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@@ -1,3 +1,60 @@ +2005-03-13 Ross Johnson <rpj at callisto.canberra.edu.au> + + * pthread_once.c (pthread_once): Completely redesigned; a change was + required to the ABI (pthread_once_t_), and resulting in a version + compatibility index increment. + + NOTES: + The design (based on pseudo code contributed by Gottlob Frege) avoids + creating a kernel object if there is no contention. See URL for details:- + http://sources.redhat.com/ml/pthreads-win32/2005/msg00029.html + This uses late initialisation similar to the technique already used for + pthreads-win32 mutexes and semaphores (from Alexander Terekhov). + + The subsequent cancelation cleanup additions (by rpj) could not be implemented + without sacrificing some of the efficiency in Gottlob's design. In particular, + although each once_control uses it's own event to block on, a global CS is + required to manage it - since the event must be either re-usable or + re-creatable under cancelation. This is not needed in the non-cancelable + design because it is able to mark the event as closed (forever). + + When uncontested, a CS operation is equivalent to an Interlocked operation + in speed. So, in the final design with cancelability, an uncontested + once_control operation involves a minimum of five interlocked operations + (including the LeaveCS operation). + + ALTERNATIVES: + An alternative design from Alexander Terekhov proposed using a named mutex, + as sketched below:- + + if (!once_control) { // May be in TLS + named_mutex::guard guard(&once_control2); + if (!once_control2) { + <init> + once_control2 = true; + } + once_control = true; + } + + A more detailed description of this can be found here:- + http://groups.yahoo.com/group/boost/message/15442 + + [Although the definition of a suitable PTHREAD_ONCE_INIT precludes use of the + TLS located flag, this is not critical.] + + There are three primary concerns though:- + 1) The [named] mutex is 'created' even in the uncontended case. + 2) A system wide unique name must be generated. + 3) Win32 mutexes are VERY slow even in the uncontended case. An uncontested + Win32 mutex lock operation can be 50 (or more) times slower than an + uncontested EnterCS operation. + + Ultimately, the named mutex trick is making use of the global locks maintained + by the kernel. + + * pthread.h (pthread_once_t_): One flag and an event HANDLE added. + (PTHREAD_ONCE_INIT): Additional values included. + 2005-03-08 Ross Johnson <rpj at callisto.canberra.edu.au>
* pthread_once.c (pthread_once): Redesigned to elliminate potential |