Commit-ID: 2d0350a8f0e6eb5494141c61c5c749b5155df33d Gitweb: https://git.kernel.org/tip/2d0350a8f0e6eb5494141c61c5c749b5155df33d Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> AuthorDate: Fri, 21 Sep 2018 18:31:53 -0400 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> CommitDate: Thu, 8 Nov 2018 21:44:41 -0800 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> Cc: <kernel-team@xxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> --- Documentation/RCU/Design/Data-Structures/Data-Structures.html | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) 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
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