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-<title>Apache Performance Tuning - Apache HTTP Server</title>
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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="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
-<p class="apache">Apache HTTP Server Version 2.0</p>
-<img alt="" src="../images/feather.gif" /></div>
-<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
-<div id="path">
-<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 2.0</a> > <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"> en </a> |
-<a href="../ko/misc/perf-tuning.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
-<a href="../tr/misc/perf-tuning.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </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"><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></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#maxclients">MaxClients</a></code> setting so that your server
- does not spawn so many children it starts swapping. This 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 patchlevel 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.html#allow">Allow</a></code>
- from domain</code> or <code><code class="directive"><a href="../mod/mod_access.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><Location /server-status></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>
-
- <div class="example"><p><code>
- HostnameLookups off<br />
- <Files ~ "\.(html|cgi)$"><br />
- <span class="indent">
- HostnameLookups on<br />
- </span>
- </Files>
- </code></p></div>
-
- <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 have to issue extra
- system calls to check up on symlinks. One extra call per
- filename component. For example, if you had:</p>
-
- <div class="example"><p><code>
- DocumentRoot /www/htdocs<br />
- <Directory /><br />
- <span class="indent">
- Options SymLinksIfOwnerMatch<br />
- </span>
- </Directory>
- </code></p></div>
-
- <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>
-
- <div class="example"><p><code>
- DocumentRoot /www/htdocs<br />
- <Directory /><br />
- <span class="indent">
- Options FollowSymLinks<br />
- </span>
- </Directory><br />
- <br />
- <Directory /www/htdocs><br />
- <span class="indent">
- Options -FollowSymLinks +SymLinksIfOwnerMatch<br />
- </span>
- </Directory>
- </code></p></div>
-
- <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="htacess" id="htacess">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>
-
- <div class="example"><p><code>
- DocumentRoot /www/htdocs<br />
- <Directory /><br />
- <span class="indent">
- AllowOverride all<br />
- </span>
- </Directory>
- </code></p></div>
-
- <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>
-
- <div class="example"><p><code>
- DirectoryIndex index
- </code></p></div>
-
- <p>Use a complete list of options:</p>
-
- <div class="example"><p><code>
- DirectoryIndex index.cgi index.pl index.shtml index.html
- </code></p></div>
-
- <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 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 files, the kernel may be unable
- to reliably serve the network file through it's 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 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 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#maxrequestsperchild">MaxRequestsPerChild</a></code>
- setting. By default this is <code>0</code>,
- which means that there is no limit to the number of requests
- 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>15</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.research.digital.com/wrl/techreports/abstracts/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/beos.html">beos</a></code>, <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/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 absense.</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>
-
- <div class="example"><p><code>
- for (;;) {<br />
- <span class="indent">
- for (;;) {<br />
- <span class="indent">
- fd_set accept_fds;<br />
- <br />
- FD_ZERO (&accept_fds);<br />
- for (i = first_socket; i <= last_socket; ++i) {<br />
- <span class="indent">
- FD_SET (i, &accept_fds);<br />
- </span>
- }<br />
- rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);<br />
- if (rc < 1) continue;<br />
- new_connection = -1;<br />
- for (i = first_socket; i <= last_socket; ++i) {<br />
- <span class="indent">
- if (FD_ISSET (i, &accept_fds)) {<br />
- <span class="indent">
- new_connection = accept (i, NULL, NULL);<br />
- if (new_connection != -1) break;<br />
- </span>
- }<br />
- </span>
- }<br />
- if (new_connection != -1) break;<br />
- </span>
- }<br />
- process the new_connection;<br />
- </span>
- }
- </code></p></div>
-
- <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>
-
- <div class="example"><p><code>
- for (;;) {<br />
- <span class="indent">
- <strong>accept_mutex_on ();</strong><br />
- for (;;) {<br />
- <span class="indent">
- fd_set accept_fds;<br />
- <br />
- FD_ZERO (&accept_fds);<br />
- for (i = first_socket; i <= last_socket; ++i) {<br />
- <span class="indent">
- FD_SET (i, &accept_fds);<br />
- </span>
- }<br />
- rc = select (last_socket+1, &accept_fds, NULL, NULL, NULL);<br />
- if (rc < 1) continue;<br />
- new_connection = -1;<br />
- for (i = first_socket; i <= last_socket; ++i) {<br />
- <span class="indent">
- if (FD_ISSET (i, &accept_fds)) {<br />
- <span class="indent">
- new_connection = accept (i, NULL, NULL);<br />
- if (new_connection != -1) break;<br />
- </span>
- }<br />
- </span>
- }<br />
- if (new_connection != -1) break;<br />
- </span>
- }<br />
- <strong>accept_mutex_off ();</strong><br />
- process the new_connection;<br />
- </span>
- }
- </code></p></div>
-
- <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 directive <code class="directive"><a href="../mod/mpm_common.html#acceptmutex">AcceptMutex</a></code> can be used to
- change the selected mutex implementation at run-time.</p>
-
- <dl>
- <dt><code>AcceptMutex flock</code></dt>
-
- <dd>
- <p>This method uses the <code>flock(2)</code> system call to
- lock a lock file (located by the <code class="directive"><a href="../mod/mpm_common.html#lockfile">LockFile</a></code> directive).</p>
- </dd>
-
- <dt><code>AcceptMutex fcntl</code></dt>
-
- <dd>
- <p>This method uses the <code>fcntl(2)</code> system call to
- lock a lock file (located by the <code class="directive"><a href="../mod/mpm_common.html#lockfile">LockFile</a></code> directive).</p>
- </dd>
-
- <dt><code>AcceptMutex sysvsem</code></dt>
-
- <dd>
- <p>(1.3 or later) This method uses SysV-style semaphores to
- implement the mutex. Unfortunately SysV-style semaphores have
- some bad side-effects. One is that it's possible Apache will
- die without cleaning up the semaphore (see the
- <code>ipcs(8)</code> man page). The other is that the
- semaphore API allows for a denial of service attack by any
- CGIs running under the same uid as the webserver
- (<em>i.e.</em>, all CGIs, unless you use something like
- <code class="program"><a href="../programs/suexec.html">suexec</a></code> or <code>cgiwrapper</code>). For these
- reasons this method is not used on any architecture except
- IRIX (where the previous two are prohibitively expensive
- on most IRIX boxes).</p>
- </dd>
-
- <dt><code>AcceptMutex pthread</code></dt>
-
- <dd>
- <p>(1.3 or later) This method uses POSIX mutexes and should
- work on any architecture implementing the full POSIX threads
- specification, however appears to only work on Solaris (2.5
- or later), and even then only in certain configurations. If
- you experiment with this you should watch out for your server
- hanging and not responding. Static content only servers may
- work just fine.</p>
- </dd>
-
- <dt><code>AcceptMutex posixsem</code></dt>
-
- <dd>
- <p>(2.0 or later) This method uses POSIX semaphores. The
- semaphore ownership is not recovered if a thread in the process
- holding the mutex segfaults, resulting in a hang of the web
- server.</p>
- </dd>
-
- </dl>
-
- <p>If your system has another method of serialization which
- isn't in the above list then it may be worthwhile adding code
- for it to APR.</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 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" behaviour 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 behaviour
- 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 shutdown each direction of the
- communication independently (recall that a TCP connection is
- bi-directional, each half is independent of the other). This
- fact is often overlooked by other servers, but is correctly
- implemented in Apache as of 1.2.</p>
-
- <p>When this feature was added to Apache it caused a flurry of
- problems on various versions of Unix because of a
- 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 of accomplishing 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>
-
- <div class="example"><p><code>
- void lingering_close (int s)<br />
- {<br />
- <span class="indent">
- char junk_buffer[2048];<br />
- <br />
- /* shutdown the sending side */<br />
- shutdown (s, 1);<br />
- <br />
- signal (SIGALRM, lingering_death);<br />
- alarm (30);<br />
- <br />
- for (;;) {<br />
- <span class="indent">
- select (s for reading, 2 second timeout);<br />
- if (error) break;<br />
- if (s is ready for reading) {<br />
- <span class="indent">
- if (read (s, junk_buffer, sizeof (junk_buffer)) <= 0) {<br />
- <span class="indent">
- break;<br />
- </span>
- }<br />
- /* just toss away whatever is here */<br />
- </span>
- }<br />
- </span>
- }<br />
- <br />
- close (s);<br />
- </span>
- }
- </code></p></div>
-
- <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 behaviour 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's 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's 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"> en </a> |
-<a href="../ko/misc/perf-tuning.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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-</div><div id="footer">
-<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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