Since I'm currently between jobs and extra time on my hands until I find
a new one, I've been experimenting with some alternative encodings for
PACK file contents.
The tables below show packing statistics for a number of actual pack
files using difference pack file encodings. All of the encodings use
object names to find the base object of a DELTA object.
Encodings:
Pack_2 - Pack version 2 (Git current), 32 bit
Pack_3 - Pack version 3 (Git future), 32 bit
Gitzilla_1 - Experimental >32 bit, described below
Gitzilla_2 - Experimental >32 bit, described below
Gitzilla_1: Per object header consisting of 2 fields: the Git object
type of the object with a flag indicating if it's delta encoded, big
endian base 128 encoding of the object length. If the object is not
delta encoded, the object header is followed by the deflated object
contents. If the object is delta encoded, the object header is followed
by the 20 byte ID of the base object, and the deflated edit list (see
below). This encoding allows for the possibility of using some of the
bits in the object type field to select one of up to 15 dictionaries.
Gitzilla_2: Per object header consisting of a single field combining the
object type and length; similar to that used in current packs but big
endian encoded. The type could be encoded either as object type with
delta flag (no room for additional object types), or the same as what
version 2 packs (room for additional object types but need to walk the
delta chain to determine type of delta encode objects). If the object is
delta encoded, the object header is followed by the 20 byte ID of the
base object, and the deflated edit list (see below).
Notice that the length encoded in the per object header for delta encode
objects is that of the decoded object, not the length of the inflated
edit list.
The edit lists used in the Gitzilla encodings consist of a sequence of
one or more hunks. Each hunk specifies either a part of the base object
to copy to the current position of the result object, or a length of
(immediate) data to copy to the current position of the result object.
Each hunk type is encode as 2 fields: the first is big endian base 128
encoded length of data to copy with bit 6 of the first byte indicating
where to copy from. The second field is the big endian base 128 encoded
offset in the base object of the copy source, or the (immediate) data to
copy.
Observations:
Most of the savings are from not including the base and result object
lengths in the deflated edit lists.
Other minor savings from joining adjacent edit hunks of the same type,
where possible.
The endian of the base 128 encoding is big because it requires less work
to _decode_ than little.
TREE objects do not delta or deflate well.
Dictionaries will need to be tailored to different object types, and
possibly to other object subsets.
The ability to get the type and length of a delta'd object without
needing to walk the delta chain is a good thing.
The use of _relative_ offsets to specify base objects would save a lot
of space.
The data:
All of the PACK files except bd5109... were rsync'd from the Git project
on kernel.org, at some point. PACK bd5109... is a mash-up of Linus'
kernel and sparse projects, Thomas' kernel history, the stable 2.6.x.y
kernel projects, Git, and gitk; all from kernel.org.
The columns are:
encoding name
pack file size
size of pack file non-object overhead
sum of packed object sizes
byte difference in sum of packed object sizes and that of
version 2 packs
average byte difference per object
average byte difference per DELTA'd object
pack-1abe018fa97f8a257a4f3a072000abf99eaa925e.pack
BLOB : 237
COMMIT: 504
DELTA : 1108
TAG : 6
TREE : 64
====== ==========
Total : 1919
Gitzilla_1: 1202917 32 1202885 -6797 -3.5419 -6.1345
Gitzilla_2: 1202011 32 1201979 -7703 -4.0141 -6.9522
Pack_2 : 1209714 32 1209682 0 0.0000 0.0000
Pack_3 : 1209686 32 1209654 -28 -0.0146 -0.0253
pack-3a080dc749b452cc847d19d91981ddfcff629750.pack
BLOB : 1020
COMMIT: 5300
DELTA : 15013
TAG : 80
TREE : 564
====== ==========
Total : 21977
Gitzilla_1: 6748060 32 6748028 -95768 -4.3576 -6.3790
Gitzilla_2: 6737992 32 6737960 -105836 -4.8158 -7.0496
Pack_2 : 6843828 32 6843796 0 0.0000 0.0000
Pack_3 : 6843747 32 6843715 -81 -0.0037 -0.0054
pack-3e6247e4edf3c006a037041eef28bbca8051b006.pack
BLOB : 1062
COMMIT: 5545
DELTA : 15565
TAG : 81
TREE : 703
====== ==========
Total : 22956
Gitzilla_1: 6724940 32 6724908 -97643 -4.2535 -6.2732
Gitzilla_2: 6714303 32 6714271 -108280 -4.7168 -6.9566
Pack_2 : 6822583 32 6822551 0 0.0000 0.0000
Pack_3 : 6822458 32 6822426 -125 -0.0054 -0.0080
pack-6bdf691abdae3590f6a807d15ebe8ed523ef1e12.pack
BLOB : 33
COMMIT: 36
DELTA : 64
TAG : 0
TREE : 4
====== ==========
Total : 137
Gitzilla_1: 136041 32 136009 -273 -1.9927 -4.2656
Gitzilla_2: 135991 32 135959 -323 -2.3577 -5.0469
Pack_2 : 136314 32 136282 0 0.0000 0.0000
Pack_3 : 136314 32 136282 0 0.0000 0.0000
pack-740c99c0be6734adbd130737dec2608dc4682761.pack
BLOB : 181
COMMIT: 173
DELTA : 383
TAG : 5
TREE : 24
====== ==========
Total : 766
Gitzilla_1: 586580 32 586548 -1799 -2.3486 -4.6971
Gitzilla_2: 586264 32 586232 -2115 -2.7611 -5.5222
Pack_2 : 588379 32 588347 0 0.0000 0.0000
Pack_3 : 588370 32 588338 -9 -0.0117 -0.0235
pack-793a9e93286d6c656941977d2e5b49e28566edcd.pack
BLOB : 1158
COMMIT: 6512
DELTA : 18715
TAG : 90
TREE : 820
====== ==========
Total : 27295
Gitzilla_1: 7957552 32 7957520 -120209 -4.4041 -6.4231
Gitzilla_2: 7944784 32 7944752 -132977 -4.8718 -7.1054
Pack_2 : 8077761 32 8077729 0 0.0000 0.0000
Pack_3 : 8077566 32 8077534 -195 -0.0071 -0.0104
pack-92b2a535e63e4a97eba2a13f60c3da922372f03a.pack
BLOB : 1149
COMMIT: 5969
DELTA : 16930
TAG : 84
TREE : 750
====== ==========
Total : 24882
Gitzilla_1: 7243503 32 7243471 -108475 -4.3596 -6.4073
Gitzilla_2: 7231901 32 7231869 -120077 -4.8259 -7.0926
Pack_2 : 7351978 32 7351946 0 0.0000 0.0000
Pack_3 : 7351841 32 7351809 -137 -0.0055 -0.0081
pack-9f1f94e6f50261b5d99c04349bfa3d315cfb4118.pack
BLOB : 1190
COMMIT: 6244
DELTA : 17881
TAG : 87
TREE : 810
====== ==========
Total : 26212
Gitzilla_1: 7920476 32 7920444 -115214 -4.3955 -6.4434
Gitzilla_2: 7908256 32 7908224 -127434 -4.8617 -7.1268
Pack_2 : 8035690 32 8035658 0 0.0000 0.0000
Pack_3 : 8035529 32 8035497 -161 -0.0061 -0.0090
pack-a875e0c11c739866b000dc9b34d1b0daead84a00.pack
BLOB : 20
COMMIT: 9
DELTA : 15
TAG : 1
TREE : 3
====== ==========
Total : 48
Gitzilla_1: 71748 32 71716 -54 -1.1250 -3.6000
Gitzilla_2: 71728 32 71696 -74 -1.5417 -4.9333
Pack_2 : 71802 32 71770 0 0.0000 0.0000
Pack_3 : 71802 32 71770 0 0.0000 0.0000
pack-abbe9cdea12803663015a7e23b7ae61c64e56499.pack
BLOB : 259
COMMIT: 342
DELTA : 705
TAG : 13
TREE : 47
====== ==========
Total : 1366
Gitzilla_1: 1086337 32 1086305 -3878 -2.8389 -5.5007
Gitzilla_2: 1085773 32 1085741 -4442 -3.2518 -6.3007
Pack_2 : 1090215 32 1090183 0 0.0000 0.0000
Pack_3 : 1090215 32 1090183 0 0.0000 0.0000
pack-bd5109ab74dd67d5b0a42f26688a4fa0a579e9fa.pack
BLOB : 29212
COMMIT: 103241
DELTA : 702986
TAG : 182
TREE : 37102
====== ==========
Total : 872723
Gitzilla_1: 282786611 32 282786579 -3647429 -4.1794 -5.1885
Gitzilla_2: 282143551 32 282143519 -4290489 -4.9162 -6.1032
Pack_2 : 286434040 32 286434008 0 0.0000 0.0000
Pack_3 : 286404302 32 286404270 -29738 -0.0341 -0.0423
pack-d18e6cccf9e472ba05e108254c827cc7ac0fe2a9.pack
BLOB : 1564
COMMIT: 3118
DELTA : 8288
TAG : 51
TREE : 365
====== ==========
Total : 13386
Gitzilla_1: 5638236 32 5638204 -44212 -3.3029 -5.3345
Gitzilla_2: 5632126 32 5632094 -50322 -3.7593 -6.0717
Pack_2 : 5682448 32 5682416 0 0.0000 0.0000
Pack_3 : 5682423 32 5682391 -25 -0.0019 -0.0030
pack-d25faec39a55935d787ae66e4f4a69d3add16417.pack
BLOB : 1180
COMMIT: 6090
DELTA : 17458
TAG : 85
TREE : 761
====== ==========
Total : 25574
Gitzilla_1: 7646842 32 7646810 -112816 -4.4114 -6.4621
Gitzilla_2: 7634964 32 7634932 -124694 -4.8758 -7.1425
Pack_2 : 7759658 32 7759626 0 0.0000 0.0000
Pack_3 : 7759521 32 7759489 -137 -0.0054 -0.0078
pack-d453bb50fef8757a0d7f9fcc88429e9ce6f74077.pack
BLOB : 28
COMMIT: 35
DELTA : 46
TAG : 2
TREE : 6
====== ==========
Total : 117
Gitzilla_1: 177544 32 177512 -266 -2.2735 -5.7826
Gitzilla_2: 177478 32 177446 -332 -2.8376 -7.2174
Pack_2 : 177810 32 177778 0 0.0000 0.0000
Pack_3 : 177806 32 177774 -4 -0.0342 -0.0870
pack-d83305b8608cb685ecd025fb141bf9d1588ae9c5.pack
BLOB : 91
COMMIT: 108
DELTA : 202
TAG : 2
TREE : 26
====== ==========
Total : 429
Gitzilla_1: 473503 32 473471 -1112 -2.5921 -5.5050
Gitzilla_2: 473269 32 473237 -1346 -3.1375 -6.6634
Pack_2 : 474615 32 474583 0 0.0000 0.0000
Pack_3 : 474603 32 474571 -12 -0.0280 -0.0594
pack-ee22eefd6bad2b54358d52ad6221ecabdc79d6f5.pack
BLOB : 175
COMMIT: 210
DELTA : 541
TAG : 2
TREE : 38
====== ==========
Total : 966
Gitzilla_1: 662650 32 662618 -3230 -3.3437 -5.9704
Gitzilla_2: 662150 32 662118 -3730 -3.8613 -6.8946
Pack_2 : 665880 32 665848 0 0.0000 0.0000
Pack_3 : 665865 32 665833 -15 -0.0155 -0.0277
pack-f0c530478b9dd6e96a69482066ec66eb5f6edbbf.pack
BLOB : 1154
COMMIT: 5797
DELTA : 16300
TAG : 83
TREE : 739
====== ==========
Total : 24073
Gitzilla_1: 7212953 32 7212921 -102252 -4.2476 -6.2731
Gitzilla_2: 7201730 32 7201698 -113475 -4.7138 -6.9617
Pack_2 : 7315205 32 7315173 0 0.0000 0.0000
Pack_3 : 7315059 32 7315027 -146 -0.0061 -0.0090
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