doc: Clarify RCU data-structure comment about rcu_tree fanout

RCU Data-Structures document describes a trick to test RCU with small number of CPUs but with a taller tree. It wasn't immediately clear how the document arrived at 16 CPUs which also requires setting the FANOUT_LEAF to 2 instead of the default of 16. This commit therefore provides the needed clarification. Signed-off-by: Joel Fernandes (Google) <[email protected]> Cc: <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]>
pointlessone · Nov 9, 2018 · 2d0350a · 2d0350a
1 parent 832aa35
commit 2d0350a
Showing 1 changed file with 5 additions and 3 deletions.
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -127,9 +127,11 @@ <h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
 </p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
 accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
 32-bit systems.
-On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
-as small as 2 if you wish, which would permit only 16 CPUs, which
-is useful for testing.
+On the other hand, you can set both <tt>CONFIG_RCU_FANOUT</tt> and
+<tt>CONFIG_RCU_FANOUT_LEAF</tt> to be as small as 2, which would result
+in a 16-CPU test using a 4-level tree.
+This can be useful for testing large-system capabilities on small test
+machines.
 
 </p><p>This multi-level combining tree allows us to get most of the
 performance and scalability