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