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| <title>Descriptors and Apache - Apache HTTP Server</title> |
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| <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> |
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| <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 class="retired"><h4>Please note</h4> |
| <p>This document refers to the <strong>2.0</strong> version of Apache httpd, which <strong>is no longer maintained</strong>. Upgrade, and refer to the current version of httpd instead, documented at:</p> |
| <ul><li><a href="http://httpd.apache.org/docs/current/">Current release version of Apache HTTP Server documentation</a></li></ul><p>You may follow <a href="http://httpd.apache.org/docs/current/misc/descriptors.html">this link</a> to go to the current version of this document.</p></div><div id="preamble"><h1>Descriptors and Apache</h1> |
| <div class="toplang"> |
| <p><span>Available Languages: </span><a href="../en/misc/descriptors.html" title="English"> en </a></p> |
| </div> |
| |
| |
| <div class="warning"><h3>Warning:</h3> |
| <p>This document has not been fully updated |
| to take into account changes made in the 2.0 version of the |
| Apache HTTP Server. Some of the information may still be |
| relevant, but please use it with care.</p> |
| </div> |
| |
| <p>A <em>descriptor</em>, also commonly called a <em>file |
| handle</em> is an object that a program uses to read or write |
| an open file, or open network socket, or a variety of other |
| devices. It is represented by an integer, and you may be |
| familiar with <code>stdin</code>, <code>stdout</code>, and |
| <code>stderr</code> which are descriptors 0, 1, and 2 |
| respectively. Apache needs a descriptor for each log file, plus |
| one for each network socket that it listens on, plus a handful |
| of others. Libraries that Apache uses may also require |
| descriptors. Normal programs don't open up many descriptors at |
| all, and so there are some latent problems that you may |
| experience should you start running Apache with many |
| descriptors (<em>i.e.</em>, with many virtual hosts).</p> |
| |
| <p>The operating system enforces a limit on the number of |
| descriptors that a program can have open at a time. There are |
| typically three limits involved here. One is a kernel |
| limitation, depending on your operating system you will either |
| be able to tune the number of descriptors available to higher |
| numbers (this is frequently called <em>FD_SETSIZE</em>). Or you |
| may be stuck with a (relatively) low amount. The second limit |
| is called the <em>hard resource</em> limit, and it is sometimes |
| set by root in an obscure operating system file, but frequently |
| is the same as the kernel limit. The third limit is called the |
| <em>soft resource</em> limit. The soft limit is always less |
| than or equal to the hard limit. For example, the hard limit |
| may be 1024, but the soft limit only 64. Any user can raise |
| their soft limit up to the hard limit. Root can raise the hard |
| limit up to the system maximum limit. The soft limit is the |
| actual limit that is used when enforcing the maximum number of |
| files a process can have open.</p> |
| |
| <p>To summarize:</p> |
| |
| <div class="example"><p><code> |
| #open files <= soft limit <= hard limit <= kernel limit |
| </code></p></div> |
| |
| |
| <p>You control the hard and soft limits using the |
| <code>limit</code> (csh) or <code>ulimit</code> (sh) |
| directives. See the respective man pages for more information. |
| For example you can probably use <code>ulimit -n |
| unlimited</code> to raise your soft limit up to the hard limit. |
| You should include this command in a shell script which starts |
| your webserver.</p> |
| |
| <p>Unfortunately, it's not always this simple. As mentioned |
| above, you will probably run into some system limitations that |
| will need to be worked around somehow. Work was done in version |
| 1.2.1 to improve the situation somewhat. Here is a partial list |
| of systems and workarounds (assuming you are using 1.2.1 or |
| later).</p> |
| |
| </div> |
| <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#bsdi">BSDI 2.0</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#freebsd">FreeBSD 2.2, BSDI 2.1+</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#linux">Linux</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#solaris">Solaris through 2.5.1</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#AIX">AIX</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#sco">SCO OpenServer</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#tru64">Compaq Tru64 UNIX/Digital UNIX/OSF</a></li> |
| <li><img alt="" src="../images/down.gif" /> <a href="#others">Others</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="bsdi" id="bsdi">BSDI 2.0</a></h2> |
| |
| <p>Under BSDI 2.0 you can build Apache to support more |
| descriptors by adding <code>-DFD_SETSIZE=nnn</code> to |
| <code>EXTRA_CFLAGS</code> (where nnn is the number of |
| descriptors you wish to support, keep it less than the hard |
| limit). But it will run into trouble if more than |
| approximately 240 Listen directives are used. This may be |
| cured by rebuilding your kernel with a higher |
| FD_SETSIZE.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="freebsd" id="freebsd">FreeBSD 2.2, BSDI 2.1+</a></h2> |
| |
| <p>Similar to the BSDI 2.0 case, you should define |
| <code>FD_SETSIZE</code> and rebuild. But the extra Listen |
| limitation doesn't exist.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="linux" id="linux">Linux</a></h2> |
| |
| <p>By default Linux has a kernel maximum of 256 open |
| descriptors per process. There are several patches available |
| for the 2.0.x series which raise this to 1024 and beyond, and |
| you can find them in the "unofficial patches" section of <a href="http://www.linuxhq.com/">the Linux Information HQ</a>. |
| None of these patches are perfect, and an entirely different |
| approach is likely to be taken during the 2.1.x development. |
| Applying these patches will raise the FD_SETSIZE used to |
| compile all programs, and unless you rebuild all your |
| libraries you should avoid running any other program with a |
| soft descriptor limit above 256. As of this writing the |
| patches available for increasing the number of descriptors do |
| not take this into account. On a dedicated webserver you |
| probably won't run into trouble.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="solaris" id="solaris">Solaris through 2.5.1</a></h2> |
| |
| <p>Solaris has a kernel hard limit of 1024 (may be lower in |
| earlier versions). But it has a limitation that files using |
| the stdio library cannot have a descriptor above 255. Apache |
| uses the stdio library for the ErrorLog directive. When you |
| have more than approximately 110 virtual hosts (with an error |
| log and an access log each) you will need to build Apache |
| with <code>-DHIGH_SLACK_LINE=256</code> added to |
| <code>EXTRA_CFLAGS</code>. You will be limited to |
| approximately 240 error logs if you do this.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="AIX" id="AIX">AIX</a></h2> |
| |
| <p>AIX version 3.2?? appears to have a hard limit of 128 |
| descriptors. End of story. Version 4.1.5 has a hard limit of |
| 2000.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="sco" id="sco">SCO OpenServer</a></h2> |
| |
| <p>Edit the <code>/etc/conf/cf.d/stune</code> file or use |
| <code>/etc/conf/cf.d/configure</code> choice 7 (User and |
| Group configuration) and modify the <code>NOFILES</code> |
| kernel parameter to a suitably higher value. SCO recommends a |
| number between 60 and 11000, the default is 110. Relink and |
| reboot, and the new number of descriptors will be |
| available.</p> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="tru64" id="tru64">Compaq Tru64 UNIX/Digital UNIX/OSF</a></h2> |
| |
| <ol> |
| <li>Raise <code>open_max_soft</code> and |
| <code>open_max_hard</code> to 4096 in the proc subsystem. |
| Do a man on sysconfig, sysconfigdb, and |
| sysconfigtab.</li> |
| |
| <li>Raise <code>max-vnodes</code> to a large number which |
| is greater than the number of apache processes * 4096 |
| (Setting it to 250,000 should be good for most people). |
| Do a man on sysconfig, sysconfigdb, and |
| sysconfigtab.</li> |
| |
| <li>If you are using Tru64 5.0, 5.0A, or 5.1, define |
| <code>NO_SLACK</code> to work around a bug in the OS. |
| <code>CFLAGS="-DNO_SLACK" ./configure</code></li> |
| </ol> |
| |
| </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="others" id="others">Others</a></h2> |
| |
| <p>If you have details on another operating system, please |
| submit it through our <a href="http://httpd.apache.org/bug_report.html">Bug Report |
| Page</a>.</p> |
| |
| <p>In addition to the problems described above there are |
| problems with many libraries that Apache uses. The most common |
| example is the bind DNS resolver library that is used by pretty |
| much every unix, which fails if it ends up with a descriptor |
| above 256. We suspect there are other libraries that similar |
| limitations. So the code as of 1.2.1 takes a defensive stance |
| and tries to save descriptors less than 16 for use while |
| processing each request. This is called the <em>low slack |
| line</em>.</p> |
| |
| <p>Note that this shouldn't waste descriptors. If you really |
| are pushing the limits and Apache can't get a descriptor above |
| 16 when it wants it, it will settle for one below 16.</p> |
| |
| <p>In extreme situations you may want to lower the low slack |
| line, but you shouldn't ever need to. For example, lowering it |
| can increase the limits 240 described above under Solaris and |
| BSDI 2.0. But you'll play a delicate balancing game with the |
| descriptors needed to serve a request. Should you want to play |
| this game, the compile time parameter is |
| <code>LOW_SLACK_LINE</code> and there's a tiny bit of |
| documentation in the header file <code>httpd.h</code>.</p> |
| |
| <p>Finally, if you suspect that all this slack stuff is causing |
| you problems, you can disable it. Add <code>-DNO_SLACK</code> |
| to <code>EXTRA_CFLAGS</code> and rebuild. But please report it |
| to our <a href="http://httpd.apache.org/bug_report.html">Bug |
| Report Page</a> so that we can investigate. </p> |
| |
| </div></div> |
| <div class="bottomlang"> |
| <p><span>Available Languages: </span><a href="../en/misc/descriptors.html" title="English"> en </a></p> |
| </div><div id="footer"> |
| <p class="apache">Copyright 2013 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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