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<H1 ALIGN="CENTER">Descriptors and Apache</H1>
-<p>A <i>descriptor</i>, also commonly called a <i>file handle</i> is
+<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>,
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
+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 <i>hard resource</i> limit, and it is sometimes set by root in an
+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 <i>soft
-resource</i> limit. The soft limit is always less than or equal to
+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
<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.
+ <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> <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.
+ <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> <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.
+ <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> <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.
+ <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> <b>AIX version ??</b>
- <dd> AIX appears to have a hard limit of 128 descriptors. End of
- story.
+ <dt><STRONG>AIX STRONG>
+ <dd>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>
- <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>.
+ <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>
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>.
+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
projects / thirdparty / apache / httpd.git / commitdiff
