Expand the section on fd limits a bit.

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<H1>Apache Server Frequently Asked Questions</H1>

<P>

$Revision: 1.17 $ ($Date: 1997/04/11 03:36:58 $)

$Revision: 1.18 $ ($Date: 1997/04/11 03:43:43 $)

</P>

<H2>The Questions</H2>

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  <HR>

 </LI>

 <LI><A

      NAME="multiviews"

      NAME="fdlim"

     ><STRONG>Why can't I run more than &lt;<EM>n</EM>&gt;

      virtual hosts?</STRONG></A>

  <P>

  resource limitations.  One of these is the <EM>per</EM>-process limit

  on <STRONG>file descriptors</STRONG>, and that's almost always the

  cause of problems seen when adding virtual hosts.  In this

  case, it is not actually Apache that's encountering the problem, but 

  typically some library routine (such as <SAMP>gethostbyname()</SAMP>)

  case, it is often not actually Apache that's encountering the problem, but 

  some library routine (such as <SAMP>gethostbyname()</SAMP>)

  which needs file descriptors and doesn't complain intelligibly when it

  can't get them.

  </P>

  <P>

  Each virtual host requires several file descriptors for housekeeping

  functions, in addition to those actually used to serve files to

  clients.

  Each log file requires a file descriptor, which means that if you are

  using seperate access and error logs for each virtual host each

  virtual host  needs two file descriptors.  Each 

  <A HREF="../mod/core.html#listen"> Listen</A> directive also needs 

  a file descriptor.  

  </P>

  <P>

  Typical values for &lt;<EM>n</EM>&gt; that we've seen are in

  the neighbourhoods of 128 or 250.  When the server bumps into the file

  descriptor limit, it may dump core with a SIGSEGV, or it might just

  hang, or it may limp along and you'll see (possibly meaningful) errors

  in the error log.

  in the error log.  One common problem that occurs when you run into

  a file descriptor limit is that CGI scripts stop being executed

  properly at times.

  </P>

  <P>

  As to what you can do about this:

  </P>

  <OL>

   <LI>Reduce the number of <A HREF="../mod/core.html#listen">Listen</A> 

       directives.  If there are no other servers running on the machine 

       and all of them are running on the same port, you normally don't 

       need any Listen directives at all.

   <LI>Reduce the number of log files.  You can use 

       <A HREF="../mod/mod_log_config.html">log_mod_config</A> to log

       all requests to a single log file while including the name

       of the virtual host in the log file.

   <LI>Increase the number of file descriptors available to the server

       (see your system's documentation on the <SAMP>limit</SAMP> or

    <SAMP>ulimit</SAMP> commands)

       <SAMP>ulimit</SAMP> commands).  For some systems, information on

       how to do this is available in the <A HREF="perf.html">performance

       hints</A> page.

   </LI>

   <LI>&quot;Don't do that&quot; - try to run with fewer virtual hosts

   </LI>