More updates from 1.3 docs.

<!--#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

<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>

    <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

        with a higher FD_SETSIZE.

    <p>

    <dt> <b>FreeBSD 2.2, BSDI 2.1+</b>

    <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>

    <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

        dedicated webserver you probably won't run into trouble.

    <p>

    <dt> <b>Solaris through 2.5.1</b>

    <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.

        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>

    <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>.

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