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authordgaudet <dgaudet@unknown>1997-06-26 06:05:19 +0000
committerdgaudet <dgaudet@unknown>1997-06-26 06:05:19 +0000
commit903fcb95924bd6cd9549445d746c472b55ad1dc3 (patch)
tree55d0f341a2c971f861293cc6fab8c64256aafb57
parentb209756b03e38ce4580a03b112c7d91fde432a3d (diff)
downloadhttpd-903fcb95924bd6cd9549445d746c472b55ad1dc3.tar.gz
Take 1, here's a description of the descriptors problem and solutions.
Reviewed by: Submitted by: Obtained from: git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@78369 13f79535-47bb-0310-9956-ffa450edef68
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
+<HTML>
+<HEAD>
+<TITLE>Descriptors and Apache</TITLE>
+</HEAD>
+
+<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
+<BODY
+ BGCOLOR="#FFFFFF"
+ TEXT="#000000"
+ LINK="#0000FF"
+ VLINK="#000080"
+ ALINK="#FF0000"
+>
+<!--#include virtual="header.html" -->
+<H1 ALIGN="CENTER">Descriptors and Apache</H1>
+
+<p>A <i>descriptor</i>, also commonly called a <i>file handle</i> 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 <i>FD_SETSIZE</i>). Or you
+may be stuck with a (relatively) low amount. The second limit is called
+the <i>hard resource</i> 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 <i>soft
+resource</i> 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> <b>BSDI 2.0</b>
+ <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> <b>FreeBSD 2.2, BSDI 2.1+</b>
+ <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> <b>Linux</b>
+ <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> <b>Solaris through 2.5.1</b>
+ <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> <b>AIX version ??</b>
+ <dd> AIX appears to have a hard limit of 128 descriptors. End of
+ story.
+ </p>
+
+ <dt> <b>Others</b>
+ <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 <i>low
+slack line</i>.
+
+<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
+bit of documentation in the header file <code>httpd.h</code>.
+
+<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.
+
+<!--#include virtual="footer.html" -->
+</BODY>
+</HTML>
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