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| -rw-r--r-- | Mach/PosixSemaphores.mdwn | 9 |
1 files changed, 9 insertions, 0 deletions
diff --git a/Mach/PosixSemaphores.mdwn b/Mach/PosixSemaphores.mdwn new file mode 100644 index 00000000..c77cb08b --- /dev/null +++ b/Mach/PosixSemaphores.mdwn @@ -0,0 +1,9 @@ +Posix Semaphores are an optional part of pthreads. There is currently an implementation for Neal Walfields libpthread, which is included in the hurd sources tree. This implemention uses a mutex and a condition variable. The implmentation is in the mailing list archives at [ [http://mail.gnu.org/archive/html/bug-hurd/2002-11/msg00316.html](http://mail.gnu.org/archive/html/bug-hurd/2002-11/msg00316.html</a>)](http://mail.gnu.org/archive/html/bug-hurd/2002-11/msg00316.html). + +Neal does not want to use this implementation because it adds the overhead of a condition variable. The condition variable imposes the following penalties: 1 extra spinlock/unlock 1 an extra call to a pthread cleanup function. + +The first penalty has virtually no cost because we know that we will never spin trying to get this spin lock because we already have a mutex lock outside the condition variable serializing accesses to the condition variable. + +The second may be more of a performance penalty, but it saves reimplmenting the code in pt-cond-signal.c pt-cond-wait.c, and pthread-timedwait.c . + +-- [[Main/JamesAMorrison]] - 19 Jan 2003 |