:54     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 12/23] midx: write object ids in a chunk Derrick Stolee
2018-06-09 17:25   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 13/23] midx: write object id fanout chunk Derrick Stolee
2018-06-09 17:28   ` Duy Nguyen
2018-06-21 19:49     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 14/23] midx: write object offsets Derrick Stolee
2018-06-09 17:41   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 15/23] midx: create core.midx config setting Derrick Stolee
2018-06-07 14:03 ` [PATCH 16/23] midx: prepare midxed_git struct Derrick Stolee
2018-06-09 17:47   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 17/23] midx: read objects from multi-pack-index Derrick Stolee
2018-06-09 17:56   ` Duy Nguyen
2018-06-21 20:03     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 18/23] midx: use midx in abbreviation calculations Derrick Stolee
2018-06-09 18:01   ` Duy Nguyen
2018-06-22 18:38     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 19/23] midx: use existing midx when writing new one Derrick Stolee
2018-06-07 14:03 ` [PATCH 20/23] midx: use midx in approximate_object_count Derrick Stolee
2018-06-09 18:03   ` Duy Nguyen
2018-06-22 18:39     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 21/23] midx: prevent duplicate packfile loads Derrick Stolee
2018-06-09 18:05   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 22/23] midx: use midx to find ref-deltas Derrick Stolee
2018-06-07 14:03 ` [PATCH 23/23] midx: clear midx on repack Derrick Stolee
2018-06-09 18:13   ` Duy Nguyen
2018-06-22 18:44     ` Derrick Stolee
2018-06-07 14:06 ` [PATCH 00/23] Multi-pack-index (MIDX) Derrick Stolee
2018-06-07 14:45 ` Ævar Arnfjörð Bjarmason
2018-06-07 14:54   ` Derrick Stolee
2018-06-25 14:34 ` Derrick Stolee [this message]
2018-06-25 14:34   ` [PATCH v2 01/24] multi-pack-index: add design document Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 02/24] multi-pack-index: add format details Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 03/24] multi-pack-index: add builtin Derrick Stolee
2018-06-25 19:15     ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 04/24] multi-pack-index: add 'write' verb Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 05/24] midx: write header information to lockfile Derrick Stolee
2018-06-25 19:19     ` Junio C Hamano
2018-07-05 19:13       ` Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 06/24] multi-pack-index: load into memory Derrick Stolee
2018-06-25 19:38     ` Junio C Hamano
2018-07-05 14:19       ` Derrick Stolee
2018-07-05 18:58         ` Eric Sunshine
2018-07-06 19:20           ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 07/24] multi-pack-index: expand test data Derrick Stolee
2018-06-25 19:45     ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 08/24] packfile: generalize pack directory list Derrick Stolee
2018-06-25 19:57     ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 09/24] multi-pack-index: read packfile list Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 10/24] multi-pack-index: write pack names in chunk Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 11/24] midx: read pack names into array Derrick Stolee
2018-06-25 23:52     ` Eric Sunshine
2018-06-25 14:34   ` [PATCH v2 12/24] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 13/24] midx: write object ids in a chunk Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 14/24] midx: write object id fanout chunk Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 15/24] midx: write object offsets Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 16/24] config: create core.multiPackIndex setting Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 17/24] midx: prepare midxed_git struct Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 18/24] midx: read objects from multi-pack-index Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 19/24] midx: use midx in abbreviation calculations Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 20/24] midx: use existing midx when writing new one Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 21/24] midx: use midx in approximate_object_count Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 22/24] midx: prevent duplicate packfile loads Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 23/24] packfile: skip loading index if in multi-pack-index Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 24/24] midx: clear midx on repack Derrick Stolee
2018-07-06  0:52   ` [PATCH v3 00/24] Multi-pack-index (MIDX) Derrick Stolee
2018-07-06  0:52     ` [PATCH v3 01/24] multi-pack-index: add design document Derrick Stolee
2018-07-06  0:52     ` [PATCH v3 02/24] multi-pack-index: add format details Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 03/24] multi-pack-index: add builtin Derrick Stolee
2018-07-06  3:54       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 04/24] multi-pack-index: add 'write' verb Derrick Stolee
2018-07-06  4:07       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 05/24] midx: write header information to lockfile Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 06/24] multi-pack-index: load into memory Derrick Stolee
2018-07-06  4:19       ` Eric Sunshine
2018-07-06  5:18         ` Eric Sunshine
2018-07-09 19:08       ` Junio C Hamano
2018-07-12 16:06         ` Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 07/24] multi-pack-index: expand test data Derrick Stolee
2018-07-06  4:36       ` Eric Sunshine
2018-07-06  5:20         ` Eric Sunshine
2018-07-12 14:10         ` Derrick Stolee
2018-07-12 18:02           ` Eric Sunshine
2018-07-12 18:06             ` Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 08/24] packfile: generalize pack directory list Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 09/24] multi-pack-index: read packfile list Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 10/24] multi-pack-index: write pack names in chunk Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 11/24] midx: read pack names into array Derrick Stolee
2018-07-06  4:58       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 12/24] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 13/24] midx: write object ids in a chunk Derrick Stolee
2018-07-06  5:04       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 14/24] midx: write object id fanout chunk Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 15/24] midx: write object offsets Derrick Stolee
2018-07-06  5:27       ` Eric Sunshine
2018-07-12 16:33         ` Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 16/24] config: create core.multiPackIndex setting Derrick Stolee
2018-07-06  5:39       ` Eric Sunshine
2018-07-12 13:19         ` Derrick Stolee
2018-07-12 16:30           ` Derrick Stolee
2018-07-11  9:48       ` SZEDER Gábor
2018-07-12 13:01         ` Derrick Stolee
2018-07-12 13:31           ` SZEDER Gábor
2018-07-12 15:40             ` Derrick Stolee
2018-07-12 17:29             ` Junio C Hamano
2018-07-06  0:53     ` [PATCH v3 17/24] midx: prepare midxed_git struct Derrick Stolee
2018-07-06  5:41       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 18/24] midx: read objects from multi-pack-index Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 19/24] midx: use midx in abbreviation calculations Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 20/24] midx: use existing midx when writing new one Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 21/24] midx: use midx in approximate_object_count Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 22/24] midx: prevent duplicate packfile loads Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 23/24] packfile: skip loading index if in multi-pack-index Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 24/24] midx: clear midx on repack Derrick Stolee
2018-07-06  5:52       ` Eric Sunshine
2018-07-12 19:39     ` [PATCH v4 00/23] Multi-pack-index (MIDX) Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 01/23] multi-pack-index: add design document Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 02/23] multi-pack-index: add format details Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 03/23] multi-pack-index: add builtin Derrick Stolee
2018-07-20 18:22         ` Junio C Hamano
2018-07-20 22:15           ` brian m. carlson
2018-07-20 22:28             ` Junio C Hamano
2018-07-12 19:39       ` [PATCH v4 04/23] multi-pack-index: add 'write' verb Derrick Stolee
2018-07-12 22:56         ` Eric Sunshine
2018-07-12 19:39       ` [PATCH v4 05/23] midx: write header information to lockfile Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 06/23] multi-pack-index: load into memory Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 07/23] t5319: expand test data Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 08/23] packfile: generalize pack directory list Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 09/23] multi-pack-index: read packfile list Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 10/23] multi-pack-index: write pack names in chunk Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 11/23] midx: read pack names into array Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 12/23] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 13/23] midx: write object ids in a chunk Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 14/23] midx: write object id fanout chunk Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 15/23] midx: write object offsets Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 16/23] config: create core.multiPackIndex setting Derrick Stolee
2018-07-12 21:05         ` Junio C Hamano
2018-07-13  0:50           ` Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 17/23] midx: read objects from multi-pack-index Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 18/23] midx: use midx in abbreviation calculations Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 19/23] midx: use existing midx when writing new one Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 20/23] midx: use midx in approximate_object_count Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 21/23] midx: prevent duplicate packfile loads Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 22/23] packfile: skip loading index if in multi-pack-index Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 23/23] midx: clear midx on repack Derrick Stolee
2018-07-12 21:11       ` [PATCH v4 00/23] Multi-pack-index (MIDX) Junio C Hamano

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