What are the Different Memory Ordering Models for Atomic Variables?

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.

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