[+] Condvar and other reodering of thread primitives.txt
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Media/Condvar and other reodering of thread primitives.txt
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Media/Condvar and other reodering of thread primitives.txt
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/**
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* Footnotes:
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* "Wakes the count of currently sleeping threads without guaranteed respect for ordering."
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* "Assuming correctness of your mutex paths, this will wake all threads up-to your everyone-be-alert condition"
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* [...]
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* "Schedules a single thread for wake up without guaranteed respect for ordering.
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*
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* A simple explaination:
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*---------------------------------------------
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* Problematic broadcasts:
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*
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* Under mutex:
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* Sleep: [A, B, C]
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*
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* Under or out of mutex (it doesnt matter so long as: the write itself was under lock *or* the write condition was fenced against the conditions sleep count update via a later call to mutex::lock() ):
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* ~awake all? shutdown condition? who knows~
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* Broadcast
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*
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* Out of mutex (!!!):
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* if (~missed/incorrect if !work available check before sleep~)
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* Sleep: [D]
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* // given that WaitForSignal forces you to unlock and relock a mutex, this illogical branch should never happen
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*
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* Effect:
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* Awake: [B, C, D]
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* Sleeping: [A]
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*
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*---------------------------------------------
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* Problematic signals:
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*
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* Under mutex:
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* Sleep: [A, B, C]
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*
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* Not under mutex:
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* Signal
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*
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* Under mutex:
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* Sleep: [D]
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*
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* Effect:
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* Awake: [D]
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* Sleeping: [A, B, C]
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*
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*---------------------------------------------
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* Cause:
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* The abstract condition accounts for the amount of threads sleeping accuarely, not the order.
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* This is usually a good thing because ordering under a spinloop generally does not happen in time and/or does not matter.
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* To implement ordering, is to implement cache-thrashing and increased context-switching for an abstract idea of "correctness" that doesn't apply to real code or performance goals.
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* (spoilers: your work pool of uniform priorities couldn't care less which thread wakes up first, nor does a single waiter pattern, but your end product will certainly bleed performance with yield thrashing)
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* ( : the same can be said for semaphores; what part of waiting while an available work count is zero needs ordering?)
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*
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*---------------------------------------------
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* The fix[es]:
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* * Ensure to properly check the correctness of the sleep condition, and that the mutex is properly locked, before calling Aurora condition primitives
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* (why the fuck would you be sleeping on a variable state observer without checking its' the state, causing an unwanted deficit? this is counter to the purpose of using a condvar.)
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* * Increase the size of the condition variable to account for a counter and implement inefficient rescheduling, to fix buggy code
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* (no thanks. implement ticket primitives yourself, see: the hacky workaround.)
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*
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*---------------------------------------------
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* The hacky workaround:
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* * If you can somehow justify this, i doubt it, but if you can, you can force the slow-path order condvar+mutex+semaphore by using AuFutex[Mutex/Semaphore/Cond] with ThreadingConfig::bPreferEmulatedWakeOnAddress = true.
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* You can further eliminate the fast paths to remove fast-path reordering; but really, if you care that much, you should be implementing your own ticket primitives over AuThreading WakeOnAddress with bPreferEmulatedWakeOnAddress = true for the guarantee of a FIFO WoA interface.
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*/
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