Where are the Locks Stored in a `std::atomic` Implementation, and What are the Implications for Multiple Instances?

Where is the Lock for a std::atomic?

Question:

Is it true that larger atomic data structures use locks for their atomic operations, and if so, where is this lock stored and what are the implications for multiple instances of the atomic variable?

Answer:

The implementation of locks for std::atomic variables typically involves a hash table of mutexes or spinlocks to serialise access to each atomic object. The address of the atomic object is used as a key to retrieve the associated mutex from the hash table.

Hash collisions may occur, causing multiple atomic objects to share the same lock. While this is not a correctness issue, it can lead to performance degradation due to increased contention for the lock.

Implications for Multiple Instances:

The locking mechanism ensures that only one thread can access an atomic variable at any given time. However, it's important to note that locks are per-process, rather than per-object. This means that multiple instances of an atomic variable in different processes will have their own dedicated locks and operate independently.

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