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-<title>Descriptors and Apache - Apache HTTP Server</title>
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-<p class="apache">Apache HTTP Server Version 2.0</p>
-<img alt="" src="../images/feather.gif" /></div>
-<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
-<div id="path">
-<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.0</a> &gt; <a href="./">Miscellaneous Documentation</a></div><div id="page-content"><div id="preamble"><h1>Descriptors and Apache</h1>
-<div class="toplang">
-<p><span>Available Languages: </span><a href="../en/misc/descriptors.html" title="English">&nbsp;en&nbsp;</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 &lt;= soft limit &lt;= hard limit &lt;= 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">&nbsp;en&nbsp;</a></p>
-</div><div id="footer">
-<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
-<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></div>
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