How can `std::hardware_destructive_interference_size` and `std::hardware_constructive_interference_size` be used to optimize memory access and performance in C 17?

Understanding std::hardware_destructive_interference_size and std::hardware_constructive_interference_size

Introduction

In C 17, the addition of std::hardware_destructive_interference_size and std::hardware_constructive_interference_size provides a portable way to optimize memory access and avoid performance pitfalls.

Relationship to L1 Cache Line Size

These constants are typically related to the L1 cache line size, which is the minimum unit of data that is transferred between the CPU and cache. By aligning and organizing data structures according to these sizes, we can avoid conflicts and improve performance.

Use Cases

• Destructive interference: When multiple objects with temporally disjoint runtime access patterns are placed closely together, they can interfere with each other's cache lines, causing performance degradation. Using std::hardware_destructive_interference_size as a padding or alignment value can mitigate this issue.
• Constructive interference: When objects with temporally local runtime access patterns are placed closely together, they can share the same cache line, improving performance. std::hardware_constructive_interference_size helps determine the optimal size and alignment for these objects.

Challenges with Static Constants

These constants are defined as static constexpr, which means they are evaluated at compile time. However, this introduces a challenge: the computed values may not align perfectly with the cache line size of the target machine at runtime.

Workarounds

To address this, we can define our own constant values based on known system characteristics if available. Alternatively, we can use platform-specific hints or libraries that detect the cache line size dynamically.

Example

The following code shows a simple example that demonstrates how these constants can be used:

#include <iostream>

using namespace std;

int main() {
  // Assuming hardware_destructive_interference_size and hardware_constructive_interference_size are defined
  int x1[hardware_destructive_interference_size]; // Avoid false sharing
  int x2[hardware_destructive_interference_size / 2]; // Potential false sharing
  int y1[hardware_constructive_interference_size]; // Promote true sharing
  pair<int, int> y2; // Potential true sharing

  // Use these arrays to store data and observe the performance difference due to alignment issues

  return 0;
}

Conclusion

std::hardware_destructive_interference_size and std::hardware_constructive_interference_size provide a valuable tool for optimizing memory access and avoiding cache line conflicts. However, it is important to be aware of the challenges associated with static constants and consider appropriate workarounds to ensure optimal performance on different platforms.

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