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authorJoel Fernandes (Google) <joel@joelfernandes.org>2018-09-22 01:31:53 +0300
committerPaul E. McKenney <paulmck@linux.ibm.com>2018-11-09 08:44:41 +0300
commit2d0350a8f0e6eb5494141c61c5c749b5155df33d (patch)
tree18a46122a843ca3c86f50b33bf8bd23fa0c4e6b9 /Documentation/RCU/Design
parent832aa35a65bac800a1adbf2eab0b42427032cab8 (diff)
downloadlinux-2d0350a8f0e6eb5494141c61c5c749b5155df33d.tar.xz
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@joelfernandes.org> Cc: <kernel-team@android.com> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Diffstat (limited to 'Documentation/RCU/Design')
-rw-r--r--Documentation/RCU/Design/Data-Structures/Data-Structures.html8
1 files 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