Understanding the Meaning of Memory Orderings

Atomic variables provide safe memory access and synchronization across threads. Understanding the different memory orderings is crucial to effectively utilize them.

Relaxed:

• No memory synchronization.
• Optimized operations that may reorder reads and writes.

Sequentially Consistent (seq_cst):

• Strictest ordering.
• Prevents reordering of any memory operations around an atomic operation.
• Ensures consistent memory access across all threads.

Acquire/Release:

• A hybrid between relaxed and seq_cst.
• Acquire: Precludes reordering of ordinary loads and stores before the atomic operation. Ensures consistency for the acquiring thread.
• Release: Prevents reordering of ordinary loads and stores after the atomic operation. Helps maintain coherence for other threads.

Consume (deprecated):

• Similar to acquire, but applies dependencies.
• Reordering restrictions apply only to data pointed to by modified atomic pointers.
• Provides limited optimization potential while increasing complexity.

Acquire-Release (acq_rel):

• Combines acquire and release.
• Ensures consistent read-write operations.
• Prevents modification of old data and ensures changes are propagated to other threads.