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+<?xml version="1.0" encoding="ISO-8859-1"?>
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+<body id="manual-page"><div id="page-header">
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.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>
+<p class="apache">Apache HTTP Server Version 2.4</p>
+<img alt="" src="../images/feather.png" /></div>
+<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.4</a> &gt; <a href="./">Miscellaneous Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache Performance Tuning</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="../en/misc/perf-tuning.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="../fr/misc/perf-tuning.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="../ko/misc/perf-tuning.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="../tr/misc/perf-tuning.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
+</div>
+
+
+ <p>Apache 2.x is a general-purpose webserver, designed to
+ provide a balance of flexibility, portability, and performance.
+ Although it has not been designed specifically to set benchmark
+ records, Apache 2.x is capable of high performance in many
+ real-world situations.</p>
+
+ <p>Compared to Apache 1.3, release 2.x contains many additional
+ optimizations to increase throughput and scalability. Most of
+ these improvements are enabled by default. However, there are
+ compile-time and run-time configuration choices that can
+ significantly affect performance. This document describes the
+ options that a server administrator can configure to tune the
+ performance of an Apache 2.x installation. Some of these
+ configuration options enable the httpd to better take advantage
+ of the capabilities of the hardware and OS, while others allow
+ the administrator to trade functionality for speed.</p>
+
+ </div>
+<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#hardware">Hardware and Operating System Issues</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#runtime">Run-Time Configuration Issues</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#compiletime">Compile-Time Configuration Issues</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#trace">Appendix: Detailed Analysis of a Trace</a></li>
+</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
+<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="hardware" id="hardware">Hardware and Operating System Issues</a></h2>
+
+
+
+ <p>The single biggest hardware issue affecting webserver
+ performance is RAM. A webserver should never ever have to swap,
+ as swapping increases the latency of each request beyond a point
+ that users consider "fast enough". This causes users to hit
+ stop and reload, further increasing the load. You can, and
+ should, control the <code class="directive"><a href="../mod/mpm_common.html#maxrequestworkers">MaxRequestWorkers</a></code> setting so that your server
+ does not spawn so many children that it starts swapping. The procedure
+ for doing this is simple: determine the size of your average Apache
+ process, by looking at your process list via a tool such as
+ <code>top</code>, and divide this into your total available memory,
+ leaving some room for other processes.</p>
+
+ <p>Beyond that the rest is mundane: get a fast enough CPU, a
+ fast enough network card, and fast enough disks, where "fast
+ enough" is something that needs to be determined by
+ experimentation.</p>
+
+ <p>Operating system choice is largely a matter of local
+ concerns. But some guidelines that have proven generally
+ useful are:</p>
+
+ <ul>
+ <li>
+ <p>Run the latest stable release and patch level of the
+ operating system that you choose. Many OS suppliers have
+ introduced significant performance improvements to their
+ TCP stacks and thread libraries in recent years.</p>
+ </li>
+
+ <li>
+ <p>If your OS supports a <code>sendfile(2)</code> system
+ call, make sure you install the release and/or patches
+ needed to enable it. (With Linux, for example, this means
+ using Linux 2.4 or later. For early releases of Solaris 8,
+ you may need to apply a patch.) On systems where it is
+ available, <code>sendfile</code> enables Apache 2 to deliver
+ static content faster and with lower CPU utilization.</p>
+ </li>
+ </ul>
+
+ </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="runtime" id="runtime">Run-Time Configuration Issues</a></h2>
+
+
+
+ <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>
+
+ <h3><a name="dns" id="dns">HostnameLookups and other DNS considerations</a></h3>
+
+
+
+ <p>Prior to Apache 1.3, <code class="directive"><a href="../mod/core.html#hostnamelookups">HostnameLookups</a></code> defaulted to <code>On</code>.
+ This adds latency to every request because it requires a
+ DNS lookup to complete before the request is finished. In
+ Apache 1.3 this setting defaults to <code>Off</code>. If you need
+ to have addresses in your log files resolved to hostnames, use the
+ <code class="program"><a href="../programs/logresolve.html">logresolve</a></code>
+ program that comes with Apache, or one of the numerous log
+ reporting packages which are available.</p>
+
+ <p>It is recommended that you do this sort of postprocessing of
+ your log files on some machine other than the production web
+ server machine, in order that this activity not adversely affect
+ server performance.</p>
+
+ <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>
+ directives (i.e., using a hostname, or a domain name, rather than
+ an IP address) then you will pay for
+ two DNS lookups (a reverse, followed by a forward lookup
+ to make sure that the reverse is not being spoofed). For best
+ performance, therefore, use IP addresses, rather than names, when
+ using these directives, if possible.</p>
+
+ <p>Note that it's possible to scope the directives, such as
+ within a <code>&lt;Location "/server-status"&gt;</code> section.
+ In this case the DNS lookups are only performed on requests
+ matching the criteria. Here's an example which disables lookups
+ except for <code>.html</code> and <code>.cgi</code> files:</p>
+
+ <pre class="prettyprint lang-config">HostnameLookups off
+&lt;Files ~ "\.(html|cgi)$"&gt;
+ HostnameLookups on
+&lt;/Files&gt;</pre>
+
+
+ <p>But even still, if you just need DNS names in some CGIs you
+ could consider doing the <code>gethostbyname</code> call in the
+ specific CGIs that need it.</p>
+
+
+
+ <h3><a name="symlinks" id="symlinks">FollowSymLinks and SymLinksIfOwnerMatch</a></h3>
+
+
+
+ <p>Wherever in your URL-space you do not have an <code>Options
+ FollowSymLinks</code>, or you do have an <code>Options
+ SymLinksIfOwnerMatch</code>, Apache will need to issue extra
+ system calls to check up on symlinks. (One extra call per
+ filename component.) For example, if you had:</p>
+
+ <pre class="prettyprint lang-config">DocumentRoot "/www/htdocs"
+&lt;Directory "/"&gt;
+ Options SymLinksIfOwnerMatch
+&lt;/Directory&gt;</pre>
+
+
+ <p>and a request is made for the URI <code>/index.html</code>,
+ then Apache will perform <code>lstat(2)</code> on
+ <code>/www</code>, <code>/www/htdocs</code>, and
+ <code>/www/htdocs/index.html</code>. The results of these
+ <code>lstats</code> are never cached, so they will occur on
+ every single request. If you really desire the symlinks
+ security checking, you can do something like this:</p>
+
+ <pre class="prettyprint lang-config">DocumentRoot "/www/htdocs"
+&lt;Directory "/"&gt;
+ Options FollowSymLinks
+&lt;/Directory&gt;
+
+&lt;Directory "/www/htdocs"&gt;
+ Options -FollowSymLinks +SymLinksIfOwnerMatch
+&lt;/Directory&gt;</pre>
+
+
+ <p>This at least avoids the extra checks for the
+ <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code> path.
+ Note that you'll need to add similar sections if you
+ have any <code class="directive"><a href="../mod/mod_alias.html#alias">Alias</a></code> or
+ <code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> paths
+ outside of your document root. For highest performance,
+ and no symlink protection, set <code>FollowSymLinks</code>
+ everywhere, and never set <code>SymLinksIfOwnerMatch</code>.</p>
+
+
+
+ <h3><a name="htaccess" id="htaccess">AllowOverride</a></h3>
+
+
+
+ <p>Wherever in your URL-space you allow overrides (typically
+ <code>.htaccess</code> files), Apache will attempt to open
+ <code>.htaccess</code> for each filename component. For
+ example,</p>
+
+ <pre class="prettyprint lang-config">DocumentRoot "/www/htdocs"
+&lt;Directory "/"&gt;
+ AllowOverride all
+&lt;/Directory&gt;</pre>
+
+
+ <p>and a request is made for the URI <code>/index.html</code>.
+ Then Apache will attempt to open <code>/.htaccess</code>,
+ <code>/www/.htaccess</code>, and
+ <code>/www/htdocs/.htaccess</code>. The solutions are similar
+ to the previous case of <code>Options FollowSymLinks</code>.
+ For highest performance use <code>AllowOverride None</code>
+ everywhere in your filesystem.</p>
+
+
+
+ <h3><a name="negotiation" id="negotiation">Negotiation</a></h3>
+
+
+
+ <p>If at all possible, avoid content negotiation if you're
+ really interested in every last ounce of performance. In
+ practice the benefits of negotiation outweigh the performance
+ penalties. There's one case where you can speed up the server.
+ Instead of using a wildcard such as:</p>
+
+ <pre class="prettyprint lang-config">DirectoryIndex index</pre>
+
+
+ <p>Use a complete list of options:</p>
+
+ <pre class="prettyprint lang-config">DirectoryIndex index.cgi index.pl index.shtml index.html</pre>
+
+
+ <p>where you list the most common choice first.</p>
+
+ <p>Also note that explicitly creating a <code>type-map</code>
+ file provides better performance than using
+ <code>MultiViews</code>, as the necessary information can be
+ determined by reading this single file, rather than having to
+ scan the directory for files.</p>
+
+ <p>If your site needs content negotiation, consider using
+ <code>type-map</code> files, rather than the <code>Options
+ MultiViews</code> directive to accomplish the negotiation. See the
+ <a href="../content-negotiation.html">Content Negotiation</a>
+ documentation for a full discussion of the methods of negotiation,
+ and instructions for creating <code>type-map</code> files.</p>
+
+
+
+ <h3>Memory-mapping</h3>
+
+
+
+ <p>In situations where Apache 2.x needs to look at the contents
+ of a file being delivered--for example, when doing server-side-include
+ processing--it normally memory-maps the file if the OS supports
+ some form of <code>mmap(2)</code>.</p>
+
+ <p>On some platforms, this memory-mapping improves performance.
+ However, there are cases where memory-mapping can hurt the performance
+ or even the stability of the httpd:</p>
+
+ <ul>
+ <li>
+ <p>On some operating systems, <code>mmap</code> does not scale
+ as well as <code>read(2)</code> when the number of CPUs increases.
+ On multiprocessor Solaris servers, for example, Apache 2.x sometimes
+ delivers server-parsed files faster when <code>mmap</code> is disabled.</p>
+ </li>
+
+ <li>
+ <p>If you memory-map a file located on an NFS-mounted filesystem
+ and a process on another NFS client machine deletes or truncates
+ the file, your process may get a bus error the next time it tries
+ to access the mapped file content.</p>
+ </li>
+ </ul>
+
+ <p>For installations where either of these factors applies, you
+ should use <code>EnableMMAP off</code> to disable the memory-mapping
+ of delivered files. (Note: This directive can be overridden on
+ a per-directory basis.)</p>
+
+
+
+ <h3>Sendfile</h3>
+
+
+
+ <p>In situations where Apache 2.x can ignore the contents of the file
+ to be delivered -- for example, when serving static file content --
+ it normally uses the kernel sendfile support for the file if the OS
+ supports the <code>sendfile(2)</code> operation.</p>
+
+ <p>On most platforms, using sendfile improves performance by eliminating
+ separate read and send mechanics. However, there are cases where using
+ sendfile can harm the stability of the httpd:</p>
+
+ <ul>
+ <li>
+ <p>Some platforms may have broken sendfile support that the build
+ system did not detect, especially if the binaries were built on
+ another box and moved to such a machine with broken sendfile support.</p>
+ </li>
+ <li>
+ <p>With an NFS-mounted filesystem, the kernel may be unable
+ to reliably serve the network file through its own cache.</p>
+ </li>
+ </ul>
+
+ <p>For installations where either of these factors applies, you
+ should use <code>EnableSendfile off</code> to disable sendfile
+ delivery of file contents. (Note: This directive can be overridden
+ on a per-directory basis.)</p>
+
+
+
+ <h3><a name="process" id="process">Process Creation</a></h3>
+
+
+
+ <p>Prior to Apache 1.3 the <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>, <code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code>, and <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> settings all had drastic effects on
+ benchmark results. In particular, Apache required a "ramp-up"
+ period in order to reach a number of children sufficient to serve
+ the load being applied. After the initial spawning of
+ <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> children,
+ only one child per second would be created to satisfy the
+ <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>
+ setting. So a server being accessed by 100 simultaneous
+ clients, using the default <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> of <code>5</code> would take on
+ the order of 95 seconds to spawn enough children to handle
+ the load. This works fine in practice on real-life servers
+ because they aren't restarted frequently. But it does really
+ poorly on benchmarks which might only run for ten minutes.</p>
+
+ <p>The one-per-second rule was implemented in an effort to
+ avoid swamping the machine with the startup of new children. If
+ the machine is busy spawning children, it can't service
+ requests. But it has such a drastic effect on the perceived
+ performance of Apache that it had to be replaced. As of Apache
+ 1.3, the code will relax the one-per-second rule. It will spawn
+ one, wait a second, then spawn two, wait a second, then spawn
+ four, and it will continue exponentially until it is spawning
+ 32 children per second. It will stop whenever it satisfies the
+ <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>
+ setting.</p>
+
+ <p>This appears to be responsive enough that it's almost
+ unnecessary to twiddle the <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>, <code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code> and <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> knobs. When more than 4 children are
+ spawned per second, a message will be emitted to the
+ <code class="directive"><a href="../mod/core.html#errorlog">ErrorLog</a></code>. If you
+ see a lot of these errors, then consider tuning these settings.
+ Use the <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> output as a guide.</p>
+
+ <p>Related to process creation is process death induced by the
+ <code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code>
+ setting. By default this is <code>0</code>,
+ which means that there is no limit to the number of connections
+ handled per child. If your configuration currently has this set
+ to some very low number, such as <code>30</code>, you may want to bump this
+ up significantly. If you are running SunOS or an old version of
+ Solaris, limit this to <code>10000</code> or so because of memory leaks.</p>
+
+ <p>When keep-alives are in use, children will be kept busy
+ doing nothing waiting for more requests on the already open
+ connection. The default <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code> of <code>5</code>
+ seconds attempts to minimize this effect. The tradeoff here is
+ between network bandwidth and server resources. In no event
+ should you raise this above about <code>60</code> seconds, as <a href="http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-95-4.html">
+ most of the benefits are lost</a>.</p>
+
+
+
+ </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></h2>
+
+
+
+ <h3>Choosing an MPM</h3>
+
+
+
+ <p>Apache 2.x supports pluggable concurrency models, called
+ <a href="../mpm.html">Multi-Processing Modules</a> (MPMs).
+ When building Apache, you must choose an MPM to use. There
+ are platform-specific MPMs for some platforms:
+ <code class="module"><a href="../mod/mpm_netware.html">mpm_netware</a></code>,
+ <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
+ general Unix-type systems, there are several MPMs from which
+ to choose. The choice of MPM can affect the speed and scalability
+ of the httpd:</p>
+
+ <ul>
+
+ <li>The <code class="module"><a href="../mod/worker.html">worker</a></code> MPM uses multiple child
+ processes with many threads each. Each thread handles
+ one connection at a time. Worker generally is a good
+ choice for high-traffic servers because it has a smaller
+ memory footprint than the prefork MPM.</li>
+
+ <li>The <code class="module"><a href="../mod/event.html">event</a></code> MPM is threaded like the
+ Worker MPM, but is designed to allow more requests to be
+ served simultaneously by passing off some processing work
+ to supporting threads, freeing up the main threads to work
+ on new requests.</li>
+
+ <li>The <code class="module"><a href="../mod/prefork.html">prefork</a></code> MPM uses multiple child
+ 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 (&amp;accept_fds);
+ for (i = first_socket; i &lt;= last_socket; ++i) {
+ FD_SET (i, &amp;accept_fds);
+ }
+ rc = select (last_socket+1, &amp;accept_fds, NULL, NULL, NULL);
+ if (rc &lt; 1) continue;
+ new_connection = -1;
+ for (i = first_socket; i &lt;= last_socket; ++i) {
+ if (FD_ISSET (i, &amp;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 (&amp;accept_fds);
+ for (i = first_socket; i &lt;= last_socket; ++i) {
+ FD_SET (i, &amp;accept_fds);
+ }
+ rc = select (last_socket+1, &amp;accept_fds, NULL, NULL, NULL);
+ if (rc &lt; 1) continue;
+ new_connection = -1;
+ for (i = first_socket; i &lt;= last_socket; ++i) {
+ if (FD_ISSET (i, &amp;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 the 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)) &lt;= 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></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>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="../en/misc/perf-tuning.html" title="English">&nbsp;en&nbsp;</a> |
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