What are the Differences Between Memory Ordering Options in Multi-threaded Programming?

Understanding the Differences Between Memory Ordering

In multi-threaded programming, atomic operations are used to ensure that data is accessed and modified in a controlled and consistent manner. However, the specific ordering of memory operations can have a significant impact on the program's behavior. The memory_order enumeration provides different ordering options, each with its own characteristics and implications.

memory_order_relaxed

This mode offers minimal synchronization and allows the compiler and hardware to optimize memory access and modify the order of operations. It is the least restrictive and comes with no guarantees of ordering beyond the individual atomic operation itself.

memory_order_consume

Contrary to the incorrect understanding presented in the question, memory_order_consume is effectively a more relaxed version of memory_order_acquire. It establishes a synchronization point for reading the atomic variable, but it does not guarantee ordering of relaxed variables written before it. However, if a thread modifies variable X and releases it using this mode, other threads consuming variable Y will see X released as well.

memory_order_acquire

This mode ensures that the read of the atomic variable is synchronized and that any relaxed writes that occurred before it are also synchronized. It does not necessarily mean that all atomic variables on all threads are synchronized.

memory_order_release

memory_order_release pushes the atomic store to other threads, but only if they read the variable using memory_order_consume or memory_order_acquire. It does not guarantee that other threads will immediately see the change.

memory_order_acq_rel

This mode combines memory_order_acquire and memory_order_release. It ensures that a read is not performed with an old value and that writes are released in order.

memory_order_seq_cst

memory_order_seq_cst is the most restrictive mode, enforcing sequential consistency. It guarantees that all atomic operations appear in the order they are executed, both within a single thread and across threads. It also forces updates to be visible in other threads, even if those updates were made using memory_order_relaxed.

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