Added EnableMMAP to performance tuning guide

         <a

        href="../mod/core.html#hostnamelookups">HostnameLookups</a><br />

         <a

        href="../mod/core.html#enablemmap">EnableMMAP</a><br />

         <a

        href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a><br />

         <a

        href="../mod/prefork.html#maxspareservers">MaxSpareServers</a><br />

    determined by reading this single file, rather than having to

    scan the directory for files.</p>

    <h4>Memory-mapping</h4>

    <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 mmap(2).

    </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>On some operating systems, mmap does not scale as well as

      read(2) when the number of CPUs increases.  On multiprocessor

      Solaris servers, for example, Apache 2.0 sometimes delivers

      server-parsed files faster when mmap is disabled.</li>

      <li>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.</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>

    <h4>Process Creation</h4>

    <p>Prior to Apache 1.3 the <code>MinSpareServers</code>,