Add docs
Document the various ways to start profiling.
diff --git a/docs/reference/gtk/running.sgml b/docs/reference/gtk/running.sgml
index ae17857..044ab55 100644
--- a/docs/reference/gtk/running.sgml
+++ b/docs/reference/gtk/running.sgml
@@ -574,6 +574,32 @@
</refsect2>
+<refsect2 id="profiling">
+ <title>Profiling</title>
+
+ <para>
+ GTK supports profiling with sysprof. It exports timing information
+ about frameclock phases and various characteristics of GskRenders
+ in a format that can be displayed by sysprof or GNOME Builder.
+ </para>
+ <para>
+ A simple way to capture data is to set the <envar>GTK_TRACE</envar>
+ environment variable. When it is set, GTK will write profiling
+ data to a file called
+ <filename>gtk.<replaceable>PID</replaceable>.syscap</filename>.
+ </para>
+ <para>
+ When launching the application from sysprof, it will set the
+ <envar>SYSPROF_TRACE_FD</envar> environment variable to point
+ GTK at a file descriptor to write profiling data to.
+ </para>
+ <para>
+ When GtkApplication registers with D-Bus, it exports the
+ <literal>org.gnome.Sysprof2.Profiler</literal> interface
+ that lets sysprof request profiling data at runtime.
+ </para>
+</refsect2>
+
</refsect1>
</refentry>