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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">

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<H1 ALIGN="CENTER">Descriptors and Apache</H1>



<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>,

<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 (<EM>i.e.</EM>, 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 <EM>FD_SETSIZE</EM>).  Or you

may be stuck with a (relatively) low amount.  The second limit is called

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 <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

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>



    <h1 align="CENTER">Descriptors and Apache</h1>



    <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>, <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 (<em>i.e.</em>, with many virtual hosts).</p>



    <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 <em>FD_SETSIZE</em>). Or you

    may be stuck with a (relatively) low amount. The second limit

    is called 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

    <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 limit is the

    actual limit that is used when enforcing the maximum number of

    files a process can have open.</p>



    <p>To summarize:</p>



    <center>

<pre>

  #open files  &lt;=  soft limit  &lt;=  hard limit  &lt;=  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><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><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><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>.

</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>



    <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):</p>



    <dl>

      <dt><strong>BSDI 2.0</strong></dt>



      <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.</dd>



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



      <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.</dd>



      <dt><strong>Linux</strong></dt>



      <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><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><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><STRONG>SCO OpenServer</STRONG> 

    <DD>Edit the

    <CODE>/etc/conf/cf.d/stune</CODE> file or use 

    <CODE>/etc/conf/cf.d/configure</CODE> choice 7

    (User and Group configuration) and modify the <CODE>NOFILES</CODE> kernel

    parameter to a suitably higher value.  SCO recommends a number

    between 60 and 11000, the default is 110.  Relink and reboot, 

    and the new number of descriptors will be available.



    <P>



    <DT><STRONG>Compaq Tru64 UNIX/Digital UNIX/OSF</STRONG>

    <DD><OL>

    <LI>Raise <code>open_max_soft</code> and <code>open_max_hard</code>

        to 4096 in the proc subsystem.

        Do a man on sysconfig, sysconfigdb, and sysconfigtab.

    <LI>Raise <code>max-vnodes</code> to a large number which is greater 

        than the number of apache processes * 4096

      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.</dd>



      <dt><strong>Solaris through 2.5.1</strong></dt>



      <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.</dd>



      <dt><strong>AIX</strong></dt>



      <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.</dd>



      <dt><strong>SCO OpenServer</strong></dt>



      <dd>Edit the <code>/etc/conf/cf.d/stune</code> file or use

      <code>/etc/conf/cf.d/configure</code> choice 7 (User and

      Group configuration) and modify the <code>NOFILES</code>

      kernel parameter to a suitably higher value. SCO recommends a

      number between 60 and 11000, the default is 110. Relink and

      reboot, and the new number of descriptors will be

      available.</dd>



      <dt><strong>Compaq Tru64 UNIX/Digital UNIX/OSF</strong></dt>



      <dd>

        <ol>

          <li>Raise <code>open_max_soft</code> and

          <code>open_max_hard</code> to 4096 in the proc subsystem.

          Do a man on sysconfig, sysconfigdb, and

          sysconfigtab.</li>



          <li>Raise <code>max-vnodes</code> to a large number which

          is greater than the number of apache processes * 4096

          (Setting it to 250,000 should be good for most people).

        Do a man on sysconfig, sysconfigdb, and sysconfigtab.

    <LI>If you are using Tru64 5.0, 5.0A, or 5.1, define 

        <code>NO_SLACK</code> to work around a bug in the OS.

        <code>CFLAGS="-DNO_SLACK" ./configure</code>

    </OL>



    <P>



    <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>



<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 <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

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.

          Do a man on sysconfig, sysconfigdb, and

          sysconfigtab.</li>



          <li>If you are using Tru64 5.0, 5.0A, or 5.1, define

          <code>NO_SLACK</code> to work around a bug in the OS.

          <code>CFLAGS="-DNO_SLACK" ./configure</code></li>

        </ol>

      </dd>



      <dt><strong>Others</strong></dt>



      <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>.</dd>

    </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 <em>low slack

    line</em>.</p>



    <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>



    <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>



    <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. 

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