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deepmind.google·6d ago
AI Engineering
Lobsters·29d ago·5 min read
Why false sharing alignment should be 128 bytes on x64
TL;DR
False sharing occurs when multiple atomic variables share the same cache line, leading to inefficient CPU coordination. To mitigate this, aligning atomic variables to 128 bytes instead of the standard 64 bytes is recommended.
✦ Why It Matters
Align atomic variables to 128 bytes to reduce false sharing and improve multi-threaded application performance.
Key Takeaways
- Identify atomic variables that may share cache lines.
- Use alignment directives to separate atomic variables by 128 bytes.
- Review performance analysis tools to measure false sharing impact.
- Adopt libraries like crossbeam-utils for better multi-threading support.
Full Summary
False sharing is a performance issue that arises when multiple atomic variables are located on the same cache line, causing excessive coordination between CPU cores. Each update to an atomic variable makes the entire cache line dirty, leading to inefficient memory access patterns.
While the standard cache line size for x86_64 is 64 bytes, libraries like crossbeam-utils and Facebook's folly suggest using 128 bytes for alignment. This increased spacing helps ensure that atomic variables do not share cache lines, thereby reducing contention.
The recommendation is based on performance analysis and tuning practices that highlight the benefits of minimizing false sharing. By adopting this alignment strategy, developers can enhance the efficiency of multi-threaded applications, particularly in high-performance computing scenarios.