Do Hardware Memory Barriers Speed Up Visibility in Atomic Operations?

Does Hardware Memory Barrier Impact Visibility Speed in Atomic Operations?

Problem Description:

In a producer-consumer queue, can incorporating an unnecessary memory fence or stronger memory order enhance the visibility of atomic operations? The goal is to potentially reduce latency at the expense of possible throughput degradation.

Hardware Memory Barrier Functionality:

Hardware installs memory fences to enforce certain memory orders specified by the C memory model. These fences ensure that specific memory operations occur in a defined sequence and are visible to all other cores.

Impact on Atomic Operation Visibility:

While hardware memory barriers guarantee necessary visibility for atomic operations with the proper memory order, they generally do not significantly impact the latency of visibility. This is because CPUs prioritize demand loads, which retrieve data from memory when needed.

Why Barriers Do Not Improve Latency:

1. Store Buffer Drain: Barriers do not directly affect the speed at which the store buffer commits data to cache. The CPU already attempts to commit to cache as soon as possible to make data globally visible.
2. Load Prioritization: CPUs prioritize demand loads over other memory accesses. A barrier before a load could only delay it, not accelerate it.
3. Out-of-Order Execution: Out-of-order CPUs prioritize oldest-first, so later loads are unlikely to fill outstanding load buffers before the current load completes its request.

Exceptions and Caveats:

In certain scenarios, barriers may have minor secondary effects on visibility latency:

• Reducing cache miss penalties by preemptively writing back data to a higher cache level.
• PowerPC non-seq_cst operations can enable store-forwarding between logical cores.

Conclusion:

Unnecessary memory fences or stronger memory orders generally do not improve the latency of atomic operation visibility. CPUs optimize load and store operations to minimize latency without relying on barriers. Careful profiling is recommended to identify any potential bottlenecks and optimize code accordingly.

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