When should I use _mm_sfence, _mm_lfence, and _mm_mfence in multi-threaded programming?

Determining the Appropriate Usage of _mm_sfence, _mm_lfence, and _mm_mfence

Introduction

In multi-threaded environments, it's crucial to maintain coherence and avoid memory ordering issues. To address these concerns, Intel provides intrinsic functions such as _mm_sfence, _mm_lfence, and _mm_mfence. However, understanding their proper usage can be challenging. This article aims to clarify the purpose and scenarios where these functions should be employed.

Understanding Store Ordering

NT Stores

_mm_sfence is primarily used in conjunction with NT (non-temporal) stores, a type of memory operation that offers increased performance for large data transfers. However, these stores have weaker memory ordering semantics, meaning there's no guarantee that other threads will observe the data immediately after it's written.

Normal Stores

Normal stores, on the other hand, have stronger memory ordering semantics. Therefore, _mm_sfence is not typically required in conjunction with normal stores.

Usage Recommendations

_mm_sfence

• Use _mm_sfence to enforce strong ordering semantics after NT store operations to ensure that the data is visible to other threads.
• It's particularly useful in scenarios where multiple threads may rely on the availability of recently written data.

_mm_lfence

• _mm_lfence has limited practical use as a load fence.
• It is more relevant in microarchitectural optimizations, such as preventing execution beyond a certain point until retirement.

_mm_mfence

• _mm_mfence is primarily useful in scenarios where you need to implement sequential consistency without using C 11 std::atomic.
• However, it's generally recommended to rely on C 11 std::atomic for memory ordering operations.

Additional Considerations

• Memory barriers do not accelerate the writing process; they merely ensure that following operations are delayed until the data is coherent in memory.
• _mm_sfence is considered the most practical barrier to use manually in C .
• The use-cases for _mm_lfence are more obscure and tend to be specific to low-level hardware optimizations.

Conclusion

Understanding the appropriate usage of _mm_sfence, _mm_lfence, and _mm_mfence is essential for effective memory management in multi-threaded code. By carefully evaluating the specific requirements of your application and utilizing these functions when necessary, you can avoid race conditions and ensure proper memory ordering, leading to reliable and high-performing code.

The above is the detailed content of When should I use _mm_sfence, _mm_lfence, and _mm_mfence in multi-threaded programming?. For more information, please follow other related articles on the PHP Chinese website!