Add arena-specific and selective dss allocation.

Add the "arenas.extend" mallctl, so that it is possible to create new
arenas that are outside the set that jemalloc automatically multiplexes
threads onto.

Add the ALLOCM_ARENA() flag for {,r,d}allocm(), so that it is possible
to explicitly allocate from a particular arena.

Add the "opt.dss" mallctl, which controls the default precedence of dss
allocation relative to mmap allocation.

Add the "arena.<i>.dss" mallctl, which makes it possible to set the
default dss precedence on a per arena or global basis.

Add the "arena.<i>.purge" mallctl, which obsoletes "arenas.purge".

Add the "stats.arenas.<i>.dss" mallctl.

doc/jemalloc.xml.in

@@ -368,6 +368,15 @@ for (i = 0; i < nbins; i++) {
             object.  This constraint can apply to both growth and
             shrinkage.</para></listitem>
           </varlistentry>
+          <varlistentry>
+            <term><constant>ALLOCM_ARENA(<parameter>a</parameter>)
+            </constant></term>
+
+            <listitem><para>Use the arena specified by the index
+            <parameter>a</parameter>.  This macro does not validate that
+            <parameter>a</parameter> specifies an arena in the valid
+            range.</para></listitem>
+          </varlistentry>
         </variablelist>
       </para>
 
@@ -785,15 +794,29 @@ for (i = 0; i < nbins; i++) {
         chunk size is 4 MiB (2^22).</para></listitem>
       </varlistentry>
 
+      <varlistentry id="opt.dss">
+        <term>
+          <mallctl>opt.dss</mallctl>
+          (<type>const char *</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para>dss (<citerefentry><refentrytitle>sbrk</refentrytitle>
+        <manvolnum>2</manvolnum></citerefentry>) allocation precedence as
+        related to <citerefentry><refentrytitle>mmap</refentrytitle>
+        <manvolnum>2</manvolnum></citerefentry> allocation.  The following
+        settings are supported: &ldquo;disabled&rdquo;, &ldquo;primary&rdquo;,
+        and &ldquo;secondary&rdquo; (default).</para></listitem>
+      </varlistentry>
+
       <varlistentry id="opt.narenas">
         <term>
           <mallctl>opt.narenas</mallctl>
           (<type>size_t</type>)
           <literal>r-</literal>
         </term>
-        <listitem><para>Maximum number of arenas to use.  The default maximum
-        number of arenas is four times the number of CPUs, or one if there is a
-        single CPU.</para></listitem>
+        <listitem><para>Maximum number of arenas to use for automatic
+        multiplexing of threads and arenas.  The default is four times the
+        number of CPUs, or one if there is a single CPU.</para></listitem>
       </varlistentry>
 
       <varlistentry id="opt.lg_dirty_mult">
@@ -1149,11 +1172,8 @@ malloc_conf = "xmalloc:true";]]></programlisting>
           <literal>rw</literal>
         </term>
         <listitem><para>Get or set the arena associated with the calling
-        thread.  The arena index must be less than the maximum number of arenas
-        (see the <link
-        linkend="arenas.narenas"><mallctl>arenas.narenas</mallctl></link>
-        mallctl).  If the specified arena was not initialized beforehand (see
-        the <link
+        thread.  If the specified arena was not initialized beforehand (see the
+        <link
         linkend="arenas.initialized"><mallctl>arenas.initialized</mallctl></link>
         mallctl), it will be automatically initialized as a side effect of
         calling this interface.</para></listitem>
@@ -1245,13 +1265,40 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         the developer may find manual flushing useful.</para></listitem>
       </varlistentry>
 
+      <varlistentry id="arena.i.purge">
+        <term>
+          <mallctl>arena.&lt;i&gt;.purge</mallctl>
+          (<type>unsigned</type>)
+          <literal>--</literal>
+        </term>
+        <listitem><para>Purge unused dirty pages for arena &lt;i&gt;, or for
+        all arenas if &lt;i&gt; equals <link
+        linkend="arenas.narenas"><mallctl>arenas.narenas</mallctl></link>.
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry id="arena.i.dss">
+        <term>
+          <mallctl>arena.&lt;i&gt;.dss</mallctl>
+          (<type>const char *</type>)
+          <literal>rw</literal>
+        </term>
+        <listitem><para>Set the precedence of dss allocation as related to mmap
+        allocation for arena &lt;i&gt;, or for all arenas if &lt;i&gt; equals
+        <link
+        linkend="arenas.narenas"><mallctl>arenas.narenas</mallctl></link>.  See
+        <link linkend="opt.dss"><mallctl>opt.dss</mallctl></link> for supported
+        settings.
+        </para></listitem>
+      </varlistentry>
+
       <varlistentry id="arenas.narenas">
         <term>
           <mallctl>arenas.narenas</mallctl>
           (<type>unsigned</type>)
           <literal>r-</literal>
         </term>
-        <listitem><para>Maximum number of arenas.</para></listitem>
+        <listitem><para>Current limit on number of arenas.</para></listitem>
       </varlistentry>
 
       <varlistentry id="arenas.initialized">
@@ -1370,6 +1417,16 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         for all arenas if none is specified.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term>
+          <mallctl>arenas.extend</mallctl>
+          (<type>unsigned</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para>Extend the array of arenas by appending a new arena,
+        and returning the new arena index.</para></listitem>
+      </varlistentry>
+
       <varlistentry id="prof.active">
         <term>
           <mallctl>prof.active</mallctl>
@@ -1538,6 +1595,20 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         </para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term>
+          <mallctl>stats.arenas.&lt;i&gt;.dss</mallctl>
+          (<type>const char *</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para>dss (<citerefentry><refentrytitle>sbrk</refentrytitle>
+        <manvolnum>2</manvolnum></citerefentry>) allocation precedence as
+        related to <citerefentry><refentrytitle>mmap</refentrytitle>
+        <manvolnum>2</manvolnum></citerefentry> allocation.  See <link
+        linkend="opt.dss"><mallctl>opt.dss</mallctl></link> for details.
+        </para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term>
           <mallctl>stats.arenas.&lt;i&gt;.nthreads</mallctl>