Do Unnecessary Memory Barriers Improve Latency for Atomic Operations?

Does a Memory Barrier Enhance the Visibility of Atomic Operations Beyond Guaranteeing Consistency?

Modern hardware memory models enforce memory coherency and ensure visibility of atomic operations through the use of memory barriers. However, it is often assumed that adding additional memory barriers beyond those necessary for memory order requirements can improve latency.

Hardware Impact of Memory Barriers

x86 Architecture: On x86 platforms, memory barriers have no significant impact on the latency of atomic operations between cores. Memory barriers merely force the current core to wait for actions that would have occurred naturally, such as store buffer draining.

ARM Architecture: On ARM, memory barriers can introduce some overhead due to additional instructions, but this is typically minimal.

Effect on Latency

Adding unnecessary memory barriers or stronger memory orders does not typically result in improved latency. In fact, it can potentially worsen throughput by introducing unnecessary delays. The expected latency remains the same with or without additional barriers.

Exceptions

In rare cases, certain scenarios (e.g., conflict misses in cache) could theoretically benefit from additional barriers. However, these cases are highly specific and require careful profiling to identify.

Optimizing Memory Access

Uncontrolled barrier usage can have negative consequences for performance. Instead, consider the following optimization techniques:

• Prefetching: Preload data into the cache to reduce latency.
• Loop Unrolling: Break down large loops to expose more parallelism.
• SIMD Instructions: Leverage SIMD instructions for efficient data processing.

Conclusion

While memory barriers are crucial for ensuring correct memory behavior, adding unnecessary barriers does not improve visibility or latency for atomic operations. Careful profiling is recommended to identify any potential exceptions to this general guideline.

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