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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>
This commit is contained in:
Joel Fernandes (Google) 2018-09-21 18:31:53 -04:00 committed by Paul E. McKenney
parent 832aa35a65
commit 2d0350a8f0
1 changed files with 5 additions and 3 deletions
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8
Documentation/RCU/Design/Data-Structures/Data-Structures.html
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@ -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