On Fri, Sep 21, 2018 at 06:31:53PM -0400, Joel Fernandes (Google) wrote:
> RCU Data-Structures document describes a trick to test RCU with small
> number of CPUs but with a larger 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. Clarify that.
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Good catch, applied, thank you! You know, those two Kconfig options
are so closely linked in my mind that I doubt if I would ever have
noticed this issue. So good show!
I of course could not resist doing a bit of editing. Could you please
check the following to make sure that I didn't mess something up?
Thanx, Paul
------------------------------------------------------------------------
commit 9870c5b0f7b41a7b6085c20c22b320681800dabc
Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Date: Fri Sep 21 18:31:53 2018 -0400
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) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index 1d2051c0c3fc..476b1ac38e4c 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -127,9 +127,11 @@ CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
</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
