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Do threads blocked by a std::mutex::lock() or a condition variable sleep in a way that frees the core for other processes, or am I required to manually put these threads to sleep? And if true, would std::mutex::try_lock() allow for a way to spin the thread without sleeping?
The reason I ask: I want to have three states for threads in my thread pool that are unused: spinning for 2 milliseconds, then locked by a mutex for 250-ish milliseconds (assuming this lets them sleep and unhog the core), then finally being deallocated.
I want to avoid calling sleep manually if I can help it, tuning the sleep duration would be hard. So can I safely leave that to the mutex?
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Mutexes are used to protect shared resources. If the mutex is already locked by another thread, the thread waits for the mutex to become available. The thread that has locked a mutex becomes its current owner and remains the owner until the same thread has unlocked it.
If you don't, then a typical mutex is held in process memory and will simply cease to exist along with anything that might have access to it when the process terminates.
If a thread which had already locked a mutex, tries to lock the mutex again, it will enter into waiting list of that mutex which results in deadlock.
Mutexes are used to prevent multiple threads from causing a data race by accessing shared resources at the same time. Sometimes, when locking mutexes, multiple threads hold each other's lock, and the program consequently deadlocks. Four conditions are required for deadlock to occur: Mutual exclusion.
That is implementation specific; the C++ standard does not speak to it directly.
In practice, mutexes may use a combination of spin lock and full sleep. Sleeping and waking up is relatively expensive, and a compiler may write the locks to spin for a few ms before putting the thread to sleep.
No C++ implementation on a major phone, PC or big iron is going to spin lock indefinitely however. I could imagine some embedded system doing so, but have not personally encountered one.
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Yes. Such blocked threads sleep and don't take up any CPU cycles.
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