Server : Apache System : Linux server1.cgrithy.com 3.10.0-1160.95.1.el7.x86_64 #1 SMP Mon Jul 24 13:59:37 UTC 2023 x86_64 User : nobody ( 99) PHP Version : 8.1.23 Disable Function : NONE Directory : /usr/share/doc/git-1.8.3.1/technical/ |
<!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 8.6.8" /> <title>Use of index and Racy Git problem</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; } #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; } 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 overriden 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"> /*<![CDATA[*/ var asciidoc = { // Namespace. ///////////////////////////////////////////////////////////////////// // Table Of Contents generator ///////////////////////////////////////////////////////////////////// /* Author: Mihai Bazon, September 2002 * http://students.infoiasi.ro/~mishoo * * Table Of Content generator * Version: 0.4 * * Feel free to use this script under the terms of the GNU General Public * License, as long as you do not remove or alter this notice. */ /* modified by Troy D. Hanson, September 2006. License: GPL */ /* modified by Stuart Rackham, 2006, 2009. License: GPL */ // toclevels = 1..4. toc: function (toclevels) { function getText(el) { var text = ""; for (var i = el.firstChild; i != null; i = i.nextSibling) { if (i.nodeType == 3 /* Node.TEXT_NODE */) // IE doesn't speak constants. text += i.data; else if (i.firstChild != null) text += getText(i); } return text; } function TocEntry(el, text, toclevel) { this.element = el; this.text = text; this.toclevel = toclevel; } function tocEntries(el, toclevels) { var result = new Array; var re = new RegExp('[hH]([1-'+(toclevels+1)+'])'); // 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>Use of index and Racy Git problem</h1> </div> <div id="content"> <div class="sect1"> <h2 id="_background">Background</h2> <div class="sectionbody"> <div class="paragraph"><p>The index is one of the most important data structures in Git. It represents a virtual working tree state by recording list of paths and their object names and serves as a staging area to write out the next tree object to be committed. The state is "virtual" in the sense that it does not necessarily have to, and often does not, match the files in the working tree.</p></div> <div class="paragraph"><p>There are cases Git needs to examine the differences between the virtual working tree state in the index and the files in the working tree. The most obvious case is when the user asks <code>git diff</code> (or its low level implementation, <code>git diff-files</code>) or <code>git-ls-files --modified</code>. In addition, Git internally checks if the files in the working tree are different from what are recorded in the index to avoid stomping on local changes in them during patch application, switching branches, and merging.</p></div> <div class="paragraph"><p>In order to speed up this comparison between the files in the working tree and the index entries, the index entries record the information obtained from the filesystem via <code>lstat(2)</code> system call when they were last updated. When checking if they differ, Git first runs <code>lstat(2)</code> on the files and compares the result with this information (this is what was originally done by the <code>ce_match_stat()</code> function, but the current code does it in <code>ce_match_stat_basic()</code> function). If some of these "cached stat information" fields do not match, Git can tell that the files are modified without even looking at their contents.</p></div> <div class="paragraph"><p>Note: not all members in <code>struct stat</code> obtained via <code>lstat(2)</code> are used for this comparison. For example, <code>st_atime</code> obviously is not useful. Currently, Git compares the file type (regular files vs symbolic links) and executable bits (only for regular files) from <code>st_mode</code> member, <code>st_mtime</code> and <code>st_ctime</code> timestamps, <code>st_uid</code>, <code>st_gid</code>, <code>st_ino</code>, and <code>st_size</code> members. With a <code>USE_STDEV</code> compile-time option, <code>st_dev</code> is also compared, but this is not enabled by default because this member is not stable on network filesystems. With <code>USE_NSEC</code> compile-time option, <code>st_mtim.tv_nsec</code> and <code>st_ctim.tv_nsec</code> members are also compared, but this is not enabled by default because in-core timestamps can have finer granularity than on-disk timestamps, resulting in meaningless changes when an inode is evicted from the inode cache. See commit 8ce13b0 of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git ([PATCH] Sync in core time granuality with filesystems, 2005-01-04).</p></div> </div> </div> <div class="sect1"> <h2 id="_racy_git">Racy Git</h2> <div class="sectionbody"> <div class="paragraph"><p>There is one slight problem with the optimization based on the cached stat information. Consider this sequence:</p></div> <div class="literalblock"> <div class="content"> <pre><code>: modify 'foo' $ git update-index 'foo' : modify 'foo' again, in-place, without changing its size</code></pre> </div></div> <div class="paragraph"><p>The first <code>update-index</code> computes the object name of the contents of file <code>foo</code> and updates the index entry for <code>foo</code> along with the <code>struct stat</code> information. If the modification that follows it happens very fast so that the file’s <code>st_mtime</code> timestamp does not change, after this sequence, the cached stat information the index entry records still exactly match what you would see in the filesystem, even though the file <code>foo</code> is now different. This way, Git can incorrectly think files in the working tree are unmodified even though they actually are. This is called the "racy Git" problem (discovered by Pasky), and the entries that appear clean when they may not be because of this problem are called "racily clean".</p></div> <div class="paragraph"><p>To avoid this problem, Git does two things:</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> When the cached stat information says the file has not been modified, and the <code>st_mtime</code> is the same as (or newer than) the timestamp of the index file itself (which is the time <code>git update-index foo</code> finished running in the above example), it also compares the contents with the object registered in the index entry to make sure they match. </p> </li> <li> <p> When the index file is updated that contains racily clean entries, cached <code>st_size</code> information is truncated to zero before writing a new version of the index file. </p> </li> </ol></div> <div class="paragraph"><p>Because the index file itself is written after collecting all the stat information from updated paths, <code>st_mtime</code> timestamp of it is usually the same as or newer than any of the paths the index contains. And no matter how quick the modification that follows <code>git update-index foo</code> finishes, the resulting <code>st_mtime</code> timestamp on <code>foo</code> cannot get a value earlier than the index file. Therefore, index entries that can be racily clean are limited to the ones that have the same timestamp as the index file itself.</p></div> <div class="paragraph"><p>The callers that want to check if an index entry matches the corresponding file in the working tree continue to call <code>ce_match_stat()</code>, but with this change, <code>ce_match_stat()</code> uses <code>ce_modified_check_fs()</code> to see if racily clean ones are actually clean after comparing the cached stat information using <code>ce_match_stat_basic()</code>.</p></div> <div class="paragraph"><p>The problem the latter solves is this sequence:</p></div> <div class="literalblock"> <div class="content"> <pre><code>$ git update-index 'foo' : modify 'foo' in-place without changing its size : wait for enough time $ git update-index 'bar'</code></pre> </div></div> <div class="paragraph"><p>Without the latter, the timestamp of the index file gets a newer value, and falsely clean entry <code>foo</code> would not be caught by the timestamp comparison check done with the former logic anymore. The latter makes sure that the cached stat information for <code>foo</code> would never match with the file in the working tree, so later checks by <code>ce_match_stat_basic()</code> would report that the index entry does not match the file and Git does not have to fall back on more expensive <code>ce_modified_check_fs()</code>.</p></div> </div> </div> <div class="sect1"> <h2 id="_runtime_penalty">Runtime penalty</h2> <div class="sectionbody"> <div class="paragraph"><p>The runtime penalty of falling back to <code>ce_modified_check_fs()</code> from <code>ce_match_stat()</code> can be very expensive when there are many racily clean entries. An obvious way to artificially create this situation is to give the same timestamp to all the files in the working tree in a large project, run <code>git update-index</code> on them, and give the same timestamp to the index file:</p></div> <div class="literalblock"> <div class="content"> <pre><code>$ date >.datestamp $ git ls-files | xargs touch -r .datestamp $ git ls-files | git update-index --stdin $ touch -r .datestamp .git/index</code></pre> </div></div> <div class="paragraph"><p>This will make all index entries racily clean. The linux-2.6 project, for example, there are over 20,000 files in the working tree. On my Athlon 64 X2 3800+, after the above:</p></div> <div class="literalblock"> <div class="content"> <pre><code>$ /usr/bin/time git diff-files 1.68user 0.54system 0:02.22elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+67111minor)pagefaults 0swaps $ git update-index MAINTAINERS $ /usr/bin/time git diff-files 0.02user 0.12system 0:00.14elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+935minor)pagefaults 0swaps</code></pre> </div></div> <div class="paragraph"><p>Running <code>git update-index</code> in the middle checked the racily clean entries, and left the cached <code>st_mtime</code> for all the paths intact because they were actually clean (so this step took about the same amount of time as the first <code>git diff-files</code>). After that, they are not racily clean anymore but are truly clean, so the second invocation of <code>git diff-files</code> fully took advantage of the cached stat information.</p></div> </div> </div> <div class="sect1"> <h2 id="_avoiding_runtime_penalty">Avoiding runtime penalty</h2> <div class="sectionbody"> <div class="paragraph"><p>In order to avoid the above runtime penalty, post 1.4.2 Git used to have a code that made sure the index file got timestamp newer than the youngest files in the index when there are many young files with the same timestamp as the resulting index file would otherwise would have by waiting before finishing writing the index file out.</p></div> <div class="paragraph"><p>I suspected that in practice the situation where many paths in the index are all racily clean was quite rare. The only code paths that can record recent timestamp for large number of paths are:</p></div> <div class="olist arabic"><ol class="arabic"> <li> <p> Initial <code>git add .</code> of a large project. </p> </li> <li> <p> <code>git checkout</code> of a large project from an empty index into an unpopulated working tree. </p> </li> </ol></div> <div class="paragraph"><p>Note: switching branches with <code>git checkout</code> keeps the cached stat information of existing working tree files that are the same between the current branch and the new branch, which are all older than the resulting index file, and they will not become racily clean. Only the files that are actually checked out can become racily clean.</p></div> <div class="paragraph"><p>In a large project where raciness avoidance cost really matters, however, the initial computation of all object names in the index takes more than one second, and the index file is written out after all that happens. Therefore the timestamp of the index file will be more than one seconds later than the youngest file in the working tree. This means that in these cases there actually will not be any racily clean entry in the resulting index.</p></div> <div class="paragraph"><p>Based on this discussion, the current code does not use the "workaround" to avoid the runtime penalty that does not exist in practice anymore. This was done with commit 0fc82cff on Aug 15, 2006.</p></div> </div> </div> </div> <div id="footnotes"><hr /></div> <div id="footer"> <div id="footer-text"> Last updated 2013-06-10 20:01:55 UTC </div> </div> </body> </html>