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chromium / external / gitster / git-htmldocs / a90214fb857c988a9e8dff46626dc715918a9330 / . / technical / pack-heuristics.html
blob: 16e93370e5124794568ae28aaac2750861115147 [file] [log] [blame]
Junio C Hamano944ce252018-05-30 22:25:26[diff] [blame]1<?xml version="1.0" encoding="UTF-8"?>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]6<meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" />
Junio C Hamano944ce252018-05-30 22:25:26[diff] [blame]7<meta name="generator" content="AsciiDoc 8.6.10" />
Junio C Hamanobc8d4782014-01-13 23:35:50[diff] [blame]8<title>Concerning Git&#8217;s Packing Heuristics</title>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]9<style type="text/css">
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]10/* Shared CSS for AsciiDoc xhtml11 and html5 backends */
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]25}
26
27body {
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48
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]49h1, h2, h3, h4, h5, h6 {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]51 margin-top: 1.2em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]68h5 {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]71
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]73 margin-left: 0;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]88ul > li { color: #aaa; }
89ul > li > * { color: black; }
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]90
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]91.monospaced, code, pre {
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]95 padding: 0;
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Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]98pre {
99 white-space: pre-wrap;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]101
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]102#author {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]103 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]104 font-weight: bold;
105 font-size: 1.1em;
106}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]107#email {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]108}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]109#revnumber, #revdate, #revremark {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]110}
111
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]112#footer {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]113 font-size: small;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]118#footer-text {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]119 float: left;
120 padding-bottom: 0.5em;
121}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]122#footer-badges {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]123 float: right;
124 padding-bottom: 0.5em;
125}
126
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]127#preamble {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]128 margin-top: 1.5em;
129 margin-bottom: 1.5em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]131div.imageblock, div.exampleblock, div.verseblock,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]132div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock,
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]151 font-weight: bold;
152 text-align: left;
153 margin-top: 1.0em;
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]172 border: 1px solid #dddddd;
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Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]174 padding: 0.5em;
175}
176
177div.listingblock > div.content {
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]178 border: 1px solid #dddddd;
179 border-left: 5px solid #f0f0f0;
180 background: #f8f8f8;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]181 padding: 0.5em;
182}
183
184div.quoteblock, div.verseblock {
185 padding-left: 1.0em;
186 margin-left: 1.0em;
187 margin-right: 10%;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]188 border-left: 5px solid #f0f0f0;
189 color: #888;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]190}
191
192div.quoteblock > div.attribution {
193 padding-top: 0.5em;
194 text-align: right;
195}
196
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]197div.verseblock > pre.content {
198 font-family: inherit;
199 font-size: inherit;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]200}
201div.verseblock > div.attribution {
202 padding-top: 0.75em;
203 text-align: left;
204}
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206div.verseblock + div.attribution {
207 text-align: left;
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209
210div.admonitionblock .icon {
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212 font-size: 1.1em;
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223div.exampleblock > div.content {
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227
228div.imageblock div.content { padding-left: 0; }
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]229span.image img { border-style: none; vertical-align: text-bottom; }
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]230a.image:visited { color: white; }
231
232dl {
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234 margin-bottom: 0.8em;
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239 font-style: normal;
240 color: navy;
241}
242dd > *:first-child {
243 margin-top: 0.1em;
244}
245
246ul, ol {
247 list-style-position: outside;
248}
249ol.arabic {
250 list-style-type: decimal;
251}
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253 list-style-type: lower-alpha;
254}
255ol.upperalpha {
256 list-style-type: upper-alpha;
257}
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259 list-style-type: lower-roman;
260}
261ol.upperroman {
262 list-style-type: upper-roman;
263}
264
265div.compact ul, div.compact ol,
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268 margin-top: 0.1em;
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271
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]272tfoot {
273 font-weight: bold;
274}
275td > div.verse {
276 white-space: pre;
277}
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]278
279div.hdlist {
280 margin-top: 0.8em;
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283div.hdlist tr {
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285}
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292 padding-right: 0.8em;
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302
303.comment {
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308 font-size: 0.8em;
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323
324#footnotes hr {
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333
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]334div.colist td {
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339div.colist td img {
340 margin-top: 0.3em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]341}
342
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]343@media print {
344 #footer-badges { display: none; }
345}
346
347#toc {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]348 margin-bottom: 2.5em;
349}
350
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]351#toctitle {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]352 color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]353 font-size: 1.1em;
354 font-weight: bold;
355 margin-top: 1.0em;
356 margin-bottom: 0.1em;
357}
358
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]359div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]360 margin-top: 0;
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373 font-size: 0.9em;
374}
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]375
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]376span.aqua { color: aqua; }
377span.black { color: black; }
378span.blue { color: blue; }
379span.fuchsia { color: fuchsia; }
380span.gray { color: gray; }
381span.green { color: green; }
382span.lime { color: lime; }
383span.maroon { color: maroon; }
384span.navy { color: navy; }
385span.olive { color: olive; }
386span.purple { color: purple; }
387span.red { color: red; }
388span.silver { color: silver; }
389span.teal { color: teal; }
390span.white { color: white; }
391span.yellow { color: yellow; }
392
393span.aqua-background { background: aqua; }
394span.black-background { background: black; }
395span.blue-background { background: blue; }
396span.fuchsia-background { background: fuchsia; }
397span.gray-background { background: gray; }
398span.green-background { background: green; }
399span.lime-background { background: lime; }
400span.maroon-background { background: maroon; }
401span.navy-background { background: navy; }
402span.olive-background { background: olive; }
403span.purple-background { background: purple; }
404span.red-background { background: red; }
405span.silver-background { background: silver; }
406span.teal-background { background: teal; }
407span.white-background { background: white; }
408span.yellow-background { background: yellow; }
409
410span.big { font-size: 2em; }
411span.small { font-size: 0.6em; }
412
413span.underline { text-decoration: underline; }
414span.overline { text-decoration: overline; }
415span.line-through { text-decoration: line-through; }
416
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]417div.unbreakable { page-break-inside: avoid; }
418
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]419
420/*
421 * xhtml11 specific
422 *
423 * */
424
425div.tableblock {
426 margin-top: 1.0em;
427 margin-bottom: 1.5em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]428}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]429div.tableblock > table {
430 border: 3px solid #527bbd;
431}
432thead, p.table.header {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]433 font-weight: bold;
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]434 color: #527bbd;
435}
436p.table {
437 margin-top: 0;
438}
439/* Because the table frame attribute is overriden by CSS in most browsers. */
440div.tableblock > table[frame="void"] {
441 border-style: none;
442}
443div.tableblock > table[frame="hsides"] {
444 border-left-style: none;
445 border-right-style: none;
446}
447div.tableblock > table[frame="vsides"] {
448 border-top-style: none;
449 border-bottom-style: none;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]450}
451
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]452
453/*
454 * html5 specific
455 *
456 * */
457
458table.tableblock {
459 margin-top: 1.0em;
460 margin-bottom: 1.5em;
461}
462thead, p.tableblock.header {
463 font-weight: bold;
464 color: #527bbd;
465}
466p.tableblock {
467 margin-top: 0;
468}
469table.tableblock {
470 border-width: 3px;
471 border-spacing: 0px;
472 border-style: solid;
473 border-color: #527bbd;
474 border-collapse: collapse;
475}
476th.tableblock, td.tableblock {
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478 padding: 4px;
479 border-style: solid;
480 border-color: #527bbd;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]481}
482
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]483table.tableblock.frame-topbot {
484 border-left-style: hidden;
485 border-right-style: hidden;
486}
487table.tableblock.frame-sides {
488 border-top-style: hidden;
489 border-bottom-style: hidden;
490}
491table.tableblock.frame-none {
492 border-style: hidden;
493}
494
495th.tableblock.halign-left, td.tableblock.halign-left {
496 text-align: left;
497}
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500}
501th.tableblock.halign-right, td.tableblock.halign-right {
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]502 text-align: right;
503}
504
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]505th.tableblock.valign-top, td.tableblock.valign-top {
506 vertical-align: top;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]507}
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]508th.tableblock.valign-middle, td.tableblock.valign-middle {
509 vertical-align: middle;
510}
511th.tableblock.valign-bottom, td.tableblock.valign-bottom {
512 vertical-align: bottom;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]513}
514
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]515
516/*
517 * manpage specific
518 *
519 * */
520
521body.manpage h1 {
522 padding-top: 0.5em;
523 padding-bottom: 0.5em;
524 border-top: 2px solid silver;
525 border-bottom: 2px solid silver;
526}
527body.manpage h2 {
528 border-style: none;
529}
530body.manpage div.sectionbody {
531 margin-left: 3em;
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]532}
533
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]534@media print {
535 body.manpage div#toc { display: none; }
536}
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]537
538
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]539</style>
540<script type="text/javascript">
541/*<![CDATA[*/
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]542var asciidoc = { // Namespace.
543
544/////////////////////////////////////////////////////////////////////
545// Table Of Contents generator
546/////////////////////////////////////////////////////////////////////
547
548/* Author: Mihai Bazon, September 2002
549 * http://students.infoiasi.ro/~mishoo
550 *
551 * Table Of Content generator
552 * Version: 0.4
553 *
554 * Feel free to use this script under the terms of the GNU General Public
555 * License, as long as you do not remove or alter this notice.
556 */
557
558 /* modified by Troy D. Hanson, September 2006. License: GPL */
559 /* modified by Stuart Rackham, 2006, 2009. License: GPL */
560
561// toclevels = 1..4.
562toc: function (toclevels) {
563
564 function getText(el) {
565 var text = "";
566 for (var i = el.firstChild; i != null; i = i.nextSibling) {
567 if (i.nodeType == 3 /* Node.TEXT_NODE */) // IE doesn't speak constants.
568 text += i.data;
569 else if (i.firstChild != null)
570 text += getText(i);
571 }
572 return text;
573 }
574
575 function TocEntry(el, text, toclevel) {
576 this.element = el;
577 this.text = text;
578 this.toclevel = toclevel;
579 }
580
581 function tocEntries(el, toclevels) {
582 var result = new Array;
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]583 var re = new RegExp('[hH]([1-'+(toclevels+1)+'])');
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]584 // Function that scans the DOM tree for header elements (the DOM2
585 // nodeIterator API would be a better technique but not supported by all
586 // browsers).
587 var iterate = function (el) {
588 for (var i = el.firstChild; i != null; i = i.nextSibling) {
589 if (i.nodeType == 1 /* Node.ELEMENT_NODE */) {
590 var mo = re.exec(i.tagName);
591 if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") {
592 result[result.length] = new TocEntry(i, getText(i), mo[1]-1);
593 }
594 iterate(i);
595 }
596 }
597 }
598 iterate(el);
599 return result;
600 }
601
602 var toc = document.getElementById("toc");
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]603 if (!toc) {
604 return;
605 }
606
607 // Delete existing TOC entries in case we're reloading the TOC.
608 var tocEntriesToRemove = [];
609 var i;
610 for (i = 0; i < toc.childNodes.length; i++) {
611 var entry = toc.childNodes[i];
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]612 if (entry.nodeName.toLowerCase() == 'div'
Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]613 && entry.getAttribute("class")
614 && entry.getAttribute("class").match(/^toclevel/))
615 tocEntriesToRemove.push(entry);
616 }
617 for (i = 0; i < tocEntriesToRemove.length; i++) {
618 toc.removeChild(tocEntriesToRemove[i]);
619 }
620
621 // Rebuild TOC entries.
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]622 var entries = tocEntries(document.getElementById("content"), toclevels);
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Junio C Hamano9d971152012-12-19 00:43:11[diff] [blame]735<body class="article">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]736<div id="header">
Junio C Hamanobc8d4782014-01-13 23:35:50[diff] [blame]737<h1>Concerning Git&#8217;s Packing Heuristics</h1>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]738</div>
739<div id="content">
Junio C Hamanobc8d4782014-01-13 23:35:50[diff] [blame]740<div id="preamble">
741<div class="sectionbody">
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]742<div class="literalblock">
743<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]744<pre><code>Oh, here's a really stupid question:</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]745</div></div>
746<div class="literalblock">
747<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]748<pre><code> Where do I go
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]749 to learn the details
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]750of Git's packing heuristics?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]751</div></div>
752<div class="paragraph"><p>Be careful what you ask!</p></div>
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]753<div class="paragraph"><p>Followers of the Git, please open the Git IRC Log and turn to
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]754February 10, 2006.</p></div>
755<div class="paragraph"><p>It&#8217;s a rare occasion, and we are joined by the King Git Himself,
756Linus Torvalds (linus). Nathaniel Smith, (njs`), has the floor
757and seeks enlightenment. Others are present, but silent.</p></div>
758<div class="paragraph"><p>Let&#8217;s listen in!</p></div>
759<div class="literalblock">
760<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]761<pre><code>&lt;njs`&gt; Oh, here's a really stupid question -- where do I go to
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]762 learn the details of Git's packing heuristics? google avails
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]763 me not, reading the source didn't help a lot, and wading
764 through the whole mailing list seems less efficient than any
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]765 of that.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]766</div></div>
767<div class="paragraph"><p>It is a bold start! A plea for help combined with a simultaneous
768tri-part attack on some of the tried and true mainstays in the quest
769for enlightenment. Brash accusations of google being useless. Hubris!
770Maligning the source. Heresy! Disdain for the mailing list archives.
771Woe.</p></div>
772<div class="literalblock">
773<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]774<pre><code>&lt;pasky&gt; yes, the packing-related delta stuff is somewhat
775 mysterious even for me ;)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]776</div></div>
777<div class="paragraph"><p>Ah! Modesty after all.</p></div>
778<div class="literalblock">
779<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]780<pre><code>&lt;linus&gt; njs, I don't think the docs exist. That's something where
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]781 I don't think anybody else than me even really got involved.
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]782 Most of the rest of Git others have been busy with (especially
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]783 Junio), but packing nobody touched after I did it.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]784</div></div>
785<div class="paragraph"><p>It&#8217;s cryptic, yet vague. Linus in style for sure. Wise men
786interpret this as an apology. A few argue it is merely a
787statement of fact.</p></div>
788<div class="literalblock">
789<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]790<pre><code>&lt;njs`&gt; I guess the next step is "read the source again", but I
791 have to build up a certain level of gumption first :-)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]792</div></div>
793<div class="paragraph"><p>Indeed! On both points.</p></div>
794<div class="literalblock">
795<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]796<pre><code>&lt;linus&gt; The packing heuristic is actually really really simple.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]797</div></div>
798<div class="paragraph"><p>Bait&#8230;</p></div>
799<div class="literalblock">
800<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]801<pre><code>&lt;linus&gt; But strange.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]802</div></div>
803<div class="paragraph"><p>And switch. That ought to do it!</p></div>
804<div class="literalblock">
805<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]806<pre><code>&lt;linus&gt; Remember: Git really doesn't follow files. So what it does is
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]807 - generate a list of all objects
808 - sort the list according to magic heuristics
809 - walk the list, using a sliding window, seeing if an object
810 can be diffed against another object in the window
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]811 - write out the list in recency order</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]812</div></div>
813<div class="paragraph"><p>The traditional understatement:</p></div>
814<div class="literalblock">
815<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]816<pre><code>&lt;njs`&gt; I suspect that what I'm missing is the precise definition of
817 the word "magic"</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]818</div></div>
819<div class="paragraph"><p>The traditional insight:</p></div>
820<div class="literalblock">
821<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]822<pre><code>&lt;pasky&gt; yes</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]823</div></div>
824<div class="paragraph"><p>And Babel-like confusion flowed.</p></div>
825<div class="literalblock">
826<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]827<pre><code>&lt;njs`&gt; oh, hmm, and I'm not sure what this sliding window means either</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]828</div></div>
829<div class="literalblock">
830<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]831<pre><code>&lt;pasky&gt; iirc, it appeared to me to be just the sha1 of the object
832 when reading the code casually ...</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]833</div></div>
834<div class="olist lowerroman"><ol class="lowerroman">
835<li>
836<p>
837which simply doesn&#8217;t sound as a very good heuristics, though ;)
838</p>
839<div class="literalblock">
840<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]841<pre><code>&lt;njs`&gt; .....and recency order. okay, I think it's clear I didn't
842 even realize how much I wasn't realizing :-)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]843</div></div>
844</li>
845</ol></div>
846<div class="paragraph"><p>Ah, grasshopper! And thus the enlightenment begins anew.</p></div>
847<div class="literalblock">
848<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]849<pre><code>&lt;linus&gt; The "magic" is actually in theory totally arbitrary.
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]850 ANY order will give you a working pack, but no, it's not
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]851 ordered by SHA-1.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]852</div></div>
853<div class="literalblock">
854<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]855<pre><code>Before talking about the ordering for the sliding delta
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]856window, let's talk about the recency order. That's more
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]857important in one way.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]858</div></div>
859<div class="literalblock">
860<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]861<pre><code>&lt;njs`&gt; Right, but if all you want is a working way to pack things
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]862 together, you could just use cat and save yourself some
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]863 trouble...</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]864</div></div>
865<div class="paragraph"><p>Waaait for it&#8230;.</p></div>
866<div class="literalblock">
867<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]868<pre><code>&lt;linus&gt; The recency ordering (which is basically: put objects
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]869 _physically_ into the pack in the order that they are
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]870 "reachable" from the head) is important.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]871</div></div>
872<div class="literalblock">
873<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]874<pre><code>&lt;njs`&gt; okay</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]875</div></div>
876<div class="literalblock">
877<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]878<pre><code>&lt;linus&gt; It's important because that's the thing that gives packs
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]879 good locality. It keeps the objects close to the head (whether
880 they are old or new, but they are _reachable_ from the head)
881 at the head of the pack. So packs actually have absolutely
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]882 _wonderful_ IO patterns.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]883</div></div>
884<div class="paragraph"><p>Read that again, because it is important.</p></div>
885<div class="literalblock">
886<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]887<pre><code>&lt;linus&gt; But recency ordering is totally useless for deciding how
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]888 to actually generate the deltas, so the delta ordering is
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]889 something else.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]890</div></div>
891<div class="literalblock">
892<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]893<pre><code>The delta ordering is (wait for it):
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]894- first sort by the "basename" of the object, as defined by
895 the name the object was _first_ reached through when
896 generating the object list
897- within the same basename, sort by size of the object
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]898- but always sort different types separately (commits first).</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]899</div></div>
900<div class="literalblock">
901<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]902<pre><code>That's not exactly it, but it's very close.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]903</div></div>
904<div class="literalblock">
905<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]906<pre><code>&lt;njs`&gt; The "_first_ reached" thing is not too important, just you
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]907 need some way to break ties since the same objects may be
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]908 reachable many ways, yes?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]909</div></div>
910<div class="paragraph"><p>And as if to clarify:</p></div>
911<div class="literalblock">
912<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]913<pre><code>&lt;linus&gt; The point is that it's all really just any random
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]914 heuristic, and the ordering is totally unimportant for
915 correctness, but it helps a lot if the heuristic gives
916 "clumping" for things that are likely to delta well against
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]917 each other.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]918</div></div>
919<div class="paragraph"><p>It is an important point, so secretly, I did my own research and have
920included my results below. To be fair, it has changed some over time.
921And through the magic of Revisionistic History, I draw upon this entry
922from The Git IRC Logs on my father&#8217;s birthday, March 1:</p></div>
923<div class="literalblock">
924<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]925<pre><code>&lt;gitster&gt; The quote from the above linus should be rewritten a
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]926 bit (wait for it):
927 - first sort by type. Different objects never delta with
928 each other.
929 - then sort by filename/dirname. hash of the basename
930 occupies the top BITS_PER_INT-DIR_BITS bits, and bottom
931 DIR_BITS are for the hash of leading path elements.
932 - then if we are doing "thin" pack, the objects we are _not_
933 going to pack but we know about are sorted earlier than
934 other objects.
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]935 - and finally sort by size, larger to smaller.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]936</div></div>
937<div class="paragraph"><p>In one swell-foop, clarification and obscurification! Nonetheless,
938authoritative. Cryptic, yet concise. It even solicits notions of
939quotes from The Source Code. Clearly, more study is needed.</p></div>
940<div class="literalblock">
941<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]942<pre><code>&lt;gitster&gt; That's the sort order. What this means is:
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]943 - we do not delta different object types.
944 - we prefer to delta the objects with the same full path, but
945 allow files with the same name from different directories.
946 - we always prefer to delta against objects we are not going
947 to send, if there are some.
948 - we prefer to delta against larger objects, so that we have
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]949 lots of removals.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]950</div></div>
951<div class="literalblock">
952<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]953<pre><code>The penultimate rule is for "thin" packs. It is used when
954the other side is known to have such objects.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]955</div></div>
956<div class="paragraph"><p>There it is again. "Thin" packs. I&#8217;m thinking to myself, "What
957is a <em>thin</em> pack?" So I ask:</p></div>
958<div class="literalblock">
959<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]960<pre><code>&lt;jdl&gt; What is a "thin" pack?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]961</div></div>
962<div class="literalblock">
963<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]964<pre><code>&lt;gitster&gt; Use of --objects-edge to rev-list as the upstream of
965 pack-objects. The pack transfer protocol negotiates that.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]966</div></div>
967<div class="paragraph"><p>Woo hoo! Cleared that <em>right</em> up!</p></div>
968<div class="literalblock">
969<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]970<pre><code>&lt;gitster&gt; There are two directions - push and fetch.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]971</div></div>
972<div class="paragraph"><p>There! Did you see it? It is not <em>"push" and "pull"</em>! How often the
973confusion has started here. So casually mentioned, too!</p></div>
974<div class="literalblock">
975<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]976<pre><code>&lt;gitster&gt; For push, git-send-pack invokes git-receive-pack on the
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]977 other end. The receive-pack says "I have up to these commits".
978 send-pack looks at them, and computes what are missing from
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]979 the other end. So "thin" could be the default there.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]980</div></div>
981<div class="literalblock">
982<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]983<pre><code>In the other direction, fetch, git-fetch-pack and
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]984git-clone-pack invokes git-upload-pack on the other end
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]985(via ssh or by talking to the daemon).</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]986</div></div>
987<div class="literalblock">
988<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]989<pre><code>There are two cases: fetch-pack with -k and clone-pack is one,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]990fetch-pack without -k is the other. clone-pack and fetch-pack
991with -k will keep the downloaded packfile without expanded, so
992we do not use thin pack transfer. Otherwise, the generated
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]993pack will have delta without base object in the same pack.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]994</div></div>
995<div class="literalblock">
996<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]997<pre><code>But fetch-pack without -k will explode the received pack into
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]998individual objects, so we automatically ask upload-pack to
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]999give us a thin pack if upload-pack supports it.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1000</div></div>
1001<div class="paragraph"><p>OK then.</p></div>
1002<div class="paragraph"><p>Uh.</p></div>
1003<div class="paragraph"><p>Let&#8217;s return to the previous conversation still in progress.</p></div>
1004<div class="literalblock">
1005<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1006<pre><code>&lt;njs`&gt; and "basename" means something like "the tail of end of
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1007 path of file objects and dir objects, as per basename(3), and
1008 we just declare all commit and tag objects to have the same
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1009 basename" or something?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1010</div></div>
1011<div class="paragraph"><p>Luckily, that too is a point that gitster clarified for us!</p></div>
1012<div class="paragraph"><p>If I might add, the trick is to make files that <em>might</em> be similar be
1013located close to each other in the hash buckets based on their file
1014names. It used to be that "foo/Makefile", "bar/baz/quux/Makefile" and
1015"Makefile" all landed in the same bucket due to their common basename,
1016"Makefile". However, now they land in "close" buckets.</p></div>
1017<div class="paragraph"><p>The algorithm allows not just for the <em>same</em> bucket, but for <em>close</em>
1018buckets to be considered delta candidates. The rationale is
1019essentially that files, like Makefiles, often have very similar
1020content no matter what directory they live in.</p></div>
1021<div class="literalblock">
1022<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1023<pre><code>&lt;linus&gt; I played around with different delta algorithms, and with
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1024 making the "delta window" bigger, but having too big of a
1025 sliding window makes it very expensive to generate the pack:
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1026 you need to compare every object with a _ton_ of other objects.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1027</div></div>
1028<div class="literalblock">
1029<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1030<pre><code>There are a number of other trivial heuristics too, which
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1031basically boil down to "don't bother even trying to delta this
1032pair" if we can tell before-hand that the delta isn't worth it
1033(due to size differences, where we can take a previous delta
1034result into account to decide that "ok, no point in trying
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1035that one, it will be worse").</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1036</div></div>
1037<div class="literalblock">
1038<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1039<pre><code>End result: packing is actually very size efficient. It's
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1040somewhat CPU-wasteful, but on the other hand, since you're
1041really only supposed to do it maybe once a month (and you can
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1042do it during the night), nobody really seems to care.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1043</div></div>
1044<div class="paragraph"><p>Nice Engineering Touch, there. Find when it doesn&#8217;t matter, and
1045proclaim it a non-issue. Good style too!</p></div>
1046<div class="literalblock">
1047<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1048<pre><code>&lt;njs`&gt; So, just to repeat to see if I'm following, we start by
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1049 getting a list of the objects we want to pack, we sort it by
1050 this heuristic (basically lexicographically on the tuple
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1051 (type, basename, size)).</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1052</div></div>
1053<div class="literalblock">
1054<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1055<pre><code>Then we walk through this list, and calculate a delta of
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1056each object against the last n (tunable parameter) objects,
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1057and pick the smallest of these deltas.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1058</div></div>
1059<div class="paragraph"><p>Vastly simplified, but the essence is there!</p></div>
1060<div class="literalblock">
1061<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1062<pre><code>&lt;linus&gt; Correct.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1063</div></div>
1064<div class="literalblock">
1065<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1066<pre><code>&lt;njs`&gt; And then once we have picked a delta or fulltext to
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1067 represent each object, we re-sort by recency, and write them
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1068 out in that order.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1069</div></div>
1070<div class="literalblock">
1071<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1072<pre><code>&lt;linus&gt; Yup. Some other small details:</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1073</div></div>
1074<div class="paragraph"><p>And of course there is the "Other Shoe" Factor too.</p></div>
1075<div class="literalblock">
1076<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1077<pre><code>&lt;linus&gt; - We limit the delta depth to another magic value (right
1078 now both the window and delta depth magic values are just "10")</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1079</div></div>
1080<div class="literalblock">
1081<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1082<pre><code>&lt;njs`&gt; Hrm, my intuition is that you'd end up with really _bad_ IO
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1083 patterns, because the things you want are near by, but to
1084 actually reconstruct them you may have to jump all over in
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1085 random ways.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1086</div></div>
1087<div class="literalblock">
1088<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1089<pre><code>&lt;linus&gt; - When we write out a delta, and we haven't yet written
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1090 out the object it is a delta against, we write out the base
1091 object first. And no, when we reconstruct them, we actually
1092 get nice IO patterns, because:
1093 - larger objects tend to be "more recent" (Linus' law: files grow)
1094 - we actively try to generate deltas from a larger object to a
1095 smaller one
1096 - this means that the top-of-tree very seldom has deltas
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1097 (i.e. deltas in _practice_ are "backwards deltas")</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1098</div></div>
1099<div class="paragraph"><p>Again, we should reread that whole paragraph. Not just because
1100Linus has slipped Linus&#8217;s Law in there on us, but because it is
1101important. Let&#8217;s make sure we clarify some of the points here:</p></div>
1102<div class="literalblock">
1103<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1104<pre><code>&lt;njs`&gt; So the point is just that in practice, delta order and
1105 recency order match each other quite well.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1106</div></div>
1107<div class="literalblock">
1108<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1109<pre><code>&lt;linus&gt; Yes. There's another nice side to this (and yes, it was
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1110 designed that way ;):
1111 - the reason we generate deltas against the larger object is
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1112 actually a big space saver too!</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1113</div></div>
1114<div class="literalblock">
1115<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1116<pre><code>&lt;njs`&gt; Hmm, but your last comment (if "we haven't yet written out
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1117 the object it is a delta against, we write out the base object
1118 first"), seems like it would make these facts mostly
1119 irrelevant because even if in practice you would not have to
1120 wander around much, in fact you just brute-force say that in
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1121 the cases where you might have to wander, don't do that :-)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1122</div></div>
1123<div class="literalblock">
1124<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1125<pre><code>&lt;linus&gt; Yes and no. Notice the rule: we only write out the base
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1126 object first if the delta against it was more recent. That
1127 means that you can actually have deltas that refer to a base
1128 object that is _not_ close to the delta object, but that only
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1129 happens when the delta is needed to generate an _old_ object.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1130</div></div>
1131<div class="literalblock">
1132<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1133<pre><code>&lt;linus&gt; See?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1134</div></div>
1135<div class="paragraph"><p>Yeah, no. I missed that on the first two or three readings myself.</p></div>
1136<div class="literalblock">
1137<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1138<pre><code>&lt;linus&gt; This keeps the front of the pack dense. The front of the
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1139 pack never contains data that isn't relevant to a "recent"
1140 object. The size optimization comes from our use of xdelta
1141 (but is true for many other delta algorithms): removing data
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1142 is cheaper (in size) than adding data.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1143</div></div>
1144<div class="literalblock">
1145<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1146<pre><code>When you remove data, you only need to say "copy bytes n--m".
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1147In contrast, in a delta that _adds_ data, you have to say "add
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1148these bytes: 'actual data goes here'"</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1149</div></div>
1150<div class="ulist"><ul>
1151<li>
1152<p>
1153njs` has quit: Read error: 104 (Connection reset by peer)
1154</p>
1155<div class="literalblock">
1156<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1157<pre><code>&lt;linus&gt; Uhhuh. I hope I didn't blow njs` mind.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1158</div></div>
1159</li>
1160<li>
1161<p>
1162njs` has joined channel #git
1163</p>
1164<div class="literalblock">
1165<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1166<pre><code>&lt;pasky&gt; :)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1167</div></div>
1168</li>
1169</ul></div>
1170<div class="paragraph"><p>The silent observers are amused. Of course.</p></div>
1171<div class="paragraph"><p>And as if njs` was expected to be omniscient:</p></div>
1172<div class="literalblock">
1173<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1174<pre><code>&lt;linus&gt; njs - did you miss anything?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1175</div></div>
1176<div class="paragraph"><p>OK, I&#8217;ll spell it out. That&#8217;s Geek Humor. If njs` was not actually
1177connected for a little bit there, how would he know if missed anything
1178while he was disconnected? He&#8217;s a benevolent dictator with a sense of
1179humor! Well noted!</p></div>
1180<div class="literalblock">
1181<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1182<pre><code>&lt;njs`&gt; Stupid router. Or gremlins, or whatever.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1183</div></div>
1184<div class="paragraph"><p>It&#8217;s a cheap shot at Cisco. Take 'em when you can.</p></div>
1185<div class="literalblock">
1186<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1187<pre><code>&lt;njs`&gt; Yes and no. Notice the rule: we only write out the base
1188 object first if the delta against it was more recent.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1189</div></div>
1190<div class="literalblock">
1191<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1192<pre><code>I'm getting lost in all these orders, let me re-read :-)
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1193So the write-out order is from most recent to least recent?
1194(Conceivably it could be the opposite way too, I'm not sure if
1195we've said) though my connection back at home is logging, so I
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1196can just read what you said there :-)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1197</div></div>
1198<div class="paragraph"><p>And for those of you paying attention, the Omniscient Trick has just
1199been detailed!</p></div>
1200<div class="literalblock">
1201<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1202<pre><code>&lt;linus&gt; Yes, we always write out most recent first</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1203</div></div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1204<div class="literalblock">
1205<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1206<pre><code>&lt;njs`&gt; And, yeah, I got the part about deeper-in-history stuff
1207 having worse IO characteristics, one sort of doesn't care.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1208</div></div>
1209<div class="literalblock">
1210<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1211<pre><code>&lt;linus&gt; With the caveat that if the "most recent" needs an older
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1212 object to delta against (hey, shrinking sometimes does
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1213 happen), we write out the old object with the delta.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1214</div></div>
1215<div class="literalblock">
1216<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1217<pre><code>&lt;njs`&gt; (if only it happened more...)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1218</div></div>
1219<div class="literalblock">
1220<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1221<pre><code>&lt;linus&gt; Anyway, the pack-file could easily be denser still, but
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]1222 because it's used both for streaming (the Git protocol) and
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1223 for on-disk, it has a few pessimizations.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1224</div></div>
1225<div class="paragraph"><p>Actually, it is a made-up word. But it is a made-up word being
1226used as setup for a later optimization, which is a real word:</p></div>
1227<div class="literalblock">
1228<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1229<pre><code>&lt;linus&gt; In particular, while the pack-file is then compressed,
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1230 it's compressed just one object at a time, so the actual
1231 compression factor is less than it could be in theory. But it
1232 means that it's all nice random-access with a simple index to
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1233 do "object name-&gt;location in packfile" translation.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1234</div></div>
1235<div class="literalblock">
1236<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1237<pre><code>&lt;njs`&gt; I'm assuming the real win for delta-ing large-&gt;small is
1238 more homogeneous statistics for gzip to run over?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1239</div></div>
1240<div class="literalblock">
1241<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1242<pre><code>(You have to put the bytes in one place or another, but
1243putting them in a larger blob wins on compression)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1244</div></div>
1245<div class="literalblock">
1246<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1247<pre><code>Actually, what is the compression strategy -- each delta
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1248individually gzipped, the whole file gzipped, somewhere in
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1249between, no compression at all, ....?</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1250</div></div>
1251<div class="literalblock">
1252<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1253<pre><code>Right.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1254</div></div>
1255<div class="paragraph"><p>Reality IRC sets in. For example:</p></div>
1256<div class="literalblock">
1257<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1258<pre><code>&lt;pasky&gt; I'll read the rest in the morning, I really have to go
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1259 sleep or there's no hope whatsoever for me at the today's
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1260 exam... g'nite all.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1261</div></div>
1262<div class="paragraph"><p>Heh.</p></div>
1263<div class="literalblock">
1264<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1265<pre><code>&lt;linus&gt; pasky: g'nite</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1266</div></div>
1267<div class="literalblock">
1268<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1269<pre><code>&lt;njs`&gt; pasky: 'luck</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1270</div></div>
1271<div class="literalblock">
1272<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1273<pre><code>&lt;linus&gt; Right: large-&gt;small matters exactly because of compression
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1274 behaviour. If it was non-compressed, it probably wouldn't make
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1275 any difference.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1276</div></div>
1277<div class="literalblock">
1278<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1279<pre><code>&lt;njs`&gt; yeah</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1280</div></div>
1281<div class="literalblock">
1282<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1283<pre><code>&lt;linus&gt; Anyway: I'm not even trying to claim that the pack-files
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1284 are perfect, but they do tend to have a nice balance of
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1285 density vs ease-of use.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1286</div></div>
1287<div class="paragraph"><p>Gasp! OK, saved. That&#8217;s a fair Engineering trade off. Close call!
1288In fact, Linus reflects on some Basic Engineering Fundamentals,
1289design options, etc.</p></div>
1290<div class="literalblock">
1291<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1292<pre><code>&lt;linus&gt; More importantly, they allow Git to still _conceptually_
1293 never deal with deltas at all, and be a "whole object" store.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1294</div></div>
1295<div class="literalblock">
1296<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1297<pre><code>Which has some problems (we discussed bad huge-file
Junio C Hamano076ffcc2013-02-06 05:13:21[diff] [blame]1298behaviour on the Git lists the other day), but it does mean
1299that the basic Git concepts are really really simple and
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1300straightforward.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1301</div></div>
1302<div class="literalblock">
1303<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1304<pre><code>It's all been quite stable.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1305</div></div>
1306<div class="literalblock">
1307<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1308<pre><code>Which I think is very much a result of having very simple
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1309basic ideas, so that there's never any confusion about what's
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1310going on.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1311</div></div>
1312<div class="literalblock">
1313<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1314<pre><code>Bugs happen, but they are "simple" bugs. And bugs that
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1315actually get some object store detail wrong are almost always
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1316so obvious that they never go anywhere.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1317</div></div>
1318<div class="literalblock">
1319<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1320<pre><code>&lt;njs`&gt; Yeah.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1321</div></div>
1322<div class="paragraph"><p>Nuff said.</p></div>
1323<div class="literalblock">
1324<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1325<pre><code>&lt;linus&gt; Anyway. I'm off for bed. It's not 6AM here, but I've got
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1326 three kids, and have to get up early in the morning to send
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1327 them off. I need my beauty sleep.</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1328</div></div>
1329<div class="literalblock">
1330<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1331<pre><code>&lt;njs`&gt; :-)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1332</div></div>
1333<div class="literalblock">
1334<div class="content">
Junio C Hamanoc14e6ad2014-10-31 20:25:53[diff] [blame]1335<pre><code>&lt;njs`&gt; appreciate the infodump, I really was failing to find the
1336 details on Git packs :-)</code></pre>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1337</div></div>
1338<div class="paragraph"><p>And now you know the rest of the story.</p></div>
1339</div>
Junio C Hamanobc8d4782014-01-13 23:35:50[diff] [blame]1340</div>
1341</div>
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1342<div id="footnotes"><hr /></div>
1343<div id="footer">
1344<div id="footer-text">
Junio C Hamano2ef0ba32018-01-26 23:13:53[diff] [blame]1345Last updated
Junio C Hamano69bb2b52018-11-18 12:44:26[diff] [blame]1346 2018-01-27 08:11:04 JST
Junio C Hamanof2b74942012-11-20 21:06:26[diff] [blame]1347</div>
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