How Do std::hardware_destructive_interference_size and std::hardware_constructive_interference_size Help Optimize Memory Access?

Understanding std::hardware_destructive_interference_size and std::hardware_constructive_interference_size

Introduction

C 17 introduced two static constexpr constants, std::hardware_destructive_interference_size and std::hardware_constructive_interference_size, to provide information about the cache-line size. However, these constants have a broader purpose beyond simply getting the L1 cache-line size.

Relationship to L1 Cache Line Size

The intention of these constants is to provide values that represent the optimal offset or limit for data structures to avoid false-sharing or promote true-sharing, respectively. While in theory, these values should align well with the L1 cache-line size, it's not guaranteed to be the case in practice.

Use Cases

These constants can be used in various scenarios:

• Avoiding Destructive Interference (False-Sharing): By ensuring that objects that experience temporally disjoint access patterns are placed far enough apart in memory (equivalent to hardware_destructive_interference_size), false-sharing can be mitigated.
• Promoting Constructive Interference (True-Sharing): By allocating objects within a size and alignment that aligns with hardware_constructive_interference_size, it can help to ensure that the objects are placed close together in memory, promoting data sharing and reducing cache misses.

Limitations and Precautions

These constants are defined at compile time and do not necessarily represent the actual cache-line size at runtime. Different machines can have different cache-line sizes.

If maximizing performance is a critical requirement, it's advisable to define a precise cache-line size value using preprocessor macros or by using platform-specific libraries that detect the cache-line size at runtime.

Example Program:

The example program provided demonstrates how these constants can be used effectively. It demonstrates false-sharing by allocating an array of int wrappers with different alignments and a pair of ints with different alignments, showcasing the impact on performance.

The program also includes a utility function, cache_line_size(), which serves as a fallback or can be redefined during compilation to use a known L1 cache-line size if available.

By understanding these constants and using them appropriately, you can optimize your code for efficient memory access and improved performance.

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