This commit adds a couple of paragraphs to the description of RCU's combining tree explaining how the combining tree keeps lock contention acceptably low, despite RCU grace periods being global operations.
Signed-off-by: Paul E. McKenney <[email protected]> --- .../Design/Data-Structures/Data-Structures.html | 23 ++++++++++++++++++++++ .../Design/Data-Structures/Data-Structures.htmlx | 23 ++++++++++++++++++++++ 2 files changed, 46 insertions(+) diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html index ba9fbb5177f6..d15744b87b99 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -100,6 +100,29 @@ 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. +</p><p>This multi-level combining tree allows us to get most of the +performance and scalability +benefits of partitioning, even though RCU grace-period detection is +inherently a global operation. +The trick here is that only the last CPU to report a quiescent state +into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt> +structure at the next level up the tree. +This means that at the leaf-level <tt>rcu_node</tt> structure, only +one access out of sixteen will progress up the tree. +For the internal <tt>rcu_node</tt> structures, the situation is even +more extreme: Only one access out of sixty-four will progress up +the tree. +Because the vast majority of the CPUs do not progress up the tree, +the lock contention remains roughly constant up the tree. +No matter how many CPUs there are in the system, at most 64 quiescent-state +reports per grace period will progress all the way to the root +<tt>rcu_node</tt> structure, thus ensuring that the lock contention +on that root <tt>rcu_node</tt> structure remains acceptably low. + +</p><p>In effect, the combining tree acts like a big shock absorber, +keeping lock contention under control at all tree levels regardless +of the level of loading on the system. + </p><p>The Linux kernel actually supports multiple flavors of RCU running concurrently, so RCU builds separate data structures for each flavor. diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx b/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx index c08fd8e9574a..8e88e3e7e2ef 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx @@ -121,6 +121,29 @@ 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. +</p><p>This multi-level combining tree allows us to get most of the +performance and scalability +benefits of partitioning, even though RCU grace-period detection is +inherently a global operation. +The trick here is that only the last CPU to report a quiescent state +into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt> +structure at the next level up the tree. +This means that at the leaf-level <tt>rcu_node</tt> structure, only +one access out of sixteen will progress up the tree. +For the internal <tt>rcu_node</tt> structures, the situation is even +more extreme: Only one access out of sixty-four will progress up +the tree. +Because the vast majority of the CPUs do not progress up the tree, +the lock contention remains roughly constant up the tree. +No matter how many CPUs there are in the system, at most 64 quiescent-state +reports per grace period will progress all the way to the root +<tt>rcu_node</tt> structure, thus ensuring that the lock contention +on that root <tt>rcu_node</tt> structure remains acceptably low. + +</p><p>In effect, the combining tree acts like a big shock absorber, +keeping lock contention under control at all tree levels regardless +of the level of loading on the system. + </p><p>The Linux kernel actually supports multiple flavors of RCU running concurrently, so RCU builds separate data structures for each flavor. -- 2.5.2
