On 2008.08.14 17:06:13 -0700, Linus Torvalds wrote:
> The "hash" we use for looking things up is also pretty much a joke, and it
> has no overflow capability, it just replaces the old entry with a new one.
So I added some stupid tracing to cache_or_unpack entry to see how often
we reread the same stuff. The whole thing just logs the base_offset in
case of a cache miss. I've gc'ed my linux-2.6.git before the run, so
that there's only a single packed_git around (at least I hope so), and I
can ignore that for the tracing.
The whole log for a "git rev-list --objects HEAD" has about 1.2M
entries, while the output of the rev-list command has about 870k lines.
Some postprocessing of the trace shows that the majority of objects is
read only once or twice. A few percent are read three to ten times, and
some are read more than two hundred times.
I'll attach the post-processed thing. The format is:
x y
Meaning that there were x base_offset values for which we had y cache
misses.
Björn
391805 1
110622 2
27830 3
13995 4
8583 5
5834 6
4275 7
3242 8
2514 9
2168 10
1632 11
1336 12
1197 13
947 14
788 15
704 16
565 17
514 18
422 19
348 20
304 21
276 22
227 23
233 24
180 25
160 26
145 27
106 28
123 29
109 30
86 31
91 32
72 33
63 34
55 35
73 36
61 37
56 38
48 39
44 40
47 41
36 42
44 43
47 44
32 45
36 46
27 47
19 48
34 49
28 50
22 51
21 52
26 53
18 54
19 55
16 56
22 57
16 58
16 59
11 60
13 61
19 62
17 63
8 64
21 65
8 66
8 67
16 68
9 69
12 70
11 71
8 72
5 73
6 74
9 75
6 76
9 77
7 78
8 79
7 80
8 81
6 82
5 83
13 84
9 85
8 86
4 87
5 89
6 90
3 91
7 92
4 93
5 94
5 95
5 96
4 97
3 98
7 99
2 100
4 101
4 102
7 103
4 104
4 105
5 106
3 107
1 108
4 109
1 110
1 111
1 112
6 113
5 114
2 115
5 116
2 117
2 118
2 119
7 120
1 121
4 122
3 123
3 124
3 125
4 126
1 127
2 128
2 129
2 130
1 131
4 132
1 133
4 134
1 135
2 136
4 137
1 139
3 140
3 141
5 142
5 143
4 144
1 148
2 149
3 150
1 151
2 152
6 153
1 154
2 155
2 156
3 157
2 158
1 159
3 160
2 161
4 162
2 163
5 164
2 165
2 166
2 169
2 170
1 171
1 172
1 173
1 176
2 177
2 178
2 179
1 180
3 181
3 182
1 183
1 184
1 186
1 187
1 190
1 192
1 194
2 195
3 196
1 197
1 200
1 201
1 202
1 208
2 214
2 216
2 217
3 224
1 225
1 228
1 230
2 232
2 233
1 234
2 236
1 239
1 241
1 245
1 246
2 249
2 250
1 252
1 259
1 261
2 263
1 266
2 268
1 272
1 282
