Add "thread.idle" mallctl.

This can encapsulate various internal cleaning logic, and can be used to free up
resources before a long sleep.

doc/jemalloc.xml.in

@@ -1654,6 +1654,28 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         default.</para></listitem>
       </varlistentry>
 
+      <varlistentry id="thread.idle">
+        <term>
+          <mallctl>thread.idle</mallctl>
+          (<type>void</type>)
+          <literal>--</literal>
+        </term>
+        <listitem><para>Hints to jemalloc that the calling thread will be idle
+	for some nontrivial period of time (say, on the order of seconds), and
+	that doing some cleanup operations may be beneficial.  There are no
+	guarantees as to what specific operations will be performed; currently
+	this flushes the caller's tcache and may (according to some heuristic)
+	purge its associated arena.</para>
+	<para>This is not intended to be a general-purpose background activity
+	mechanism, and threads should not wake up multiple times solely to call
+	it.  Rather, a thread waiting for a task should do a timed wait first,
+	call <link linkend="thread.idle"><mallctl>thread.idle</mallctl><link> if
+	no task appears in the timeout interval, and then do an untimed wait.
+	For such a background activity mechanism, see
+	<link linked="background_thread"><mallctl>background_thread</mallctl></link>.
+	</para></listitem>
+      </varlistentry>
+
       <varlistentry id="tcache.create">
         <term>
           <mallctl>tcache.create</mallctl>