1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
|
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
<meta name="generator" content="AsciiDoc 10.2.0" />
<title>Concerning Git’s Packing Heuristics</title>
<style type="text/css">
/* Shared CSS for AsciiDoc xhtml11 and html5 backends */
/* Default font. */
body {
font-family: Georgia,serif;
}
/* Title font. */
h1, h2, h3, h4, h5, h6,
div.title, caption.title,
thead, p.table.header,
#toctitle,
#author, #revnumber, #revdate, #revremark,
#footer {
font-family: Arial,Helvetica,sans-serif;
}
body {
margin: 1em 5% 1em 5%;
}
a {
color: blue;
text-decoration: underline;
}
a:visited {
color: fuchsia;
}
em {
font-style: italic;
color: navy;
}
strong {
font-weight: bold;
color: #083194;
}
h1, h2, h3, h4, h5, h6 {
color: #527bbd;
margin-top: 1.2em;
margin-bottom: 0.5em;
line-height: 1.3;
}
h1, h2, h3 {
border-bottom: 2px solid silver;
}
h2 {
padding-top: 0.5em;
}
h3 {
float: left;
}
h3 + * {
clear: left;
}
h5 {
font-size: 1.0em;
}
div.sectionbody {
margin-left: 0;
}
hr {
border: 1px solid silver;
}
p {
margin-top: 0.5em;
margin-bottom: 0.5em;
}
ul, ol, li > p {
margin-top: 0;
}
ul > li { color: #aaa; }
ul > li > * { color: black; }
.monospaced, code, pre {
font-family: "Courier New", Courier, monospace;
font-size: inherit;
color: navy;
padding: 0;
margin: 0;
}
pre {
white-space: pre-wrap;
}
#author {
color: #527bbd;
font-weight: bold;
font-size: 1.1em;
}
#email {
}
#revnumber, #revdate, #revremark {
}
#footer {
font-size: small;
border-top: 2px solid silver;
padding-top: 0.5em;
margin-top: 4.0em;
}
#footer-text {
float: left;
padding-bottom: 0.5em;
}
#footer-badges {
float: right;
padding-bottom: 0.5em;
}
#preamble {
margin-top: 1.5em;
margin-bottom: 1.5em;
}
div.imageblock, div.exampleblock, div.verseblock,
div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
div.admonitionblock {
margin-top: 1.0em;
margin-bottom: 1.5em;
}
div.admonitionblock {
margin-top: 2.0em;
margin-bottom: 2.0em;
margin-right: 10%;
color: #606060;
}
div.content { /* Block element content. */
padding: 0;
}
/* Block element titles. */
div.title, caption.title {
color: #527bbd;
font-weight: bold;
text-align: left;
margin-top: 1.0em;
margin-bottom: 0.5em;
}
div.title + * {
margin-top: 0;
}
td div.title:first-child {
margin-top: 0.0em;
}
div.content div.title:first-child {
margin-top: 0.0em;
}
div.content + div.title {
margin-top: 0.0em;
}
div.sidebarblock > div.content {
background: #ffffee;
border: 1px solid #dddddd;
border-left: 4px solid #f0f0f0;
padding: 0.5em;
}
div.listingblock > div.content {
border: 1px solid #dddddd;
border-left: 5px solid #f0f0f0;
background: #f8f8f8;
padding: 0.5em;
}
div.quoteblock, div.verseblock {
padding-left: 1.0em;
margin-left: 1.0em;
margin-right: 10%;
border-left: 5px solid #f0f0f0;
color: #888;
}
div.quoteblock > div.attribution {
padding-top: 0.5em;
text-align: right;
}
div.verseblock > pre.content {
font-family: inherit;
font-size: inherit;
}
div.verseblock > div.attribution {
padding-top: 0.75em;
text-align: left;
}
/* DEPRECATED: Pre version 8.2.7 verse style literal block. */
div.verseblock + div.attribution {
text-align: left;
}
div.admonitionblock .icon {
vertical-align: top;
font-size: 1.1em;
font-weight: bold;
text-decoration: underline;
color: #527bbd;
padding-right: 0.5em;
}
div.admonitionblock td.content {
padding-left: 0.5em;
border-left: 3px solid #dddddd;
}
div.exampleblock > div.content {
border-left: 3px solid #dddddd;
padding-left: 0.5em;
}
div.imageblock div.content { padding-left: 0; }
span.image img { border-style: none; vertical-align: text-bottom; }
a.image:visited { color: white; }
dl {
margin-top: 0.8em;
margin-bottom: 0.8em;
}
dt {
margin-top: 0.5em;
margin-bottom: 0;
font-style: normal;
color: navy;
}
dd > *:first-child {
margin-top: 0.1em;
}
ul, ol {
list-style-position: outside;
}
ol.arabic {
list-style-type: decimal;
}
ol.loweralpha {
list-style-type: lower-alpha;
}
ol.upperalpha {
list-style-type: upper-alpha;
}
ol.lowerroman {
list-style-type: lower-roman;
}
ol.upperroman {
list-style-type: upper-roman;
}
div.compact ul, div.compact ol,
div.compact p, div.compact p,
div.compact div, div.compact div {
margin-top: 0.1em;
margin-bottom: 0.1em;
}
tfoot {
font-weight: bold;
}
td > div.verse {
white-space: pre;
}
div.hdlist {
margin-top: 0.8em;
margin-bottom: 0.8em;
}
div.hdlist tr {
padding-bottom: 15px;
}
dt.hdlist1.strong, td.hdlist1.strong {
font-weight: bold;
}
td.hdlist1 {
vertical-align: top;
font-style: normal;
padding-right: 0.8em;
color: navy;
}
td.hdlist2 {
vertical-align: top;
}
div.hdlist.compact tr {
margin: 0;
padding-bottom: 0;
}
.comment {
background: yellow;
}
.footnote, .footnoteref {
font-size: 0.8em;
}
span.footnote, span.footnoteref {
vertical-align: super;
}
#footnotes {
margin: 20px 0 20px 0;
padding: 7px 0 0 0;
}
#footnotes div.footnote {
margin: 0 0 5px 0;
}
#footnotes hr {
border: none;
border-top: 1px solid silver;
height: 1px;
text-align: left;
margin-left: 0;
width: 20%;
min-width: 100px;
}
div.colist td {
padding-right: 0.5em;
padding-bottom: 0.3em;
vertical-align: top;
}
div.colist td img {
margin-top: 0.3em;
}
@media print {
#footer-badges { display: none; }
}
#toc {
margin-bottom: 2.5em;
}
#toctitle {
color: #527bbd;
font-size: 1.1em;
font-weight: bold;
margin-top: 1.0em;
margin-bottom: 0.1em;
}
div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
margin-top: 0;
margin-bottom: 0;
}
div.toclevel2 {
margin-left: 2em;
font-size: 0.9em;
}
div.toclevel3 {
margin-left: 4em;
font-size: 0.9em;
}
div.toclevel4 {
margin-left: 6em;
font-size: 0.9em;
}
span.aqua { color: aqua; }
span.black { color: black; }
span.blue { color: blue; }
span.fuchsia { color: fuchsia; }
span.gray { color: gray; }
span.green { color: green; }
span.lime { color: lime; }
span.maroon { color: maroon; }
span.navy { color: navy; }
span.olive { color: olive; }
span.purple { color: purple; }
span.red { color: red; }
span.silver { color: silver; }
span.teal { color: teal; }
span.white { color: white; }
span.yellow { color: yellow; }
span.aqua-background { background: aqua; }
span.black-background { background: black; }
span.blue-background { background: blue; }
span.fuchsia-background { background: fuchsia; }
span.gray-background { background: gray; }
span.green-background { background: green; }
span.lime-background { background: lime; }
span.maroon-background { background: maroon; }
span.navy-background { background: navy; }
span.olive-background { background: olive; }
span.purple-background { background: purple; }
span.red-background { background: red; }
span.silver-background { background: silver; }
span.teal-background { background: teal; }
span.white-background { background: white; }
span.yellow-background { background: yellow; }
span.big { font-size: 2em; }
span.small { font-size: 0.6em; }
span.underline { text-decoration: underline; }
span.overline { text-decoration: overline; }
span.line-through { text-decoration: line-through; }
div.unbreakable { page-break-inside: avoid; }
/*
* xhtml11 specific
*
* */
div.tableblock {
margin-top: 1.0em;
margin-bottom: 1.5em;
}
div.tableblock > table {
border: 3px solid #527bbd;
}
thead, p.table.header {
font-weight: bold;
color: #527bbd;
}
p.table {
margin-top: 0;
}
/* Because the table frame attribute is overridden by CSS in most browsers. */
div.tableblock > table[frame="void"] {
border-style: none;
}
div.tableblock > table[frame="hsides"] {
border-left-style: none;
border-right-style: none;
}
div.tableblock > table[frame="vsides"] {
border-top-style: none;
border-bottom-style: none;
}
/*
* html5 specific
*
* */
table.tableblock {
margin-top: 1.0em;
margin-bottom: 1.5em;
}
thead, p.tableblock.header {
font-weight: bold;
color: #527bbd;
}
p.tableblock {
margin-top: 0;
}
table.tableblock {
border-width: 3px;
border-spacing: 0px;
border-style: solid;
border-color: #527bbd;
border-collapse: collapse;
}
th.tableblock, td.tableblock {
border-width: 1px;
padding: 4px;
border-style: solid;
border-color: #527bbd;
}
table.tableblock.frame-topbot {
border-left-style: hidden;
border-right-style: hidden;
}
table.tableblock.frame-sides {
border-top-style: hidden;
border-bottom-style: hidden;
}
table.tableblock.frame-none {
border-style: hidden;
}
th.tableblock.halign-left, td.tableblock.halign-left {
text-align: left;
}
th.tableblock.halign-center, td.tableblock.halign-center {
text-align: center;
}
th.tableblock.halign-right, td.tableblock.halign-right {
text-align: right;
}
th.tableblock.valign-top, td.tableblock.valign-top {
vertical-align: top;
}
th.tableblock.valign-middle, td.tableblock.valign-middle {
vertical-align: middle;
}
th.tableblock.valign-bottom, td.tableblock.valign-bottom {
vertical-align: bottom;
}
/*
* manpage specific
*
* */
body.manpage h1 {
padding-top: 0.5em;
padding-bottom: 0.5em;
border-top: 2px solid silver;
border-bottom: 2px solid silver;
}
body.manpage h2 {
border-style: none;
}
body.manpage div.sectionbody {
margin-left: 3em;
}
@media print {
body.manpage div#toc { display: none; }
}
</style>
<script type="text/javascript">
/*<+'])');
// Function that scans the DOM tree for header elements (the DOM2
// nodeIterator API would be a better technique but not supported by all
// browsers).
var iterate = function (el) {
for (var i = el.firstChild; i != null; i = i.nextSibling) {
if (i.nodeType == 1 /* Node.ELEMENT_NODE */) {
var mo = re.exec(i.tagName);
if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") {
result[result.length] = new TocEntry(i, getText(i), mo[1]-1);
}
iterate(i);
}
}
}
iterate(el);
return result;
}
var toc = document.getElementById("toc");
if (!toc) {
return;
}
// Delete existing TOC entries in case we're reloading the TOC.
var tocEntriesToRemove = [];
var i;
for (i = 0; i < toc.childNodes.length; i++) {
var entry = toc.childNodes[i];
if (entry.nodeName.toLowerCase() == 'div'
&& entry.getAttribute("class")
&& entry.getAttribute("class").match(/^toclevel/))
tocEntriesToRemove.push(entry);
}
for (i = 0; i < tocEntriesToRemove.length; i++) {
toc.removeChild(tocEntriesToRemove[i]);
}
// Rebuild TOC entries.
var entries = tocEntries(document.getElementById("content"), toclevels);
for (var i = 0; i < entries.length; ++i) {
var entry = entries[i];
if (entry.element.id == "")
entry.element.id = "_toc_" + i;
var a = document.createElement("a");
a.href = "#" + entry.element.id;
a.appendChild(document.createTextNode(entry.text));
var div = document.createElement("div");
div.appendChild(a);
div.className = "toclevel" + entry.toclevel;
toc.appendChild(div);
}
if (entries.length == 0)
toc.parentNode.removeChild(toc);
},
/////////////////////////////////////////////////////////////////////
// Footnotes generator
/////////////////////////////////////////////////////////////////////
/* Based on footnote generation code from:
* http://www.brandspankingnew.net/archive/2005/07/format_footnote.html
*/
footnotes: function () {
// Delete existing footnote entries in case we're reloading the footnodes.
var i;
var noteholder = document.getElementById("footnotes");
if (!noteholder) {
return;
}
var entriesToRemove = [];
for (i = 0; i < noteholder.childNodes.length; i++) {
var entry = noteholder.childNodes[i];
if (entry.nodeName.toLowerCase() == 'div' && entry.getAttribute("class") == "footnote")
entriesToRemove.push(entry);
}
for (i = 0; i < entriesToRemove.length; i++) {
noteholder.removeChild(entriesToRemove[i]);
}
// Rebuild footnote entries.
var cont = document.getElementById("content");
var spans = cont.getElementsByTagName("span");
var refs = {};
var n = 0;
for (i=0; i<spans.length; i++) {
if (spans[i].className == "footnote") {
n++;
var note = spans[i].getAttribute("data-note");
if (!note) {
// Use [\s\S] in place of . so multi-line matches work.
// Because JavaScript has no s (dotall) regex flag.
note = spans[i].innerHTML.match(/\s*\[([\s\S]*)]\s*/)[1];
spans[i].innerHTML =
"[<a id='_footnoteref_" + n + "' href='#_footnote_" + n +
"' title='View footnote' class='footnote'>" + n + "</a>]";
spans[i].setAttribute("data-note", note);
}
noteholder.innerHTML +=
"<div class='footnote' id='_footnote_" + n + "'>" +
"<a href='#_footnoteref_" + n + "' title='Return to text'>" +
n + "</a>. " + note + "</div>";
var id =spans[i].getAttribute("id");
if (id != null) refs["#"+id] = n;
}
}
if (n == 0)
noteholder.parentNode.removeChild(noteholder);
else {
// Process footnoterefs.
for (i=0; i<spans.length; i++) {
if (spans[i].className == "footnoteref") {
var href = spans[i].getElementsByTagName("a")[0].getAttribute("href");
href = href.match(/#.*/)[0]; // Because IE return full URL.
n = refs[href];
spans[i].innerHTML =
"[<a href='#_footnote_" + n +
"' title='View footnote' class='footnote'>" + n + "</a>]";
}
}
}
},
install: function(toclevels) {
var timerId;
function reinstall() {
asciidoc.footnotes();
if (toclevels) {
asciidoc.toc(toclevels);
}
}
function reinstallAndRemoveTimer() {
clearInterval(timerId);
reinstall();
}
timerId = setInterval(reinstall, 500);
if (document.addEventListener)
document.addEventListener("DOMContentLoaded", reinstallAndRemoveTimer, false);
else
window.onload = reinstallAndRemoveTimer;
}
}
asciidoc.install();
/*]]>*/
</script>
</head>
<body class="article">
<div id="header">
<h1>Concerning Git’s Packing Heuristics</h1>
<span id="revdate">2024-01-19</span>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="literalblock">
<div class="content">
<pre><code>Oh, here's a really stupid question:</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code> Where do I go
to learn the details
of Git's packing heuristics?</code></pre>
</div></div>
<div class="paragraph"><p>Be careful what you ask!</p></div>
<div class="paragraph"><p>Followers of the Git, please open the Git IRC Log and turn to
February 10, 2006.</p></div>
<div class="paragraph"><p>It’s a rare occasion, and we are joined by the King Git Himself,
Linus Torvalds (linus). Nathaniel Smith, (njs`), has the floor
and seeks enlightenment. Others are present, but silent.</p></div>
<div class="paragraph"><p>Let’s listen in!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Oh, here's a really stupid question -- where do I go to
learn the details of Git's packing heuristics? google avails
me not, reading the source didn't help a lot, and wading
through the whole mailing list seems less efficient than any
of that.</code></pre>
</div></div>
<div class="paragraph"><p>It is a bold start! A plea for help combined with a simultaneous
tri-part attack on some of the tried and true mainstays in the quest
for enlightenment. Brash accusations of google being useless. Hubris!
Maligning the source. Heresy! Disdain for the mailing list archives.
Woe.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><pasky> yes, the packing-related delta stuff is somewhat
mysterious even for me ;)</code></pre>
</div></div>
<div class="paragraph"><p>Ah! Modesty after all.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> njs, I don't think the docs exist. That's something where
I don't think anybody else than me even really got involved.
Most of the rest of Git others have been busy with (especially
Junio), but packing nobody touched after I did it.</code></pre>
</div></div>
<div class="paragraph"><p>It’s cryptic, yet vague. Linus in style for sure. Wise men
interpret this as an apology. A few argue it is merely a
statement of fact.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> I guess the next step is "read the source again", but I
have to build up a certain level of gumption first :-)</code></pre>
</div></div>
<div class="paragraph"><p>Indeed! On both points.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> The packing heuristic is actually really really simple.</code></pre>
</div></div>
<div class="paragraph"><p>Bait…</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> But strange.</code></pre>
</div></div>
<div class="paragraph"><p>And switch. That ought to do it!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Remember: Git really doesn't follow files. So what it does is
- generate a list of all objects
- sort the list according to magic heuristics
- walk the list, using a sliding window, seeing if an object
can be diffed against another object in the window
- write out the list in recency order</code></pre>
</div></div>
<div class="paragraph"><p>The traditional understatement:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> I suspect that what I'm missing is the precise definition of
the word "magic"</code></pre>
</div></div>
<div class="paragraph"><p>The traditional insight:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><pasky> yes</code></pre>
</div></div>
<div class="paragraph"><p>And Babel-like confusion flowed.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> oh, hmm, and I'm not sure what this sliding window means either</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><pasky> iirc, it appeared to me to be just the sha1 of the object
when reading the code casually ...</code></pre>
</div></div>
<div class="olist lowerroman"><ol class="lowerroman">
<li>
<p>
which simply doesn’t sound as a very good heuristics, though ;)
</p>
<div class="literalblock">
<div class="content">
<pre><code><njs`> .....and recency order. okay, I think it's clear I didn't
even realize how much I wasn't realizing :-)</code></pre>
</div></div>
</li>
</ol></div>
<div class="paragraph"><p>Ah, grasshopper! And thus the enlightenment begins anew.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> The "magic" is actually in theory totally arbitrary.
ANY order will give you a working pack, but no, it's not
ordered by SHA-1.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Before talking about the ordering for the sliding delta
window, let's talk about the recency order. That's more
important in one way.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Right, but if all you want is a working way to pack things
together, you could just use cat and save yourself some
trouble...</code></pre>
</div></div>
<div class="paragraph"><p>Waaait for it….</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> The recency ordering (which is basically: put objects
_physically_ into the pack in the order that they are
"reachable" from the head) is important.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> okay</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> It's important because that's the thing that gives packs
good locality. It keeps the objects close to the head (whether
they are old or new, but they are _reachable_ from the head)
at the head of the pack. So packs actually have absolutely
_wonderful_ IO patterns.</code></pre>
</div></div>
<div class="paragraph"><p>Read that again, because it is important.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> But recency ordering is totally useless for deciding how
to actually generate the deltas, so the delta ordering is
something else.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>The delta ordering is (wait for it):
- first sort by the "basename" of the object, as defined by
the name the object was _first_ reached through when
generating the object list
- within the same basename, sort by size of the object
- but always sort different types separately (commits first).</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>That's not exactly it, but it's very close.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> The "_first_ reached" thing is not too important, just you
need some way to break ties since the same objects may be
reachable many ways, yes?</code></pre>
</div></div>
<div class="paragraph"><p>And as if to clarify:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> The point is that it's all really just any random
heuristic, and the ordering is totally unimportant for
correctness, but it helps a lot if the heuristic gives
"clumping" for things that are likely to delta well against
each other.</code></pre>
</div></div>
<div class="paragraph"><p>It is an important point, so secretly, I did my own research and have
included my results below. To be fair, it has changed some over time.
And through the magic of Revisionistic History, I draw upon this entry
from The Git IRC Logs on my father’s birthday, March 1:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><gitster> The quote from the above linus should be rewritten a
bit (wait for it):
- first sort by type. Different objects never delta with
each other.
- then sort by filename/dirname. hash of the basename
occupies the top BITS_PER_INT-DIR_BITS bits, and bottom
DIR_BITS are for the hash of leading path elements.
- then if we are doing "thin" pack, the objects we are _not_
going to pack but we know about are sorted earlier than
other objects.
- and finally sort by size, larger to smaller.</code></pre>
</div></div>
<div class="paragraph"><p>In one swell-foop, clarification and obscurification! Nonetheless,
authoritative. Cryptic, yet concise. It even solicits notions of
quotes from The Source Code. Clearly, more study is needed.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><gitster> That's the sort order. What this means is:
- we do not delta different object types.
- we prefer to delta the objects with the same full path, but
allow files with the same name from different directories.
- we always prefer to delta against objects we are not going
to send, if there are some.
- we prefer to delta against larger objects, so that we have
lots of removals.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>The penultimate rule is for "thin" packs. It is used when
the other side is known to have such objects.</code></pre>
</div></div>
<div class="paragraph"><p>There it is again. "Thin" packs. I’m thinking to myself, "What
is a <em>thin</em> pack?" So I ask:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><jdl> What is a "thin" pack?</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><gitster> Use of --objects-edge to rev-list as the upstream of
pack-objects. The pack transfer protocol negotiates that.</code></pre>
</div></div>
<div class="paragraph"><p>Woo hoo! Cleared that <em>right</em> up!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><gitster> There are two directions - push and fetch.</code></pre>
</div></div>
<div class="paragraph"><p>There! Did you see it? It is not <em>"push" and "pull"</em>! How often the
confusion has started here. So casually mentioned, too!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><gitster> For push, git-send-pack invokes git-receive-pack on the
other end. The receive-pack says "I have up to these commits".
send-pack looks at them, and computes what are missing from
the other end. So "thin" could be the default there.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>In the other direction, fetch, git-fetch-pack and
git-clone-pack invokes git-upload-pack on the other end
(via ssh or by talking to the daemon).</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>There are two cases: fetch-pack with -k and clone-pack is one,
fetch-pack without -k is the other. clone-pack and fetch-pack
with -k will keep the downloaded packfile without expanded, so
we do not use thin pack transfer. Otherwise, the generated
pack will have delta without base object in the same pack.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>But fetch-pack without -k will explode the received pack into
individual objects, so we automatically ask upload-pack to
give us a thin pack if upload-pack supports it.</code></pre>
</div></div>
<div class="paragraph"><p>OK then.</p></div>
<div class="paragraph"><p>Uh.</p></div>
<div class="paragraph"><p>Let’s return to the previous conversation still in progress.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> and "basename" means something like "the tail of end of
path of file objects and dir objects, as per basename(3), and
we just declare all commit and tag objects to have the same
basename" or something?</code></pre>
</div></div>
<div class="paragraph"><p>Luckily, that too is a point that gitster clarified for us!</p></div>
<div class="paragraph"><p>If I might add, the trick is to make files that <em>might</em> be similar be
located close to each other in the hash buckets based on their file
names. It used to be that "foo/Makefile", "bar/baz/quux/Makefile" and
"Makefile" all landed in the same bucket due to their common basename,
"Makefile". However, now they land in "close" buckets.</p></div>
<div class="paragraph"><p>The algorithm allows not just for the <em>same</em> bucket, but for <em>close</em>
buckets to be considered delta candidates. The rationale is
essentially that files, like Makefiles, often have very similar
content no matter what directory they live in.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> I played around with different delta algorithms, and with
making the "delta window" bigger, but having too big of a
sliding window makes it very expensive to generate the pack:
you need to compare every object with a _ton_ of other objects.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>There are a number of other trivial heuristics too, which
basically boil down to "don't bother even trying to delta this
pair" if we can tell before-hand that the delta isn't worth it
(due to size differences, where we can take a previous delta
result into account to decide that "ok, no point in trying
that one, it will be worse").</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>End result: packing is actually very size efficient. It's
somewhat CPU-wasteful, but on the other hand, since you're
really only supposed to do it maybe once a month (and you can
do it during the night), nobody really seems to care.</code></pre>
</div></div>
<div class="paragraph"><p>Nice Engineering Touch, there. Find when it doesn’t matter, and
proclaim it a non-issue. Good style too!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> So, just to repeat to see if I'm following, we start by
getting a list of the objects we want to pack, we sort it by
this heuristic (basically lexicographically on the tuple
(type, basename, size)).</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Then we walk through this list, and calculate a delta of
each object against the last n (tunable parameter) objects,
and pick the smallest of these deltas.</code></pre>
</div></div>
<div class="paragraph"><p>Vastly simplified, but the essence is there!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Correct.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> And then once we have picked a delta or fulltext to
represent each object, we re-sort by recency, and write them
out in that order.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Yup. Some other small details:</code></pre>
</div></div>
<div class="paragraph"><p>And of course there is the "Other Shoe" Factor too.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> - We limit the delta depth to another magic value (right
now both the window and delta depth magic values are just "10")</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Hrm, my intuition is that you'd end up with really _bad_ IO
patterns, because the things you want are near by, but to
actually reconstruct them you may have to jump all over in
random ways.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> - When we write out a delta, and we haven't yet written
out the object it is a delta against, we write out the base
object first. And no, when we reconstruct them, we actually
get nice IO patterns, because:
- larger objects tend to be "more recent" (Linus' law: files grow)
- we actively try to generate deltas from a larger object to a
smaller one
- this means that the top-of-tree very seldom has deltas
(i.e. deltas in _practice_ are "backwards deltas")</code></pre>
</div></div>
<div class="paragraph"><p>Again, we should reread that whole paragraph. Not just because
Linus has slipped Linus’s Law in there on us, but because it is
important. Let’s make sure we clarify some of the points here:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> So the point is just that in practice, delta order and
recency order match each other quite well.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Yes. There's another nice side to this (and yes, it was
designed that way ;):
- the reason we generate deltas against the larger object is
actually a big space saver too!</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Hmm, but your last comment (if "we haven't yet written out
the object it is a delta against, we write out the base object
first"), seems like it would make these facts mostly
irrelevant because even if in practice you would not have to
wander around much, in fact you just brute-force say that in
the cases where you might have to wander, don't do that :-)</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Yes and no. Notice the rule: we only write out the base
object first if the delta against it was more recent. That
means that you can actually have deltas that refer to a base
object that is _not_ close to the delta object, but that only
happens when the delta is needed to generate an _old_ object.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> See?</code></pre>
</div></div>
<div class="paragraph"><p>Yeah, no. I missed that on the first two or three readings myself.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> This keeps the front of the pack dense. The front of the
pack never contains data that isn't relevant to a "recent"
object. The size optimization comes from our use of xdelta
(but is true for many other delta algorithms): removing data
is cheaper (in size) than adding data.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>When you remove data, you only need to say "copy bytes n--m".
In contrast, in a delta that _adds_ data, you have to say "add
these bytes: 'actual data goes here'"</code></pre>
</div></div>
<div class="ulist"><ul>
<li>
<p>
njs` has quit: Read error: 104 (Connection reset by peer)
</p>
<div class="literalblock">
<div class="content">
<pre><code><linus> Uhhuh. I hope I didn't blow njs` mind.</code></pre>
</div></div>
</li>
<li>
<p>
njs` has joined channel #git
</p>
<div class="literalblock">
<div class="content">
<pre><code><pasky> :)</code></pre>
</div></div>
</li>
</ul></div>
<div class="paragraph"><p>The silent observers are amused. Of course.</p></div>
<div class="paragraph"><p>And as if njs` was expected to be omniscient:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> njs - did you miss anything?</code></pre>
</div></div>
<div class="paragraph"><p>OK, I’ll spell it out. That’s Geek Humor. If njs` was not actually
connected for a little bit there, how would he know if missed anything
while he was disconnected? He’s a benevolent dictator with a sense of
humor! Well noted!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Stupid router. Or gremlins, or whatever.</code></pre>
</div></div>
<div class="paragraph"><p>It’s a cheap shot at Cisco. Take 'em when you can.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Yes and no. Notice the rule: we only write out the base
object first if the delta against it was more recent.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>I'm getting lost in all these orders, let me re-read :-)
So the write-out order is from most recent to least recent?
(Conceivably it could be the opposite way too, I'm not sure if
we've said) though my connection back at home is logging, so I
can just read what you said there :-)</code></pre>
</div></div>
<div class="paragraph"><p>And for those of you paying attention, the Omniscient Trick has just
been detailed!</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Yes, we always write out most recent first</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> And, yeah, I got the part about deeper-in-history stuff
having worse IO characteristics, one sort of doesn't care.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> With the caveat that if the "most recent" needs an older
object to delta against (hey, shrinking sometimes does
happen), we write out the old object with the delta.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> (if only it happened more...)</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Anyway, the pack-file could easily be denser still, but
because it's used both for streaming (the Git protocol) and
for on-disk, it has a few pessimizations.</code></pre>
</div></div>
<div class="paragraph"><p>Actually, it is a made-up word. But it is a made-up word being
used as setup for a later optimization, which is a real word:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> In particular, while the pack-file is then compressed,
it's compressed just one object at a time, so the actual
compression factor is less than it could be in theory. But it
means that it's all nice random-access with a simple index to
do "object name->location in packfile" translation.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> I'm assuming the real win for delta-ing large->small is
more homogeneous statistics for gzip to run over?</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>(You have to put the bytes in one place or another, but
putting them in a larger blob wins on compression)</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Actually, what is the compression strategy -- each delta
individually gzipped, the whole file gzipped, somewhere in
between, no compression at all, ....?</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Right.</code></pre>
</div></div>
<div class="paragraph"><p>Reality IRC sets in. For example:</p></div>
<div class="literalblock">
<div class="content">
<pre><code><pasky> I'll read the rest in the morning, I really have to go
sleep or there's no hope whatsoever for me at the today's
exam... g'nite all.</code></pre>
</div></div>
<div class="paragraph"><p>Heh.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> pasky: g'nite</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> pasky: 'luck</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Right: large->small matters exactly because of compression
behaviour. If it was non-compressed, it probably wouldn't make
any difference.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> yeah</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Anyway: I'm not even trying to claim that the pack-files
are perfect, but they do tend to have a nice balance of
density vs ease-of use.</code></pre>
</div></div>
<div class="paragraph"><p>Gasp! OK, saved. That’s a fair Engineering trade off. Close call!
In fact, Linus reflects on some Basic Engineering Fundamentals,
design options, etc.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> More importantly, they allow Git to still _conceptually_
never deal with deltas at all, and be a "whole object" store.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Which has some problems (we discussed bad huge-file
behaviour on the Git lists the other day), but it does mean
that the basic Git concepts are really really simple and
straightforward.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>It's all been quite stable.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Which I think is very much a result of having very simple
basic ideas, so that there's never any confusion about what's
going on.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code>Bugs happen, but they are "simple" bugs. And bugs that
actually get some object store detail wrong are almost always
so obvious that they never go anywhere.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> Yeah.</code></pre>
</div></div>
<div class="paragraph"><p>Nuff said.</p></div>
<div class="literalblock">
<div class="content">
<pre><code><linus> Anyway. I'm off for bed. It's not 6AM here, but I've got
three kids, and have to get up early in the morning to send
them off. I need my beauty sleep.</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> :-)</code></pre>
</div></div>
<div class="literalblock">
<div class="content">
<pre><code><njs`> appreciate the infodump, I really was failing to find the
details on Git packs :-)</code></pre>
</div></div>
<div class="paragraph"><p>And now you know the rest of the story.</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated
2020-03-10 15:02:33 PDT
</div>
</div>
</body>
</html>
|
