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<H1 ALIGN="CENTER">Descriptors and Apache</H1>
<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 (i.e. with many virtual hosts).
<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>To summarize:
<center><pre>
#open files <= soft limit <= hard limit <= kernel limit
</pre></center>
<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>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):
<dl>
<dt><STRONG>BSDI 2.0</STRONG>
<dd>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>
<dt><STRONG>FreeBSD 2.2, BSDI 2.1+</STRONG>
<dd>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>
<dt><STRONG>Linux</STRONG>
<dd>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>
<dt><STRONG>Solaris through 2.5.1</STRONG>
<dd>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>
<dt><STRONG>AIX version ??</STRONG>
<dd>AIX appears to have a hard limit of 128 descriptors. End of
story.
<p>
<dt><STRONG>Others</STRONG>
<dd>If you have details on another operating system, please submit
it through our <a href="http://www.apache.org/bug_report.html">Bug
Report Page</a>.
<p>
</dl>
<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>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>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
<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://www.apache.org/bug_report.html">Bug
Report Page</a> so that
we can investigate.
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