Why Use `memory_order_seq_cst` to Set a Stop Flag When Checking It with `memory_order_relaxed`?

Why Set the Stop Flag Using memory_order_seq_cst If You Check It with memory_order_relaxed?

When setting up multi-threaded applications, it's important to consider the memory order when accessing shared data. In situations where you have a stop flag that multiple worker threads check to determine when to stop working, you might wonder why it's recommended to use memory_order_seq_cst when setting the stop flag, even though the worker threads are checking it with memory_order_relaxed.

Understanding Memory Ordering

memory_order_seq_cst (sequentially consistent) is the strongest memory ordering, ensuring that operations appear in program order and are visible to all threads immediately. memory_order_relaxed, on the other hand, is the weakest memory ordering, allowing threads to see operations in a different order than they were performed.

Rationale for Using memory_order_seq_cst for Stop Flag

While it may seem that using memory_order_relaxed for both setting and checking the stop flag would suffice, there are a few reasons why using memory_order_seq_cst for setting the stop flag is recommended:

1. Visibility: Using memory_order_seq_cst ensures that the store operation for setting the stop flag becomes visible to all threads as soon as it's executed. This means that any thread that checks the flag will immediately see the updated value, eliminating the risk of a worker thread continuing to run after the stop flag has been set.

2. Coherency: memory_order_seq_cst guarantees that all threads see the same value for the stop flag. This prevents any inconsistent behavior or data race conditions that could occur if different threads were seeing different versions of the flag.

Impact on Latency

It's worth noting that there is no significant latency benefit to using memory_order_seq_cst. The ISO C standard allows implementations to implement atomic operations with different latencies depending on the memory ordering, but on modern hardware, the difference is typically negligible.

Conclusion

While memory_order_relaxed can be used for checking the stop flag, it's generally recommended to set the stop flag using memory_order_seq_cst. This ensures immediate visibility and coherency of the shared stop flag, minimizing the risk of race conditions and ensuring consistent behavior across all worker threads.

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