Pad Structs to Avoid False Sharing Between Threads

Align Hot Fields to Cache Lines

When each thread increments its own counter in a shared array, placing counters 8 bytes apart packs eight of them into one 64-byte line. Padding or aligning each counter to 64 bytes gives each thread a private line.

Padding Patterns

Use alignas(64) on struct fields or insert explicit char padding arrays. Verify sizeof and offsets with offsetof before trusting layout.

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• Over-padding increases memory but buys parallelism
• C++ std::hardware_destructive_interference_size documents line size when available
• False sharing also hits kernel per-CPU statistics arrays
• Pair padding fixes with perf c2c validation

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

• Trace first: walk one failing input step by step
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will pad a struct array so threads no longer false-share, then benchmark before and after. This exercise requires measuring contention drops, not just showing the layout change.