Module Name: src
Committed By: ad
Date: Mon Mar 23 21:26:00 UTC 2009
Modified Files:
src/share/man/man9: rwlock.9
Log Message:
Add "PERFORMANCE CONSIDERATIONS"
To generate a diff of this commit:
cvs rdiff -u -r1.10 -r1.11 src/share/man/man9/rwlock.9
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/share/man/man9/rwlock.9
diff -u src/share/man/man9/rwlock.9:1.10 src/share/man/man9/rwlock.9:1.11
--- src/share/man/man9/rwlock.9:1.10 Wed Apr 30 13:10:58 2008
+++ src/share/man/man9/rwlock.9 Mon Mar 23 21:26:00 2009
@@ -1,6 +1,6 @@
-.\" $NetBSD: rwlock.9,v 1.10 2008/04/30 13:10:58 martin Exp $
+.\" $NetBSD: rwlock.9,v 1.11 2009/03/23 21:26:00 ad Exp $
.\"
-.\" Copyright (c) 2006, 2007 The NetBSD Foundation, Inc.
+.\" Copyright (c) 2006, 2007, 2009 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
@@ -27,7 +27,7 @@
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
-.Dd December 4, 2007
+.Dd March 23, 2009
.Dt RWLOCK 9
.Os
.Sh NAME
@@ -74,10 +74,6 @@
.Pp
In addition to the capabilities provided by mutexes, RW locks distinguish
between read (shared) and write (exclusive) access.
-RW locks are intended to provide protection for kernel data
-or objects that are read much more frequently than updated.
-For objects that are updated as frequently as they are read, mutexes should
-be used to guarantee atomic access.
.Pp
RW locks are in one of three distinct states at any given time:
.Bl -tag -width cdosrunrundo
@@ -182,6 +178,27 @@
These functions must never be used to make locking decisions at run time:
they are provided only for diagnostic purposes.
.El
+.Sh PERFORMANCE CONSIDERATIONS
+RW locks are subject to high cache contention on multiprocessor systems,
+and scale poorly when the write:read ratio is not strongly in favour of
+readers.
+Ideally, RW locks should only be used in settings when the following three
+conditions are met:
+.Bl -bullet
+.It
+The data object(s) protected by the RW lock are read much more frequently
+than written.
+.It
+The read-side hold time for the RW lock is long (in the order of thousands
+of processor clock cycles).
+.It
+Strong synchronization semantics are required: there is no scope for
+lockless, lazy or optimisitic synchronization.
+.El
+.Pp
+Generally speaking, it is better to organise code paths and/or
+data flows such that fewer and weaker synchronization points are required
+to ensure correct operation.
.Sh CODE REFERENCES
This section describes places within the
.Nx
