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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
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<title>Apache Performance Tuning - Apache HTTP Server Version 2.5</title>
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<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="../style/css/prettify.css" />
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<script src="../style/scripts/prettify.min.js" type="text/javascript">
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</script>
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<link href="../images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.5</p>
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<img alt="" src="../images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Miscellaneous Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache Performance Tuning</h1>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="../en/misc/perf-tuning.html" title="English"> en </a> |
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<a href="../fr/misc/perf-tuning.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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<a href="../ko/misc/perf-tuning.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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<a href="../tr/misc/perf-tuning.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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</div>
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<div class="warning"><h3>Warning</h3>
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<p>This document is partially out of date and might be inaccurate.</p>
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</div>
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<p>Apache 2.4 is a general-purpose webserver, designed to
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provide a balance of flexibility, portability, and performance.
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Although it has not been designed specifically to set benchmark
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records, Apache 2.4 is capable of high performance in many
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real-world situations.</p>
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<p>This document describes the options that a server administrator
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can configure to tune the performance of an Apache 2.4 installation.
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Some of these configuration options enable the httpd to better take
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advantage of the capabilities of the hardware and OS, while others allow
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the administrator to trade functionality for speed.</p>
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</div>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#hardware">Hardware and Operating System Issues</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#runtime">Run-Time Configuration Issues</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#compiletime">Compile-Time Configuration Issues</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#trace">Appendix: Detailed Analysis of a Trace</a></li>
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</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="hardware" id="hardware">Hardware and Operating System Issues</a> <a title="Permanent link" href="#hardware" class="permalink">¶</a></h2>
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<p>The single biggest hardware issue affecting webserver
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performance is RAM. A webserver should never ever have to swap,
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as swapping increases the latency of each request beyond a point
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that users consider "fast enough". This causes users to hit
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stop and reload, further increasing the load. You can, and
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should, control the <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> setting so that your server
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does not spawn so many children that it starts swapping. The procedure
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for doing this is simple: determine the size of your average Apache
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process, by looking at your process list via a tool such as
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<code>top</code>, and divide this into your total available memory,
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leaving some room for other processes.</p>
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<p>Beyond that the rest is mundane: get a fast enough CPU, a
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fast enough network card, and fast enough disks, where "fast
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enough" is something that needs to be determined by
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experimentation.</p>
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<p>Operating system choice is largely a matter of local
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concerns. But some guidelines that have proven generally
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useful are:</p>
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<ul>
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<li>
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<p>Run the latest stable release and patch level of the
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operating system that you choose. Many OS suppliers have
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introduced significant performance improvements to their
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TCP stacks and thread libraries in recent years.</p>
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</li>
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<li>
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<p>If your OS supports a <code>sendfile(2)</code> system
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call, make sure you install the release and/or patches
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needed to enable it. (With Linux, for example, this means
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using Linux 2.4 or later. For early releases of Solaris 8,
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you may need to apply a patch.) On systems where it is
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available, <code>sendfile</code> enables Apache to deliver
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static content faster and with lower CPU utilization.</p>
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</li>
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</ul>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="runtime" id="runtime">Run-Time Configuration Issues</a> <a title="Permanent link" href="#runtime" class="permalink">¶</a></h2>
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<table class="related"><tr><th>Related Modules</th><th>Related Directives</th></tr><tr><td><ul><li><code class="module"><a href="../mod/mod_dir.html">mod_dir</a></code></li><li><code class="module"><a href="../mod/mpm_common.html">mpm_common</a></code></li><li><code class="module"><a href="../mod/mod_status.html">mod_status</a></code></li></ul></td><td><ul><li><code class="directive"><a href="../mod/core.html#allowoverride">AllowOverride</a></code></li><li><code class="directive"><a href="../mod/mod_dir.html#directoryindex">DirectoryIndex</a></code></li><li><code class="directive"><a href="../mod/core.html#hostnamelookups">HostnameLookups</a></code></li><li><code class="directive"><a href="../mod/core.html#enablemmap">EnableMMAP</a></code></li><li><code class="directive"><a href="../mod/core.html#enablesendfile">EnableSendfile</a></code></li><li><code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code></li><li><code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code></li><li><code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code></li><li><code class="directive"><a href="../mod/core.html#options">Options</a></code></li><li><code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code></li></ul></td></tr></table>
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<h3><a name="dns" id="dns">HostnameLookups and other DNS considerations</a></h3>
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<p>Prior to Apache 1.3, <code class="directive"><a href="../mod/core.html#hostnamelookups">HostnameLookups</a></code> defaulted to <code>On</code>.
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causing an extra latency penalty for every request due to a
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DNS lookup to complete before the request was finished.
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In Apache 2.4 this setting defaults to <code>Off</code>. If you need
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to have addresses in your log files resolved to hostnames, please
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consider post-processing rather than forcing Apache to do it in the first
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place. It is recommended that you do this sort of post-processing of
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your log files on some machine other than the production web
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server machine, in order that this activity not adversely affect
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server performance.</p>
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<p>If you use any <code><code class="directive"><a href="../mod/mod_access_compat.html#allow">Allow</a></code> from domain</code> or <code><code class="directive"><a href="../mod/mod_access_compat.html#deny">Deny</a></code> from domain</code>
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directives (i.e., using a hostname, or a domain name, rather than
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an IP address) then you will pay for
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two DNS lookups (a reverse, followed by a forward lookup
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to make sure that the reverse is not being spoofed). For best
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performance, whenever it is possible, use IP addresses rather
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than domain names.</p>
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<div class="warning"><h3>Warning:</h3>
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<p>Please use the <code class="directive"><a href="../mod/mod_authz_core.html#require">Require</a></code> directive with Apache 2.4;
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more info in the related <a href="../upgrading.html">upgrading guide</a>.</p>
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</div>
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<p>Note that it's possible to scope the directives, such as
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within a <code><Location "/server-status"></code> section.
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In this case the DNS lookups are only performed on requests
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matching the criteria. Here's an example which disables lookups
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except for <code>.html</code> and <code>.cgi</code> files:</p>
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<pre class="prettyprint lang-config"><Files ~ "\.(html|cgi)$">
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HostnameLookups on
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</Files></pre>
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<p>But even still, if you just need DNS names in some CGIs you
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could consider doing the <code>gethostbyname</code> call in the
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specific CGIs that need it.</p>
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<h3><a name="symlinks" id="symlinks">FollowSymLinks and SymLinksIfOwnerMatch</a></h3>
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<p>Wherever in your URL-space you do not have an <code>Options
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FollowSymLinks</code>, or you do have an <code>Options
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SymLinksIfOwnerMatch</code>, Apache will need to issue extra
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system calls to check up on symlinks. (One extra call per
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filename component.) For example, if you had:</p>
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<pre class="prettyprint lang-config">DocumentRoot "/www/htdocs"
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<Directory "/">
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Options SymLinksIfOwnerMatch
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</Directory></pre>
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|
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<p>and a request is made for the URI <code>/index.html</code>,
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then Apache will perform <code>lstat(2)</code> on
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<code>/www</code>, <code>/www/htdocs</code>, and
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<code>/www/htdocs/index.html</code>. The results of these
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<code>lstats</code> are never cached, so they will occur on
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every single request. If you really desire the symlinks
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security checking, you can do something like this:</p>
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<pre class="prettyprint lang-config">DocumentRoot "/www/htdocs"
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<Directory "/">
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Options FollowSymLinks
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</Directory>
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<Directory "/www/htdocs">
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Options -FollowSymLinks +SymLinksIfOwnerMatch
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</Directory></pre>
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<p>This at least avoids the extra checks for the
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<code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> path.
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Note that you'll need to add similar sections if you
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have any <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code> or
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<code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> paths
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outside of your document root. For highest performance,
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and no symlink protection, set <code>FollowSymLinks</code>
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everywhere, and never set <code>SymLinksIfOwnerMatch</code>.</p>
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<h3><a name="htaccess" id="htaccess">AllowOverride</a></h3>
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<p>Wherever in your URL-space you allow overrides (typically
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<code>.htaccess</code> files), Apache will attempt to open
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<code>.htaccess</code> for each filename component. For
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example,</p>
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<pre class="prettyprint lang-config">DocumentRoot "/www/htdocs"
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<Directory "/">
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AllowOverride all
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</Directory></pre>
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|
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<p>and a request is made for the URI <code>/index.html</code>.
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Then Apache will attempt to open <code>/.htaccess</code>,
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<code>/www/.htaccess</code>, and
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<code>/www/htdocs/.htaccess</code>. The solutions are similar
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to the previous case of <code>Options FollowSymLinks</code>.
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For highest performance use <code>AllowOverride None</code>
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everywhere in your filesystem.</p>
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<h3><a name="negotiation" id="negotiation">Negotiation</a></h3>
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<p>If at all possible, avoid content negotiation if you're
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really interested in every last ounce of performance. In
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practice the benefits of negotiation outweigh the performance
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penalties. There's one case where you can speed up the server.
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Instead of using a wildcard such as:</p>
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<pre class="prettyprint lang-config">DirectoryIndex index</pre>
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<p>Use a complete list of options:</p>
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<pre class="prettyprint lang-config">DirectoryIndex index.cgi index.pl index.shtml index.html</pre>
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<p>where you list the most common choice first.</p>
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<p>Also note that explicitly creating a <code>type-map</code>
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file provides better performance than using
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<code>MultiViews</code>, as the necessary information can be
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determined by reading this single file, rather than having to
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scan the directory for files.</p>
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<p>If your site needs content negotiation, consider using
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<code>type-map</code> files, rather than the <code>Options
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MultiViews</code> directive to accomplish the negotiation. See the
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<a href="../content-negotiation.html">Content Negotiation</a>
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documentation for a full discussion of the methods of negotiation,
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and instructions for creating <code>type-map</code> files.</p>
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<h3>Memory-mapping</h3>
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|
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<p>In situations where Apache 2.x needs to look at the contents
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of a file being delivered--for example, when doing server-side-include
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processing--it normally memory-maps the file if the OS supports
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some form of <code>mmap(2)</code>.</p>
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<p>On some platforms, this memory-mapping improves performance.
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However, there are cases where memory-mapping can hurt the performance
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or even the stability of the httpd:</p>
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<ul>
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<li>
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<p>On some operating systems, <code>mmap</code> does not scale
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as well as <code>read(2)</code> when the number of CPUs increases.
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On multiprocessor Solaris servers, for example, Apache 2.x sometimes
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delivers server-parsed files faster when <code>mmap</code> is disabled.</p>
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</li>
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<li>
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<p>If you memory-map a file located on an NFS-mounted filesystem
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and a process on another NFS client machine deletes or truncates
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the file, your process may get a bus error the next time it tries
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to access the mapped file content.</p>
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</li>
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</ul>
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<p>For installations where either of these factors applies, you
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should use <code>EnableMMAP off</code> to disable the memory-mapping
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of delivered files. (Note: This directive can be overridden on
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a per-directory basis.)</p>
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<h3>Sendfile</h3>
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|
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<p>In situations where Apache 2.x can ignore the contents of the file
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to be delivered -- for example, when serving static file content --
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it normally uses the kernel sendfile support for the file if the OS
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supports the <code>sendfile(2)</code> operation.</p>
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<p>On most platforms, using sendfile improves performance by eliminating
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separate read and send mechanics. However, there are cases where using
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sendfile can harm the stability of the httpd:</p>
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|
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<ul>
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<li>
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<p>Some platforms may have broken sendfile support that the build
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system did not detect, especially if the binaries were built on
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another box and moved to such a machine with broken sendfile support.</p>
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</li>
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<li>
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<p>With an NFS-mounted filesystem, the kernel may be unable
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to reliably serve the network file through its own cache.</p>
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</li>
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</ul>
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<p>For installations where either of these factors applies, you
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should use <code>EnableSendfile off</code> to disable sendfile
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delivery of file contents. (Note: This directive can be overridden
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on a per-directory basis.)</p>
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<h3><a name="process" id="process">Recycle child processes</a></h3>
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<p><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code>
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limits the numbers of connections that a child process can handle during
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|
its lifetime (by default set to <code>0</code> - unlimited). This affects all
|
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the <a href="../mpm.html#defaults">MPMs</a>, even the ones using threads.
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For example, each process created by the <code class="module"><a href="../mod/worker.html">worker</a></code> MPM spawns
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multiple threads that will handle connections, but this does not influence
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the overall count. It only means that the sum of requests handled by all the
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threads spawned by a single process will be counted against the
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<code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code> value.</p>
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<p><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code> should
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not have any limit in the optimal use case, since there should not be any
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reason to force a process kill other than software bugs causing memory leaks
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|
or excessive CPU usage.</p>
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<p>When keep-alives are in use, a process (or a thread spawned by a process)
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|
will be kept busy doing nothing but waiting for more requests on the already open
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connection. The default <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code> of <code>5</code>
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seconds attempts to minimize this effect. The tradeoff here is
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between network bandwidth and server resources.</p>
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|
|
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="compiletime" id="compiletime">Compile-Time Configuration Issues</a> <a title="Permanent link" href="#compiletime" class="permalink">¶</a></h2>
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|
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<h3>Choosing an MPM</h3>
|
|
|
|
|
|
|
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<p>Apache 2.x supports pluggable concurrency models, called
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<a href="../mpm.html">Multi-Processing Modules</a> (MPMs).
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When building Apache, you must choose an MPM to use. There
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|
are platform-specific MPMs for some platforms:
|
|
<code class="module"><a href="../mod/mpm_netware.html">mpm_netware</a></code>,
|
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<code class="module"><a href="../mod/mpmt_os2.html">mpmt_os2</a></code>, and <code class="module"><a href="../mod/mpm_winnt.html">mpm_winnt</a></code>. For
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general Unix-type systems, there are several MPMs from which
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to choose. The choice of MPM can affect the speed and scalability
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|
of the httpd:</p>
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|
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|
<ul>
|
|
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<li>The <code class="module"><a href="../mod/worker.html">worker</a></code> MPM uses multiple child
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processes with many threads each. Each thread handles
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one connection at a time. Worker generally is a good
|
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choice for high-traffic servers because it has a smaller
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memory footprint than the prefork MPM.</li>
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<li>The <code class="module"><a href="../mod/event.html">event</a></code> MPM is threaded like the
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Worker MPM, but is designed to allow more requests to be
|
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served simultaneously by passing off some processing work
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to supporting threads, freeing up the main threads to work
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on new requests.</li>
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<li>The <code class="module"><a href="../mod/prefork.html">prefork</a></code> MPM uses multiple child
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processes with one thread each. Each process handles
|
|
one connection at a time. On many systems, prefork is
|
|
comparable in speed to worker, but it uses more memory.
|
|
Prefork's threadless design has advantages over worker
|
|
in some situations: it can be used with non-thread-safe
|
|
third-party modules, and it is easier to debug on platforms
|
|
with poor thread debugging support.</li>
|
|
|
|
</ul>
|
|
|
|
<p>For more information on these and other MPMs, please
|
|
see the MPM <a href="../mpm.html">documentation</a>.</p>
|
|
|
|
|
|
|
|
<h3><a name="modules" id="modules">Modules</a></h3>
|
|
|
|
|
|
|
|
<p>Since memory usage is such an important consideration in
|
|
performance, you should attempt to eliminate modules that you are
|
|
not actually using. If you have built the modules as <a href="../dso.html">DSOs</a>, eliminating modules is a simple
|
|
matter of commenting out the associated <code class="directive"><a href="../mod/mod_so.html#loadmodule">LoadModule</a></code> directive for that module.
|
|
This allows you to experiment with removing modules and seeing
|
|
if your site still functions in their absence.</p>
|
|
|
|
<p>If, on the other hand, you have modules statically linked
|
|
into your Apache binary, you will need to recompile Apache in
|
|
order to remove unwanted modules.</p>
|
|
|
|
<p>An associated question that arises here is, of course, what
|
|
modules you need, and which ones you don't. The answer here
|
|
will, of course, vary from one web site to another. However, the
|
|
<em>minimal</em> list of modules which you can get by with tends
|
|
to include <code class="module"><a href="../mod/mod_mime.html">mod_mime</a></code>, <code class="module"><a href="../mod/mod_dir.html">mod_dir</a></code>,
|
|
and <code class="module"><a href="../mod/mod_log_config.html">mod_log_config</a></code>. <code>mod_log_config</code> is,
|
|
of course, optional, as you can run a web site without log
|
|
files. This is, however, not recommended.</p>
|
|
|
|
|
|
|
|
<h3>Atomic Operations</h3>
|
|
|
|
|
|
|
|
<p>Some modules, such as <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> and
|
|
recent development builds of the worker MPM, use APR's
|
|
atomic API. This API provides atomic operations that can
|
|
be used for lightweight thread synchronization.</p>
|
|
|
|
<p>By default, APR implements these operations using the
|
|
most efficient mechanism available on each target
|
|
OS/CPU platform. Many modern CPUs, for example, have
|
|
an instruction that does an atomic compare-and-swap (CAS)
|
|
operation in hardware. On some platforms, however, APR
|
|
defaults to a slower, mutex-based implementation of the
|
|
atomic API in order to ensure compatibility with older
|
|
CPU models that lack such instructions. If you are
|
|
building Apache for one of these platforms, and you plan
|
|
to run only on newer CPUs, you can select a faster atomic
|
|
implementation at build time by configuring Apache with
|
|
the <code>--enable-nonportable-atomics</code> option:</p>
|
|
|
|
<div class="example"><p><code>
|
|
./buildconf<br />
|
|
./configure --with-mpm=worker --enable-nonportable-atomics=yes
|
|
</code></p></div>
|
|
|
|
<p>The <code>--enable-nonportable-atomics</code> option is
|
|
relevant for the following platforms:</p>
|
|
|
|
<ul>
|
|
|
|
<li>Solaris on SPARC<br />
|
|
By default, APR uses mutex-based atomics on Solaris/SPARC.
|
|
If you configure with <code>--enable-nonportable-atomics</code>,
|
|
however, APR generates code that uses a SPARC v8plus opcode for
|
|
fast hardware compare-and-swap. If you configure Apache with
|
|
this option, the atomic operations will be more efficient
|
|
(allowing for lower CPU utilization and higher concurrency),
|
|
but the resulting executable will run only on UltraSPARC
|
|
chips.
|
|
</li>
|
|
|
|
<li>Linux on x86<br />
|
|
By default, APR uses mutex-based atomics on Linux. If you
|
|
configure with <code>--enable-nonportable-atomics</code>,
|
|
however, APR generates code that uses a 486 opcode for fast
|
|
hardware compare-and-swap. This will result in more efficient
|
|
atomic operations, but the resulting executable will run only
|
|
on 486 and later chips (and not on 386).
|
|
</li>
|
|
|
|
</ul>
|
|
|
|
|
|
|
|
<h3>mod_status and ExtendedStatus On</h3>
|
|
|
|
|
|
|
|
<p>If you include <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and you also set
|
|
<code>ExtendedStatus On</code> when building and running
|
|
Apache, then on every request Apache will perform two calls to
|
|
<code>gettimeofday(2)</code> (or <code>times(2)</code>
|
|
depending on your operating system), and (pre-1.3) several
|
|
extra calls to <code>time(2)</code>. This is all done so that
|
|
the status report contains timing indications. For highest
|
|
performance, set <code>ExtendedStatus off</code> (which is the
|
|
default).</p>
|
|
|
|
|
|
|
|
<h3>accept Serialization - Multiple Sockets</h3>
|
|
|
|
|
|
|
|
<div class="warning"><h3>Warning:</h3>
|
|
<p>This section has not been fully updated
|
|
to take into account changes made in the 2.x version of the
|
|
Apache HTTP Server. Some of the information may still be
|
|
relevant, but please use it with care.</p>
|
|
</div>
|
|
|
|
<p>This discusses a shortcoming in the Unix socket API. Suppose
|
|
your web server uses multiple <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> statements to listen on either multiple
|
|
ports or multiple addresses. In order to test each socket
|
|
to see if a connection is ready, Apache uses
|
|
<code>select(2)</code>. <code>select(2)</code> indicates that a
|
|
socket has <em>zero</em> or <em>at least one</em> connection
|
|
waiting on it. Apache's model includes multiple children, and
|
|
all the idle ones test for new connections at the same time. A
|
|
naive implementation looks something like this (these examples
|
|
do not match the code, they're contrived for pedagogical
|
|
purposes):</p>
|
|
|
|
<pre class="prettyprint lang-c"> for (;;) {
|
|
for (;;) {
|
|
fd_set accept_fds;
|
|
|
|
FD_ZERO (&accept_fds);
|
|
for (i = first_socket; i <= last_socket; ++i) {
|
|
FD_SET (i, &accept_fds);
|
|
}
|
|
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
|
|
if (rc < 1) continue;
|
|
new_connection = -1;
|
|
for (i = first_socket; i <= last_socket; ++i) {
|
|
if (FD_ISSET (i, &accept_fds)) {
|
|
new_connection = accept (i, NULL, NULL);
|
|
if (new_connection != -1) break;
|
|
}
|
|
}
|
|
if (new_connection != -1) break;
|
|
}
|
|
process_the(new_connection);
|
|
}</pre>
|
|
|
|
|
|
<p>But this naive implementation has a serious starvation problem.
|
|
Recall that multiple children execute this loop at the same
|
|
time, and so multiple children will block at
|
|
<code>select</code> when they are in between requests. All
|
|
those blocked children will awaken and return from
|
|
<code>select</code> when a single request appears on any socket.
|
|
(The number of children which awaken varies depending on the
|
|
operating system and timing issues.) They will all then fall
|
|
down into the loop and try to <code>accept</code> the
|
|
connection. But only one will succeed (assuming there's still
|
|
only one connection ready). The rest will be <em>blocked</em>
|
|
in <code>accept</code>. This effectively locks those children
|
|
into serving requests from that one socket and no other
|
|
sockets, and they'll be stuck there until enough new requests
|
|
appear on that socket to wake them all up. This starvation
|
|
problem was first documented in <a href="http://bugs.apache.org/index/full/467">PR#467</a>. There
|
|
are at least two solutions.</p>
|
|
|
|
<p>One solution is to make the sockets non-blocking. In this
|
|
case the <code>accept</code> won't block the children, and they
|
|
will be allowed to continue immediately. But this wastes CPU
|
|
time. Suppose you have ten idle children in
|
|
<code>select</code>, and one connection arrives. Then nine of
|
|
those children will wake up, try to <code>accept</code> the
|
|
connection, fail, and loop back into <code>select</code>,
|
|
accomplishing nothing. Meanwhile none of those children are
|
|
servicing requests that occurred on other sockets until they
|
|
get back up to the <code>select</code> again. Overall this
|
|
solution does not seem very fruitful unless you have as many
|
|
idle CPUs (in a multiprocessor box) as you have idle children
|
|
(not a very likely situation).</p>
|
|
|
|
<p>Another solution, the one used by Apache, is to serialize
|
|
entry into the inner loop. The loop looks like this
|
|
(differences highlighted):</p>
|
|
|
|
<pre class="prettyprint lang-c"> for (;;) {
|
|
<strong>accept_mutex_on ();</strong>
|
|
for (;;) {
|
|
fd_set accept_fds;
|
|
|
|
FD_ZERO (&accept_fds);
|
|
for (i = first_socket; i <= last_socket; ++i) {
|
|
FD_SET (i, &accept_fds);
|
|
}
|
|
rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);
|
|
if (rc < 1) continue;
|
|
new_connection = -1;
|
|
for (i = first_socket; i <= last_socket; ++i) {
|
|
if (FD_ISSET (i, &accept_fds)) {
|
|
new_connection = accept (i, NULL, NULL);
|
|
if (new_connection != -1) break;
|
|
}
|
|
}
|
|
if (new_connection != -1) break;
|
|
}
|
|
<strong>accept_mutex_off ();</strong>
|
|
process the new_connection;
|
|
}</pre>
|
|
|
|
|
|
<p><a id="serialize" name="serialize">The functions</a>
|
|
<code>accept_mutex_on</code> and <code>accept_mutex_off</code>
|
|
implement a mutual exclusion semaphore. Only one child can have
|
|
the mutex at any time. There are several choices for
|
|
implementing these mutexes. The choice is defined in
|
|
<code>src/conf.h</code> (pre-1.3) or
|
|
<code>src/include/ap_config.h</code> (1.3 or later). Some
|
|
architectures do not have any locking choice made, on these
|
|
architectures it is unsafe to use multiple
|
|
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>
|
|
directives.</p>
|
|
|
|
<p>The <code class="directive"><a href="../mod/core.html#mutex">Mutex</a></code> directive can
|
|
be used to change the mutex implementation of the
|
|
<code>mpm-accept</code> mutex at run-time. Special considerations
|
|
for different mutex implementations are documented with that
|
|
directive.</p>
|
|
|
|
<p>Another solution that has been considered but never
|
|
implemented is to partially serialize the loop -- that is, let
|
|
in a certain number of processes. This would only be of
|
|
interest on multiprocessor boxes where it's possible that multiple
|
|
children could run simultaneously, and the serialization
|
|
actually doesn't take advantage of the full bandwidth. This is
|
|
a possible area of future investigation, but priority remains
|
|
low because highly parallel web servers are not the norm.</p>
|
|
|
|
<p>Ideally you should run servers without multiple
|
|
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>
|
|
statements if you want the highest performance.
|
|
But read on.</p>
|
|
|
|
|
|
|
|
<h3>accept Serialization - Single Socket</h3>
|
|
|
|
|
|
|
|
<p>The above is fine and dandy for multiple socket servers, but
|
|
what about single socket servers? In theory they shouldn't
|
|
experience any of these same problems because all children can
|
|
just block in <code>accept(2)</code> until a connection
|
|
arrives, and no starvation results. In practice this hides
|
|
almost the same "spinning" behavior discussed above in the
|
|
non-blocking solution. The way that most TCP stacks are
|
|
implemented, the kernel actually wakes up all processes blocked
|
|
in <code>accept</code> when a single connection arrives. One of
|
|
those processes gets the connection and returns to user-space.
|
|
The rest spin in the kernel and go back to sleep when they
|
|
discover there's no connection for them. This spinning is
|
|
hidden from the user-land code, but it's there nonetheless.
|
|
This can result in the same load-spiking wasteful behavior
|
|
that a non-blocking solution to the multiple sockets case
|
|
can.</p>
|
|
|
|
<p>For this reason we have found that many architectures behave
|
|
more "nicely" if we serialize even the single socket case. So
|
|
this is actually the default in almost all cases. Crude
|
|
experiments under Linux (2.0.30 on a dual Pentium pro 166
|
|
w/128Mb RAM) have shown that the serialization of the single
|
|
socket case causes less than a 3% decrease in requests per
|
|
second over unserialized single-socket. But unserialized
|
|
single-socket showed an extra 100ms latency on each request.
|
|
This latency is probably a wash on long haul lines, and only an
|
|
issue on LANs. If you want to override the single socket
|
|
serialization, you can define
|
|
<code>SINGLE_LISTEN_UNSERIALIZED_ACCEPT</code>, and then
|
|
single-socket servers will not serialize at all.</p>
|
|
|
|
|
|
|
|
<h3>Lingering Close</h3>
|
|
|
|
|
|
|
|
<p>As discussed in <a href="http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-connection-00.txt">
|
|
draft-ietf-http-connection-00.txt</a> section 8, in order for
|
|
an HTTP server to <strong>reliably</strong> implement the
|
|
protocol, it needs to shut down each direction of the
|
|
communication independently. (Recall that a TCP connection is
|
|
bi-directional. Each half is independent of the other.)</p>
|
|
|
|
<p>When this feature was added to Apache, it caused a flurry of
|
|
problems on various versions of Unix because of shortsightedness.
|
|
The TCP specification does not state that the <code>FIN_WAIT_2</code>
|
|
state has a timeout, but it doesn't prohibit it.
|
|
On systems without the timeout, Apache 1.2 induces many sockets
|
|
stuck forever in the <code>FIN_WAIT_2</code> state. In many cases this
|
|
can be avoided by simply upgrading to the latest TCP/IP patches
|
|
supplied by the vendor. In cases where the vendor has never
|
|
released patches (<em>i.e.</em>, SunOS4 -- although folks with
|
|
a source license can patch it themselves), we have decided to
|
|
disable this feature.</p>
|
|
|
|
<p>There are two ways to accomplish this. One is the socket
|
|
option <code>SO_LINGER</code>. But as fate would have it, this
|
|
has never been implemented properly in most TCP/IP stacks. Even
|
|
on those stacks with a proper implementation (<em>i.e.</em>,
|
|
Linux 2.0.31), this method proves to be more expensive (cputime)
|
|
than the next solution.</p>
|
|
|
|
<p>For the most part, Apache implements this in a function
|
|
called <code>lingering_close</code> (in
|
|
<code>http_main.c</code>). The function looks roughly like
|
|
this:</p>
|
|
|
|
<pre class="prettyprint lang-c"> void lingering_close (int s)
|
|
{
|
|
char junk_buffer[2048];
|
|
|
|
/* shutdown the sending side */
|
|
shutdown (s, 1);
|
|
|
|
signal (SIGALRM, lingering_death);
|
|
alarm (30);
|
|
|
|
for (;;) {
|
|
select (s for reading, 2 second timeout);
|
|
if (error) break;
|
|
if (s is ready for reading) {
|
|
if (read (s, junk_buffer, sizeof (junk_buffer)) <= 0) {
|
|
break;
|
|
}
|
|
/* just toss away whatever is here */
|
|
}
|
|
}
|
|
|
|
close (s);
|
|
}</pre>
|
|
|
|
|
|
<p>This naturally adds some expense at the end of a connection,
|
|
but it is required for a reliable implementation. As HTTP/1.1
|
|
becomes more prevalent, and all connections are persistent,
|
|
this expense will be amortized over more requests. If you want
|
|
to play with fire and disable this feature, you can define
|
|
<code>NO_LINGCLOSE</code>, but this is not recommended at all.
|
|
In particular, as HTTP/1.1 pipelined persistent connections
|
|
come into use, <code>lingering_close</code> is an absolute
|
|
necessity (and <a href="http://www.w3.org/Protocols/HTTP/Performance/Pipeline.html">
|
|
pipelined connections are faster</a>, so you want to support
|
|
them).</p>
|
|
|
|
|
|
|
|
<h3>Scoreboard File</h3>
|
|
|
|
|
|
|
|
<p>Apache's parent and children communicate with each other
|
|
through something called the scoreboard. Ideally this should be
|
|
implemented in shared memory. For those operating systems that
|
|
we either have access to, or have been given detailed ports
|
|
for, it typically is implemented using shared memory. The rest
|
|
default to using an on-disk file. The on-disk file is not only
|
|
slow, but it is unreliable (and less featured). Peruse the
|
|
<code>src/main/conf.h</code> file for your architecture, and
|
|
look for either <code>USE_MMAP_SCOREBOARD</code> or
|
|
<code>USE_SHMGET_SCOREBOARD</code>. Defining one of those two
|
|
(as well as their companions <code>HAVE_MMAP</code> and
|
|
<code>HAVE_SHMGET</code> respectively) enables the supplied
|
|
shared memory code. If your system has another type of shared
|
|
memory, edit the file <code>src/main/http_main.c</code> and add
|
|
the hooks necessary to use it in Apache. (Send us back a patch
|
|
too, please.)</p>
|
|
|
|
<div class="note">Historical note: The Linux port of Apache didn't start to
|
|
use shared memory until version 1.2 of Apache. This oversight
|
|
resulted in really poor and unreliable behavior of earlier
|
|
versions of Apache on Linux.</div>
|
|
|
|
|
|
|
|
<h3>DYNAMIC_MODULE_LIMIT</h3>
|
|
|
|
|
|
|
|
<p>If you have no intention of using dynamically loaded modules
|
|
(you probably don't if you're reading this and tuning your
|
|
server for every last ounce of performance), then you should add
|
|
<code>-DDYNAMIC_MODULE_LIMIT=0</code> when building your
|
|
server. This will save RAM that's allocated only for supporting
|
|
dynamically loaded modules.</p>
|
|
|
|
|
|
|
|
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
|
|
<div class="section">
|
|
<h2><a name="trace" id="trace">Appendix: Detailed Analysis of a Trace</a> <a title="Permanent link" href="#trace" class="permalink">¶</a></h2>
|
|
|
|
|
|
|
|
<p>Here is a system call trace of Apache 2.0.38 with the worker MPM
|
|
on Solaris 8. This trace was collected using:</p>
|
|
|
|
<div class="example"><p><code>
|
|
truss -l -p <var>httpd_child_pid</var>.
|
|
</code></p></div>
|
|
|
|
<p>The <code>-l</code> option tells truss to log the ID of the
|
|
LWP (lightweight process--Solaris' form of kernel-level thread)
|
|
that invokes each system call.</p>
|
|
|
|
<p>Other systems may have different system call tracing utilities
|
|
such as <code>strace</code>, <code>ktrace</code>, or <code>par</code>.
|
|
They all produce similar output.</p>
|
|
|
|
<p>In this trace, a client has requested a 10KB static file
|
|
from the httpd. Traces of non-static requests or requests
|
|
with content negotiation look wildly different (and quite ugly
|
|
in some cases).</p>
|
|
|
|
<div class="example"><pre>/67: accept(3, 0x00200BEC, 0x00200C0C, 1) (sleeping...)
|
|
/67: accept(3, 0x00200BEC, 0x00200C0C, 1) = 9</pre></div>
|
|
|
|
<p>In this trace, the listener thread is running within LWP #67.</p>
|
|
|
|
<div class="note">Note the lack of <code>accept(2)</code> serialization. On this
|
|
particular platform, the worker MPM uses an unserialized accept by
|
|
default unless it is listening on multiple ports.</div>
|
|
|
|
<div class="example"><pre>/65: lwp_park(0x00000000, 0) = 0
|
|
/67: lwp_unpark(65, 1) = 0</pre></div>
|
|
|
|
<p>Upon accepting the connection, the listener thread wakes up
|
|
a worker thread to do the request processing. In this trace,
|
|
the worker thread that handles the request is mapped to LWP #65.</p>
|
|
|
|
<div class="example"><pre>/65: getsockname(9, 0x00200BA4, 0x00200BC4, 1) = 0</pre></div>
|
|
|
|
<p>In order to implement virtual hosts, Apache needs to know
|
|
the local socket address used to accept the connection. It
|
|
is possible to eliminate this call in many situations (such
|
|
as when there are no virtual hosts, or when
|
|
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> directives
|
|
are used which do not have wildcard addresses). But
|
|
no effort has yet been made to do these optimizations. </p>
|
|
|
|
<div class="example"><pre>/65: brk(0x002170E8) = 0
|
|
/65: brk(0x002190E8) = 0</pre></div>
|
|
|
|
<p>The <code>brk(2)</code> calls allocate memory from the heap.
|
|
It is rare to see these in a system call trace, because the httpd
|
|
uses custom memory allocators (<code>apr_pool</code> and
|
|
<code>apr_bucket_alloc</code>) for most request processing.
|
|
In this trace, the httpd has just been started, so it must
|
|
call <code>malloc(3)</code> to get the blocks of raw memory
|
|
with which to create the custom memory allocators.</p>
|
|
|
|
<div class="example"><pre>/65: fcntl(9, F_GETFL, 0x00000000) = 2
|
|
/65: fstat64(9, 0xFAF7B818) = 0
|
|
/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B910, 2190656) = 0
|
|
/65: fstat64(9, 0xFAF7B818) = 0
|
|
/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B914, 2190656) = 0
|
|
/65: setsockopt(9, 65535, 8192, 0xFAF7B918, 4, 2190656) = 0
|
|
/65: fcntl(9, F_SETFL, 0x00000082) = 0</pre></div>
|
|
|
|
<p>Next, the worker thread puts the connection to the client (file
|
|
descriptor 9) in non-blocking mode. The <code>setsockopt(2)</code>
|
|
and <code>getsockopt(2)</code> calls are a side-effect of how
|
|
Solaris' libc handles <code>fcntl(2)</code> on sockets.</p>
|
|
|
|
<div class="example"><pre>/65: read(9, " G E T / 1 0 k . h t m".., 8000) = 97</pre></div>
|
|
|
|
<p>The worker thread reads the request from the client.</p>
|
|
|
|
<div class="example"><pre>/65: stat("/var/httpd/apache/httpd-8999/htdocs/10k.html", 0xFAF7B978) = 0
|
|
/65: open("/var/httpd/apache/httpd-8999/htdocs/10k.html", O_RDONLY) = 10</pre></div>
|
|
|
|
<p>This httpd has been configured with <code>Options FollowSymLinks</code>
|
|
and <code>AllowOverride None</code>. Thus it doesn't need to
|
|
<code>lstat(2)</code> each directory in the path leading up to the
|
|
requested file, nor check for <code>.htaccess</code> files.
|
|
It simply calls <code>stat(2)</code> to verify that the file:
|
|
1) exists, and 2) is a regular file, not a directory.</p>
|
|
|
|
<div class="example"><pre>/65: sendfilev(0, 9, 0x00200F90, 2, 0xFAF7B53C) = 10269</pre></div>
|
|
|
|
<p>In this example, the httpd is able to send the HTTP response
|
|
header and the requested file with a single <code>sendfilev(2)</code>
|
|
system call. Sendfile semantics vary among operating systems. On some other
|
|
systems, it is necessary to do a <code>write(2)</code> or
|
|
<code>writev(2)</code> call to send the headers before calling
|
|
<code>sendfile(2)</code>.</p>
|
|
|
|
<div class="example"><pre>/65: write(4, " 1 2 7 . 0 . 0 . 1 - ".., 78) = 78</pre></div>
|
|
|
|
<p>This <code>write(2)</code> call records the request in the
|
|
access log. Note that one thing missing from this trace is a
|
|
<code>time(2)</code> call. Unlike Apache 1.3, Apache 2.x uses
|
|
<code>gettimeofday(3)</code> to look up the time. On some operating
|
|
systems, like Linux or Solaris, <code>gettimeofday</code> has an
|
|
optimized implementation that doesn't require as much overhead
|
|
as a typical system call.</p>
|
|
|
|
<div class="example"><pre>/65: shutdown(9, 1, 1) = 0
|
|
/65: poll(0xFAF7B980, 1, 2000) = 1
|
|
/65: read(9, 0xFAF7BC20, 512) = 0
|
|
/65: close(9) = 0</pre></div>
|
|
|
|
<p>The worker thread does a lingering close of the connection.</p>
|
|
|
|
<div class="example"><pre>/65: close(10) = 0
|
|
/65: lwp_park(0x00000000, 0) (sleeping...)</pre></div>
|
|
|
|
<p>Finally the worker thread closes the file that it has just delivered
|
|
and blocks until the listener assigns it another connection.</p>
|
|
|
|
<div class="example"><pre>/67: accept(3, 0x001FEB74, 0x001FEB94, 1) (sleeping...)</pre></div>
|
|
|
|
<p>Meanwhile, the listener thread is able to accept another connection
|
|
as soon as it has dispatched this connection to a worker thread (subject
|
|
to some flow-control logic in the worker MPM that throttles the listener
|
|
if all the available workers are busy). Though it isn't apparent from
|
|
this trace, the next <code>accept(2)</code> can (and usually does, under
|
|
high load conditions) occur in parallel with the worker thread's handling
|
|
of the just-accepted connection.</p>
|
|
|
|
</div></div>
|
|
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