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diff --git a/docs/manual/misc/perf-tuning.html.en b/docs/manual/misc/perf-tuning.html.en new file mode 100644 index 0000000..a2b3fdc --- /dev/null +++ b/docs/manual/misc/perf-tuning.html.en @@ -0,0 +1,986 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head> +<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> +<!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>Apache Performance Tuning - Apache HTTP Server Version 2.4</title> +<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> +<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> +<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" /> +<script src="../style/scripts/prettify.min.js" type="text/javascript"> +</script> + +<link href="../images/favicon.ico" rel="shortcut icon" /></head> +<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="<-" 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 2.4</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="../fr/misc/perf-tuning.html" hreflang="fr" rel="alternate" title="Français"> fr </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"><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><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> + + <pre class="prettyprint lang-config">HostnameLookups off +<Files ~ "\.(html|cgi)$"> + HostnameLookups on +</Files></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" +<Directory "/"> + Options SymLinksIfOwnerMatch +</Directory></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" +<Directory "/"> + Options FollowSymLinks +</Directory> + +<Directory "/www/htdocs"> + Options -FollowSymLinks +SymLinksIfOwnerMatch +</Directory></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" +<Directory "/"> + AllowOverride all +</Directory></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 (&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 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)) <= 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"> en </a> | +<a href="../fr/misc/perf-tuning.html" hreflang="fr" rel="alternate" title="Français"> fr </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><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. 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