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Date: Fri, 21 Sep 2018 18:14:28 -0700
From: "Paul E. McKenney"
To: "Joel Fernandes (Google)"
Cc: linux-kernel@vger.kernel.org, Jonathan Corbet ,
Josh Triplett ,
Lai Jiangshan ,
linux-doc@vger.kernel.org,
Mathieu Desnoyers ,
Steven Rostedt
Subject: Re: [PATCH RFC] Documentation: RCU: Clarify comment about fanout
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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)
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)
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)
Signed-off-by: Paul E. McKenney
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 rcu_node tree as follows:
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 CONFIG_RCU_FANOUT 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 CONFIG_RCU_FANOUT and
+CONFIG_RCU_FANOUT_LEAF 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.
This multi-level combining tree allows us to get most of the
performance and scalability