Do Memory Barriers Speed Up Atomic Operations in a Producer-Consumer Queue?

Does a Memory Barrier Enhance Visibility Speed of Atomic Operations in Addition to Guaranteeing Their Execution Order?

In the context of a producer-consumer queue, it is often the case that the data stored by the producer is intended to be visible to the consumer as quickly as possible. One might wonder if adding a hardware memory fence between the producer's store operation and the consumer's load operation would help achieve this objective.

However, adding a memory fence does not significantly impact the latency of atomic operations in a multi-core system. The reason for this is that the hardware already ensures visibility of the store operation to all other cores, regardless of whether a memory fence is present.

What Happens Without a Fence?

In the absence of a fence, the producer's store operation with a release memory order is guaranteed to become visible to all other cores at some point in the future. On x86 architectures, there are no hardware barriers present, while on ARM architectures, fences are placed before the store operation (on the producer side) and after the load operation (on the consumer side).

Even though no hardware fences are used in the case of x86, the value stored by the producer without a fence will eventually be observed by the load operation without a fence. This process may require a few unsuccessful load attempts, but it will eventually succeed.

Effects of Memory Barriers on Latency

The addition of a memory barrier does not typically reduce the latency of observing the stored value for the following reasons:

• Store Buffer Flush: Memory barriers do not force the store buffer to commit data to the cache. Instead, they stall the issuing core, preventing it from performing subsequent memory operations until the store buffer has been drained.
• Invisibility of Store Buffer: The store buffer is invisible to other cores, meaning that the store operation becomes visible when it commits to the L1 data cache.
• Inefficiency of Blind Fencing: Using memory barriers indiscriminately without careful profiling can actually harm performance due to unnecessary stalls.

Conclusion

In most cases, adding an unnecessary memory barrier between atomic operations in a producer-consumer queue does not improve latency. The hardware already guarantees visibility of atomic operations without the need for explicit memory barriers. Profiling is essential to identify situations where a memory barrier is beneficial, and it should be used only when necessary.

The above is the detailed content of Do Memory Barriers Speed Up Atomic Operations in a Producer-Consumer Queue?. For more information, please follow other related articles on the PHP Chinese website!