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197 lines
8.7 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Descriptors and Apache</title>
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<h1 align="center">Descriptors and Apache</h1>
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<p>A <em>descriptor</em>, also commonly called a <em>file
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handle</em> is an object that a program uses to read or write
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an open file, or open network socket, or a variety of other
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devices. It is represented by an integer, and you may be
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familiar with <code>stdin</code>, <code>stdout</code>, and
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<code>stderr</code> which are descriptors 0, 1, and 2
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respectively. Apache needs a descriptor for each log file, plus
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one for each network socket that it listens on, plus a handful
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of others. Libraries that Apache uses may also require
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descriptors. Normal programs don't open up many descriptors at
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all, and so there are some latent problems that you may
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experience should you start running Apache with many
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descriptors (<em>i.e.</em>, with many virtual hosts).</p>
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<p>The operating system enforces a limit on the number of
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descriptors that a program can have open at a time. There are
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typically three limits involved here. One is a kernel
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limitation, depending on your operating system you will either
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be able to tune the number of descriptors available to higher
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numbers (this is frequently called <em>FD_SETSIZE</em>). Or you
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may be stuck with a (relatively) low amount. The second limit
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is called the <em>hard resource</em> limit, and it is sometimes
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set by root in an obscure operating system file, but frequently
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is the same as the kernel limit. The third limit is called the
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<em>soft resource</em> limit. The soft limit is always less
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than or equal to the hard limit. For example, the hard limit
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may be 1024, but the soft limit only 64. Any user can raise
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their soft limit up to the hard limit. Root can raise the hard
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limit up to the system maximum limit. The soft limit is the
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actual limit that is used when enforcing the maximum number of
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files a process can have open.</p>
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<p>To summarize:</p>
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<center>
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<pre>
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#open files <= soft limit <= hard limit <= kernel limit
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</pre>
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</center>
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<p>You control the hard and soft limits using the
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<code>limit</code> (csh) or <code>ulimit</code> (sh)
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directives. See the respective man pages for more information.
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For example you can probably use <code>ulimit -n
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unlimited</code> to raise your soft limit up to the hard limit.
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You should include this command in a shell script which starts
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your webserver.</p>
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<p>Unfortunately, it's not always this simple. As mentioned
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above, you will probably run into some system limitations that
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will need to be worked around somehow. Work was done in version
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1.2.1 to improve the situation somewhat. Here is a partial list
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of systems and workarounds (assuming you are using 1.2.1 or
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later):</p>
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<dl>
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<dt><strong>BSDI 2.0</strong></dt>
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<dd>Under BSDI 2.0 you can build Apache to support more
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descriptors by adding <code>-DFD_SETSIZE=nnn</code> to
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<code>EXTRA_CFLAGS</code> (where nnn is the number of
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descriptors you wish to support, keep it less than the hard
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limit). But it will run into trouble if more than
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approximately 240 Listen directives are used. This may be
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cured by rebuilding your kernel with a higher
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FD_SETSIZE.</dd>
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<dt><strong>FreeBSD 2.2, BSDI 2.1+</strong></dt>
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<dd>Similar to the BSDI 2.0 case, you should define
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<code>FD_SETSIZE</code> and rebuild. But the extra Listen
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limitation doesn't exist.</dd>
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<dt><strong>Linux</strong></dt>
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<dd>By default Linux has a kernel maximum of 256 open
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descriptors per process. There are several patches available
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for the 2.0.x series which raise this to 1024 and beyond, and
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you can find them in the "unofficial patches" section of <a
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href="http://www.linuxhq.com/">the Linux Information HQ</a>.
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None of these patches are perfect, and an entirely different
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approach is likely to be taken during the 2.1.x development.
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Applying these patches will raise the FD_SETSIZE used to
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compile all programs, and unless you rebuild all your
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libraries you should avoid running any other program with a
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soft descriptor limit above 256. As of this writing the
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patches available for increasing the number of descriptors do
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not take this into account. On a dedicated webserver you
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probably won't run into trouble.</dd>
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<dt><strong>Solaris through 2.5.1</strong></dt>
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<dd>Solaris has a kernel hard limit of 1024 (may be lower in
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earlier versions). But it has a limitation that files using
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the stdio library cannot have a descriptor above 255. Apache
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uses the stdio library for the ErrorLog directive. When you
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have more than approximately 110 virtual hosts (with an error
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log and an access log each) you will need to build Apache
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with <code>-DHIGH_SLACK_LINE=256</code> added to
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<code>EXTRA_CFLAGS</code>. You will be limited to
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approximately 240 error logs if you do this.</dd>
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<dt><strong>AIX</strong></dt>
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<dd>AIX version 3.2?? appears to have a hard limit of 128
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descriptors. End of story. Version 4.1.5 has a hard limit of
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2000.</dd>
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<dt><strong>SCO OpenServer</strong></dt>
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<dd>Edit the <code>/etc/conf/cf.d/stune</code> file or use
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<code>/etc/conf/cf.d/configure</code> choice 7 (User and
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Group configuration) and modify the <code>NOFILES</code>
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kernel parameter to a suitably higher value. SCO recommends a
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number between 60 and 11000, the default is 110. Relink and
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reboot, and the new number of descriptors will be
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available.</dd>
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<dt><strong>Compaq Tru64 UNIX/Digital UNIX/OSF</strong></dt>
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<dd>
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<ol>
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<li>Raise <code>open_max_soft</code> and
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<code>open_max_hard</code> to 4096 in the proc subsystem.
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Do a man on sysconfig, sysconfigdb, and
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sysconfigtab.</li>
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<li>Raise <code>max-vnodes</code> to a large number which
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is greater than the number of apache processes * 4096
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(Setting it to 250,000 should be good for most people).
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Do a man on sysconfig, sysconfigdb, and
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sysconfigtab.</li>
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<li>If you are using Tru64 5.0, 5.0A, or 5.1, define
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<code>NO_SLACK</code> to work around a bug in the OS.
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<code>CFLAGS="-DNO_SLACK" ./configure</code></li>
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</ol>
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</dd>
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<dt><strong>Others</strong></dt>
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<dd>If you have details on another operating system, please
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submit it through our <a
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href="http://httpd.apache.org/bug_report.html">Bug Report
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Page</a>.</dd>
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</dl>
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<p>In addition to the problems described above there are
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problems with many libraries that Apache uses. The most common
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example is the bind DNS resolver library that is used by pretty
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much every unix, which fails if it ends up with a descriptor
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above 256. We suspect there are other libraries that similar
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limitations. So the code as of 1.2.1 takes a defensive stance
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and tries to save descriptors less than 16 for use while
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processing each request. This is called the <em>low slack
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line</em>.</p>
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<p>Note that this shouldn't waste descriptors. If you really
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are pushing the limits and Apache can't get a descriptor above
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16 when it wants it, it will settle for one below 16.</p>
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<p>In extreme situations you may want to lower the low slack
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line, but you shouldn't ever need to. For example, lowering it
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can increase the limits 240 described above under Solaris and
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BSDI 2.0. But you'll play a delicate balancing game with the
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descriptors needed to serve a request. Should you want to play
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this game, the compile time parameter is
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<code>LOW_SLACK_LINE</code> and there's a tiny bit of
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documentation in the header file <code>httpd.h</code>.</p>
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<p>Finally, if you suspect that all this slack stuff is causing
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you problems, you can disable it. Add <code>-DNO_SLACK</code>
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to <code>EXTRA_CFLAGS</code> and rebuild. But please report it
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to our <a href="http://httpd.apache.org/bug_report.html">Bug
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Report Page</a> so that we can investigate. </p>
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