Defined in file: hzLock.h

Derivative of: hzLocker

hzLockRWD is a resource lock effected as a 'wrapper' based on a single mutex. The lock is granted to the first thread that requests it and until the winning thread releases the lock, all other threads seeking the lock have to wait. Mutex locks can be blocking or non-blocking. When blockng, all waiting threads are suspended. The CPU performs a context switch (swaps out the current thread and runs another in its place). When non-blocking the waiting threads loop or 'spin', constantly testing if the lock has become available and then seeking the look. This spinning only stops when the lock is granted. Both blocking and non-blocking locks can be subjected to a timeout but if they are, the calling thread must be able to contend with being denied access to the resource controlled by the lock. While spining round a loop is clearly a waste of time, it should be noted that thread suspension is also a waste of time - and in the general case, a much worse waste of time. Context switching is very expensive. For this reason the decision was taken that hzLockRWD should not support blocking. So hzLockRWD Lock() method never blocks and hzLockRWD is exclusively a spinlock. The Lock() method has a single optional argument of timeout (as an integer number of milliseconds), with a default of -1 (not applicable). A timeout of zero will try for a lock but imeadiately return with or without it. The return is E_OK for lock granted, but also E_OK if the lock is not initialized (not made active). If the timeout expires E_TIMEOUT is returned. Note that outside these circumstances, Lock() won't return! The purpose of the hzLockRWD class is essentially diagnostic - as otherwise a mutex is too simple to require a wrapper! hzLockRWD keeps track of contentions and of time wasted by those contentions. To this end, hzLockRWD instances must be named, either by passing a hzString or char* to the constructor or later with a call to the Init() method. Indeed without being named, a hzLockRWD is inactive and does not lock anything. The global function ContentionReport(hzChain& Z) reports on contentions of all mutexes and adds up time wasted in spinlocks by all threads.

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