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Diffstat (limited to 'unsorted/PosixSemaphores.mdwn')
| -rw-r--r-- | unsorted/PosixSemaphores.mdwn | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/unsorted/PosixSemaphores.mdwn b/unsorted/PosixSemaphores.mdwn new file mode 100644 index 00000000..be5586bd --- /dev/null +++ b/unsorted/PosixSemaphores.mdwn @@ -0,0 +1,13 @@ +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 + +Moved page to Mach web. + +-- [[Main/GrantBow]] - 21 Jan 2003 |