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    and instructions for creating <code>type-map</code> files.</p>

    

    <h3>Memory-mapping</h3>

      

      <p>In situations where Apache 2.0 needs to look at the contents
      of a file being delivered--for example, when doing server-side-include
      processing--it normally memory-maps the file if the OS supports
      some form of <code>mmap(2)</code>.</p>

      <p>On some platforms, this memory-mapping improves performance.
      However, there are cases where memory-mapping can hurt the performance
      or even the stability of the httpd:</p>

      <ul>
        <li>
          <p>On some operating systems, <code>mmap</code> does not scale
          as well as <code>read(2)</code> when the number of CPUs increases.
          On multiprocessor Solaris servers, for example, Apache 2.0 sometimes
          delivers server-parsed files faster when <code>mmap</code> is disabled.</p>
        </li>

        <li>
          <p>If you memory-map a file located on an NFS-mounted filesystem
          and a process on another NFS client machine deletes or truncates
          the file, your process may get a bus error the next time it tries
          to access the mapped file content.</p>
        </li>
      </ul>

      <p>For installations where either of these factors applies, you
      should use <code>EnableMMAP off</code> to disable the memory-mapping
      of delivered files. (Note: This directive can be overridden on
      a per-directory basis.)</p>

    

    <h3>Sendfile</h3>

      

      <p>In situations where Apache 2.0 can ignore the contents of the file
      to be delivered -- for example, when serving static file content -- 
      it normally uses the kernel sendfile support the file if the OS 
      supports the <code>sendfile(2)</code> operation.</p>

      <p>On most platforms, using sendfile improves performance by eliminating
      separate read and send mechanics.  However, there are cases where using
      sendfile can harm the stability of the httpd:</p>

      <ul>
        <li>
          <p>Some platforms may have broken sendfile support that the build
          system did not detect, especially if the binaries were built on
          another box and moved to such a machine with broken sendfile support.</p>
        </li>
        <li>
          <p>With an NFS-mounted files, the kernel may be unable 
          to reliably serve the network file through it's own cache.</p>
        </li>
      </ul>

      <p>For installations where either of these factors applies, you
      should use <code>EnableSendfile off</code> to disable sendfile
      delivery of file contents. (Note: This directive can be overridden 
      on a per-directory basis.)</p>

    

    <h3><a name="process" id="process">Process Creation</a></h3>

      

      <p>Prior to Apache 1.3 the <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>, <code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code>, and <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> settings all had drastic effects on
      benchmark results. In particular, Apache required a "ramp-up"
      period in order to reach a number of children sufficient to serve
      the load being applied. After the initial spawning of
      <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> children,
      only one child per second would be created to satisfy the
      <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>
      setting. So a server being accessed by 100 simultaneous
      clients, using the default <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> of <code>5</code> would take on
      the order 95 seconds to spawn enough children to handle
      the load. This works fine in practice on real-life servers,
      because they aren't restarted frequently. But does really
      poorly on benchmarks which might only run for ten minutes.</p>

      <p>The one-per-second rule was implemented in an effort to
      avoid swamping the machine with the startup of new children. If
      the machine is busy spawning children it can't service
      requests. But it has such a drastic effect on the perceived
      performance of Apache that it had to be replaced. As of Apache
      1.3, the code will relax the one-per-second rule. It will spawn
      one, wait a second, then spawn two, wait a second, then spawn
      four, and it will continue exponentially until it is spawning
      32 children per second. It will stop whenever it satisfies the
      <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>
      setting.</p>

      <p>This appears to be responsive enough that it's almost
      unnecessary to twiddle the <code class="directive"><a href="../mod/prefork.html#minspareservers">MinSpareServers</a></code>, <code class="directive"><a href="../mod/prefork.html#maxspareservers">MaxSpareServers</a></code> and <code class="directive"><a href="../mod/mpm_common.html#startservers">StartServers</a></code> knobs. When more than 4 children are
      spawned per second, a message will be emitted to the
      <code class="directive"><a href="../mod/core.html#errorlog">ErrorLog</a></code>. If you
      see a lot of these errors then consider tuning these settings.
      Use the <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> output as a guide.</p>

    <p>Related to process creation is process death induced by the
    <code class="directive"><a href="../mod/mpm_common.html#maxrequestsperchild">MaxRequestsPerChild</a></code>
    setting. By default this is <code>0</code>,
    which means that there is no limit to the number of requests
    handled per child. If your configuration currently has this set
    to some very low number, such as <code>30</code>, you may want to bump this
    up significantly. If you are running SunOS or an old version of
    Solaris, limit this to <code>10000</code> or so because of memory leaks.</p>

    <p>When keep-alives are in use, children will be kept busy
    doing nothing waiting for more requests on the already open
    connection. The default <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code> of <code>15</code>
    seconds attempts to minimize this effect. The tradeoff here is
    between network bandwidth and server resources. In no event
    should you raise this above about <code>60</code> seconds, as <a href="http://www.research.digital.com/wrl/techreports/abstracts/95.4.html">
    most of the benefits are lost</a>.</p>

    

  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="compiletime" id="compiletime">Compile-Time Configuration Issues</a></h2>

    

    <h3>Choosing an MPM</h3>

      

      <p>Apache 2.x supports pluggable concurrency models, called
      <a href="../mpm.html">Multi-Processing Modules</a> (MPMs).
      When building Apache, you must choose an MPM to use.  There
      are platform-specific MPMs for some platforms:
      <code class="module"><a href="../mod/beos.html">beos</a></code>, <code class="module"><a href="../mod/mpm_netware.html">mpm_netware</a></code>,
      <code class="module"><a href="../mod/mpmt_os2.html">mpmt_os2</a></code>, and <code class="module"><a href="../mod/mpm_winnt.html">mpm_winnt</a></code>.  For
      general Unix-type systems, there are several MPMs from which
      to choose.  The choice of MPM can affect the speed and scalability
      of the httpd:</p>

      <ul>

        <li>The <code class="module"><a href="../mod/worker.html">worker</a></code> MPM uses multiple child
        processes with many threads each.  Each thread handles
        one connection at a time.  Worker generally is a good
        choice for high-traffic servers because it has a smaller
        memory footprint than the prefork MPM.</li>

        <li>The <code class="module"><a href="../mod/prefork.html">prefork</a></code> MPM uses multiple child
        processes with one thread each.  Each process handles
        one connection at a time.  On many systems, prefork is
        comparable in speed to worker, but it uses more memory.
        Prefork's threadless design has advantages over worker
        in some situations: it can be used with non-thread-safe
        third-party modules, and it is easier to debug on platforms
        with poor thread debugging support.</li>

      </ul>

      <p>For more information on these and other MPMs, please
      see the MPM <a href="../mpm.html">documentation</a>.</p>

    

    <h3><a name="modules" id="modules">Modules</a></h3>

        

        <p>Since memory usage is such an important consideration in
        performance, you should attempt to eliminate modules that youare
        not actually using. If you have built the modules as <a href="../dso.html">DSOs</a>, eliminating modules is a simple
        matter of commenting out the associated <code class="directive"><a href="../mod/mod_so.html#loadmodule">LoadModule</a></code> directive for that module.
        This allows you to experiment with removing modules, and seeing
        if your site still functions in their absense.</p>

        <p>If, on the other hand, you have modules statically linked
        into your Apache binary, you will need to recompile Apache in
        order to remove unwanted modules.</p>

        <p>An associated question that arises here is, of course, what
        modules you need, and which ones you don't. The answer here
        will, of course, vary from one web site to another. However, the
        <em>minimal</em> list of modules which you can get by with tends
        to include <code class="module"><a href="../mod/mod_mime.html">mod_mime</a></code>, <code class="module"><a href="../mod/mod_dir.html">mod_dir</a></code>,
        and <code class="module"><a href="../mod/mod_log_config.html">mod_log_config</a></code>. <code>mod_log_config</code> is,
        of course, optional, as you can run a web site without log
        files. This is, however, not recommended.</p>

    

    <h3>Atomic Operations</h3>

      

      <p>Some modules, such as <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> and
      recent development builds of the worker MPM, use APR's
      atomic API.  This API provides atomic operations that can
      be used for lightweight thread synchronization.</p>

      <p>By default, APR implements these operations using the
      most efficient mechanism available on each target
      OS/CPU platform.  Many modern CPUs, for example, have
      an instruction that does an atomic compare-and-swap (CAS)
      operation in hardware.  On some platforms, however, APR
      defaults to a slower, mutex-based implementation of the
      atomic API in order to ensure compatibility with older
      CPU models that lack such instructions.  If you are
      building Apache for one of these platforms, and you plan
      to run only on newer CPUs, you can select a faster atomic
      implementation at build time by configuring Apache with
      the <code>--enable-nonportable-atomics</code> option:</p>

      <div class="example"><p><code>
        ./buildconf<br />
        ./configure --with-mpm=worker --enable-nonportable-atomics=yes
      </code></p></div>

      <p>The <code>--enable-nonportable-atomics</code> option is
      relevant for the following platforms:</p>

      <ul>

        <li>Solaris on SPARC<br />
            By default, APR uses mutex-based atomics on Solaris/SPARC.
            If you configure with <code>--enable-nonportable-atomics</code>,
            however, APR generates code that uses a SPARC v8plus opcode for
            fast hardware compare-and-swap.  If you configure Apache with
            this option, the atomic operations will be more efficient
            (allowing for lower CPU utilization and higher concurrency),
            but the resulting executable will run only on UltraSPARC
            chips.
        </li>

        <li>Linux on x86<br />
            By default, APR uses mutex-based atomics on Linux.  If you
            configure with <code>--enable-nonportable-atomics</code>,
            however, APR generates code that uses a 486 opcode for fast
            hardware compare-and-swap.  This will result in more efficient
            atomic operations, but the resulting executable will run only
            on 486 and later chips (and not on 386).
        </li>

      </ul>

    

    <h3>mod_status and ExtendedStatus On</h3>

      

      <p>If you include <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and you also set
      <code>ExtendedStatus On</code> when building and running
      Apache, then on every request Apache will perform two calls to
      <code>gettimeofday(2)</code> (or <code>times(2)</code>
      depending on your operating system), and (pre-1.3) several
      extra calls to <code>time(2)</code>. This is all done so that