machine-key

Prints this machine’s hardware fingerprint — the key every engine’s history is partitioned by.

cargo bench-history machine-key

Every engine’s numbers are machine-dependent in practice — Criterion wall-clock time and all_the_time CPU time obviously, but Callgrind instruction counts and alloc_tracker allocations too, because libraries dispatch to different code paths on different microarchitectures. So every engine’s history is partitioned by this machine key, and runs from different machines are never mixed.

--verbose additionally reports the factors behind the key on standard error — the fingerprint version, the processor and memory-region counts, and the processor models — for tracing a key change to the hardware detail that moved. Facts that are merely recorded beside a run as provenance, per-processor speeds among them, are left out, so what is listed is exactly what the key depends on.

The key hashes only properties of the hardware: the processor and memory-region counts and the distinct processor models present. Readings that the same machine can report differently from one boot to the next — per-processor speeds above all, which are boot-time calibration figures rather than hardware identity — are deliberately excluded. It takes very little drift to do damage: a GitHub-hosted ARM64 Windows runner reports all four of its Cobalt 100 processors calibrated at 10678 on most boots and one of the four at 10681 on others, and hashing that reading would fork the runner’s history between the two. That is exactly the fragmentation the key exists to prevent, and the speeds add no discriminating power the processor models do not already carry. They are recorded beside a run as provenance instead whenever its host was probed, so a machine’s speed mix is answered from its stored runs rather than from its key.

The key is version-tagged, so a change to the factors it hashes forks stored history into a new partition rather than silently mixing incomparable data.

See Comparability and partitioning for how the machine key fits into the discriminant set.