diff options
| author | Joe Orton <jorton@apache.org> | 2020-08-19 07:31:23 +0000 |
|---|---|---|
| committer | Joe Orton <jorton@apache.org> | 2020-08-19 07:31:23 +0000 |
| commit | 7f6aec2cf7fb0caaa93dc98e25968d58b60107fd (patch) | |
| tree | 2218e4fdf202d9781910e0218b690acbdc92a0d5 /docs/manual/misc/perf-tuning.html.en | |
| parent | ac888899b7c2cecdda5ed98ad5dd73607b1ca5bc (diff) | |
| download | httpd-7f6aec2cf7fb0caaa93dc98e25968d58b60107fd.tar.gz | |
Follow up to r1880368 and r1880980:
Complete move to .en.utf8 suffixes in two steps:
1. From fresh checkout, renamed all existing files:
$ for f in *.html.en */*.html.en; do svn mv $f $f.utf8; done
2. Ran "./build.sh all" which also regenerates the typemaps.
[skip ci]
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1880982 13f79535-47bb-0310-9956-ffa450edef68
Diffstat (limited to 'docs/manual/misc/perf-tuning.html.en')
| -rw-r--r-- | docs/manual/misc/perf-tuning.html.en | 956 |
1 files changed, 0 insertions, 956 deletions
diff --git a/docs/manual/misc/perf-tuning.html.en b/docs/manual/misc/perf-tuning.html.en deleted file mode 100644 index 7515296b21..0000000000 --- a/docs/manual/misc/perf-tuning.html.en +++ /dev/null @@ -1,956 +0,0 @@ -<?xml version="1.0" encoding="UTF-8"?> -<!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=UTF-8" http-equiv="Content-Type" /> -<!-- - XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX - This file is generated from xml source: DO NOT EDIT - XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX - --> -<title>Apache Performance Tuning - Apache HTTP Server Version 2.5</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/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p> -<p class="apache">Apache HTTP Server Version 2.5</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 2.5</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> - - - <div class="warning"><h3>Warning</h3> - <p>This document is partially out of date and might be inaccurate.</p> - </div> - - <p>Apache 2.4 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 2.4 is capable of high performance in many - real-world situations.</p> - - <p>This document describes the options that a server administrator - can configure to tune the performance of an Apache 2.4 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><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> <a title="Permanent link" href="#hardware" class="permalink">¶</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 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> <a title="Permanent link" href="#runtime" class="permalink">¶</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>. - causing an extra latency penalty for every request due to a - DNS lookup to complete before the request was finished. - In Apache 2.4 this setting defaults to <code>Off</code>. If you need - to have addresses in your log files resolved to hostnames, please - consider post-processing rather than forcing Apache to do it in the first - place. It is recommended that you do this sort of post-processing 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, whenever it is possible, use IP addresses rather - than domain names.</p> - - <div class="warning"><h3>Warning:</h3> - <p>Please use the <code class="directive"><a href="../mod/mod_authz_core.html#require">Require</a></code> directive with Apache 2.4; - more info in the related <a href="../upgrading.html">upgrading guide</a>.</p> - </div> - - <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"><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">Recycle child processes</a></h3> - - - - <p><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code> - limits the numbers of connections that a child process can handle during - its lifetime (by default set to <code>0</code> - unlimited). This affects all - the <a href="../mpm.html#defaults">MPMs</a>, even the ones using threads. - For example, each process created by the <code class="module"><a href="../mod/worker.html">worker</a></code> MPM spawns - multiple threads that will handle connections, but this does not influence - the overall count. It only means that the sum of requests handled by all the - threads spawned by a single process will be counted against the - <code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code> value.</p> - - <p><code class="directive"><a href="../mod/mpm_common.html#maxconnectionsperchild">MaxConnectionsPerChild</a></code> should - not have any limit in the optimal use case, since there should not be any - reason to force a process kill other than software bugs causing memory leaks - or excessive CPU usage.</p> - - <p>When keep-alives are in use, a process (or a thread spawned by a process) - will be kept busy doing nothing but 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.</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> <a title="Permanent link" href="#compiletime" class="permalink">¶</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 children can - just block in <code>accept(2)</code> until a connection - arrives, and no starvation results. In practice this hides - almost the same "spinning" behavior discussed above in the - non-blocking solution. The way that most TCP stacks are - implemented, the kernel actually wakes up all processes blocked - in <code>accept</code> when a single connection arrives. One of - those processes gets the connection and returns to user-space. - The rest spin in the kernel and go back to sleep when they - discover there's no connection for them. This spinning is - hidden from the user-land code, but it's there nonetheless. - This can result in the same load-spiking wasteful behavior - that a non-blocking solution to the multiple sockets case - can.</p> - - <p>For this reason we have found that many architectures behave - more "nicely" if we serialize even the single socket case. So - this is actually the default in almost all cases. Crude - experiments under Linux (2.0.30 on a dual Pentium pro 166 - w/128Mb RAM) have shown that the serialization of the single - socket case causes less than a 3% decrease in requests per - second over unserialized single-socket. But unserialized - single-socket showed an extra 100ms latency on each request. - This latency is probably a wash on long haul lines, and only an - issue on LANs. If you want to override the single socket - serialization, you can define - <code>SINGLE_LISTEN_UNSERIALIZED_ACCEPT</code>, and then - single-socket servers will not serialize at all.</p> - - - - <h3>Lingering Close</h3> - - - - <p>As discussed in <a href="http://www.ics.uci.edu/pub/ietf/http/draft-ietf-http-connection-00.txt"> - draft-ietf-http-connection-00.txt</a> section 8, in order for - an HTTP server to <strong>reliably</strong> implement the - protocol, it needs to shut down each direction of the - communication independently. (Recall that a TCP connection is - bi-directional. Each half is independent of the other.)</p> - - <p>When this feature was added to Apache, it caused a flurry of - problems on various versions of Unix because of shortsightedness. - The TCP specification does not state that the <code>FIN_WAIT_2</code> - state has a timeout, but it doesn't prohibit it. - On systems without the timeout, Apache 1.2 induces many sockets - stuck forever in the <code>FIN_WAIT_2</code> state. In many cases this - can be avoided by simply upgrading to the latest TCP/IP patches - supplied by the vendor. In cases where the vendor has never - released patches (<em>i.e.</em>, SunOS4 -- although folks with - a source license can patch it themselves), we have decided to - disable this feature.</p> - - <p>There are two ways to accomplish this. One is the socket - option <code>SO_LINGER</code>. But as fate would have it, this - has never been implemented properly in most TCP/IP stacks. Even - on those stacks with a proper implementation (<em>i.e.</em>, - Linux 2.0.31), this method proves to be more expensive (cputime) - than the next solution.</p> - - <p>For the most part, Apache implements this in a function - called <code>lingering_close</code> (in - <code>http_main.c</code>). The function looks roughly like - this:</p> - - <pre class="prettyprint lang-c"> void lingering_close (int s) - { - char junk_buffer[2048]; - - /* shutdown the sending side */ - shutdown (s, 1); - - signal (SIGALRM, lingering_death); - alarm (30); - - for (;;) { - select (s for reading, 2 second timeout); - if (error) break; - if (s is ready for reading) { - if (read (s, junk_buffer, sizeof (junk_buffer)) <= 0) { - break; - } - /* just toss away whatever is here */ - } - } - - close (s); - }</pre> - - - <p>This naturally adds some expense at the end of a connection, - but it is required for a reliable implementation. As HTTP/1.1 - becomes more prevalent, and all connections are persistent, - this expense will be amortized over more requests. If you want - to play with fire and disable this feature, you can define - <code>NO_LINGCLOSE</code>, but this is not recommended at all. - In particular, as HTTP/1.1 pipelined persistent connections - come into use, <code>lingering_close</code> is an absolute - necessity (and <a href="http://www.w3.org/Protocols/HTTP/Performance/Pipeline.html"> - pipelined connections are faster</a>, so you want to support - them).</p> - - - - <h3>Scoreboard File</h3> - - - - <p>Apache's parent and children communicate with each other - through something called the scoreboard. Ideally this should be - implemented in shared memory. For those operating systems that - we either have access to, or have been given detailed ports - for, it typically is implemented using shared memory. The rest - default to using an on-disk file. The on-disk file is not only - slow, but it is unreliable (and less featured). Peruse the - <code>src/main/conf.h</code> file for your architecture, and - look for either <code>USE_MMAP_SCOREBOARD</code> or - <code>USE_SHMGET_SCOREBOARD</code>. Defining one of those two - (as well as their companions <code>HAVE_MMAP</code> and - <code>HAVE_SHMGET</code> respectively) enables the supplied - shared memory code. If your system has another type of shared - memory, edit the file <code>src/main/http_main.c</code> and add - the hooks necessary to use it in Apache. (Send us back a patch - too, please.)</p> - - <div class="note">Historical note: The Linux port of Apache didn't start to - use shared memory until version 1.2 of Apache. This oversight - resulted in really poor and unreliable behavior of earlier - versions of Apache on Linux.</div> - - - - <h3>DYNAMIC_MODULE_LIMIT</h3> - - - - <p>If you have no intention of using dynamically loaded modules - (you probably don't if you're reading this and tuning your - server for every last ounce of performance), then you should add - <code>-DDYNAMIC_MODULE_LIMIT=0</code> when building your - server. This will save RAM that's allocated only for supporting - dynamically loaded modules.</p> - - - - </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> -<div class="section"> -<h2><a name="trace" id="trace">Appendix: Detailed Analysis of a Trace</a> <a title="Permanent link" href="#trace" class="permalink">¶</a></h2> - - - - <p>Here is a system call trace of Apache 2.0.38 with the worker MPM - on Solaris 8. This trace was collected using:</p> - - <div class="example"><p><code> - truss -l -p <var>httpd_child_pid</var>. - </code></p></div> - - <p>The <code>-l</code> option tells truss to log the ID of the - LWP (lightweight process--Solaris' form of kernel-level thread) - that invokes each system call.</p> - - <p>Other systems may have different system call tracing utilities - such as <code>strace</code>, <code>ktrace</code>, or <code>par</code>. - They all produce similar output.</p> - - <p>In this trace, a client has requested a 10KB static file - from the httpd. Traces of non-static requests or requests - with content negotiation look wildly different (and quite ugly - in some cases).</p> - - <div class="example"><pre>/67: accept(3, 0x00200BEC, 0x00200C0C, 1) (sleeping...) -/67: accept(3, 0x00200BEC, 0x00200C0C, 1) = 9</pre></div> - - <p>In this trace, the listener thread is running within LWP #67.</p> - - <div class="note">Note the lack of <code>accept(2)</code> serialization. On this - particular platform, the worker MPM uses an unserialized accept by - default unless it is listening on multiple ports.</div> - - <div class="example"><pre>/65: lwp_park(0x00000000, 0) = 0 -/67: lwp_unpark(65, 1) = 0</pre></div> - - <p>Upon accepting the connection, the listener thread wakes up - a worker thread to do the request processing. In this trace, - the worker thread that handles the request is mapped to LWP #65.</p> - - <div class="example"><pre>/65: getsockname(9, 0x00200BA4, 0x00200BC4, 1) = 0</pre></div> - - <p>In order to implement virtual hosts, Apache needs to know - the local socket address used to accept the connection. It - is possible to eliminate this call in many situations (such - as when there are no virtual hosts, or when - <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> directives - are used which do not have wildcard addresses). But - no effort has yet been made to do these optimizations. </p> - - <div class="example"><pre>/65: brk(0x002170E8) = 0 -/65: brk(0x002190E8) = 0</pre></div> - - <p>The <code>brk(2)</code> calls allocate memory from the heap. - It is rare to see these in a system call trace, because the httpd - uses custom memory allocators (<code>apr_pool</code> and - <code>apr_bucket_alloc</code>) for most request processing. - In this trace, the httpd has just been started, so it must - call <code>malloc(3)</code> to get the blocks of raw memory - with which to create the custom memory allocators.</p> - - <div class="example"><pre>/65: fcntl(9, F_GETFL, 0x00000000) = 2 -/65: fstat64(9, 0xFAF7B818) = 0 -/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B910, 2190656) = 0 -/65: fstat64(9, 0xFAF7B818) = 0 -/65: getsockopt(9, 65535, 8192, 0xFAF7B918, 0xFAF7B914, 2190656) = 0 -/65: setsockopt(9, 65535, 8192, 0xFAF7B918, 4, 2190656) = 0 -/65: fcntl(9, F_SETFL, 0x00000082) = 0</pre></div> - - <p>Next, the worker thread puts the connection to the client (file - descriptor 9) in non-blocking mode. The <code>setsockopt(2)</code> - and <code>getsockopt(2)</code> calls are a side-effect of how - Solaris' libc handles <code>fcntl(2)</code> on sockets.</p> - - <div class="example"><pre>/65: read(9, " G E T / 1 0 k . h t m".., 8000) = 97</pre></div> - - <p>The worker thread reads the request from the client.</p> - - <div class="example"><pre>/65: stat("/var/httpd/apache/httpd-8999/htdocs/10k.html", 0xFAF7B978) = 0 -/65: open("/var/httpd/apache/httpd-8999/htdocs/10k.html", O_RDONLY) = 10</pre></div> - - <p>This httpd has been configured with <code>Options FollowSymLinks</code> - and <code>AllowOverride None</code>. Thus it doesn't need to - <code>lstat(2)</code> each directory in the path leading up to the - requested file, nor check for <code>.htaccess</code> files. - It simply calls <code>stat(2)</code> to verify that the file: - 1) exists, and 2) is a regular file, not a directory.</p> - - <div class="example"><pre>/65: sendfilev(0, 9, 0x00200F90, 2, 0xFAF7B53C) = 10269</pre></div> - - <p>In this example, the httpd is able to send the HTTP response - header and the requested file with a single <code>sendfilev(2)</code> - system call. Sendfile semantics vary among operating systems. On some other - systems, it is necessary to do a <code>write(2)</code> or - <code>writev(2)</code> call to send the headers before calling - <code>sendfile(2)</code>.</p> - - <div class="example"><pre>/65: write(4, " 1 2 7 . 0 . 0 . 1 - ".., 78) = 78</pre></div> - - <p>This <code>write(2)</code> call records the request in the - access log. Note that one thing missing from this trace is a - <code>time(2)</code> call. Unlike Apache 1.3, Apache 2.x uses - <code>gettimeofday(3)</code> to look up the time. On some operating - systems, like Linux or Solaris, <code>gettimeofday</code> has an - optimized implementation that doesn't require as much overhead - as a typical system call.</p> - - <div class="example"><pre>/65: shutdown(9, 1, 1) = 0 -/65: poll(0xFAF7B980, 1, 2000) = 1 -/65: read(9, 0xFAF7BC20, 512) = 0 -/65: close(9) = 0</pre></div> - - <p>The worker thread does a lingering close of the connection.</p> - - <div class="example"><pre>/65: close(10) = 0 -/65: lwp_park(0x00000000, 0) (sleeping...)</pre></div> - - <p>Finally the worker thread closes the file that it has just delivered - and blocks until the listener assigns it another connection.</p> - - <div class="example"><pre>/67: accept(3, 0x001FEB74, 0x001FEB94, 1) (sleeping...)</pre></div> - - <p>Meanwhile, the listener thread is able to accept another connection - as soon as it has dispatched this connection to a worker thread (subject - to some flow-control logic in the worker MPM that throttles the listener - if all the available workers are busy). Though it isn't apparent from - this trace, the next <code>accept(2)</code> can (and usually does, under - high load conditions) occur in parallel with the worker thread's handling - of the just-accepted connection.</p> - - </div></div> -<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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