From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: linux-kernel@vger.kernel.org
Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com,
bobby.prani@gmail.com,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention
Date: Tue, 23 Feb 2016 21:00:46 -0800 [thread overview]
Message-ID: <1456290047-16654-13-git-send-email-paulmck@linux.vnet.ibm.com> (raw)
In-Reply-To: <20160224050021.GA14616@linux.vnet.ibm.com>
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 <paulmck@linux.vnet.ibm.com>
---
.../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
next prev parent reply other threads:[~2016-02-24 5:18 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores Paul E. McKenney
2016-02-24 21:12 ` Mathieu Desnoyers
2016-02-24 21:40 ` Paul E. McKenney
2016-02-25 6:41 ` Jianyu Zhan
2016-02-25 14:08 ` Paul E. McKenney
2016-02-25 8:21 ` Peter Zijlstra
2016-02-25 14:07 ` Paul E. McKenney
2016-02-25 14:48 ` Peter Zijlstra
2016-02-25 15:42 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference() Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 08/14] documentation: Add alternative release-acquire outcome Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 09/14] documentation: Add documentation for RCU's major data structures Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier() Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs Paul E. McKenney
2016-02-24 5:00 ` Paul E. McKenney [this message]
2016-02-24 5:00 ` [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example Paul E. McKenney
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