Display walltime benchmarks with subnanosecond precision

[the8472]

With modern CPUs running at more than one cycle per nanosecond the current precision is insufficient to resolve differences worth several cycles per iteration.

Granted, walltime benchmarks often are noisy but occasionally, especially when no allocations are involved, the difference really is just a few cycles.

example results when benchmarking 1-4 serialized ADD instructions and an empty bench body

running 4 tests
test add  ... bench:           0.24 ns/iter (+/- 0.00)
test add2 ... bench:           0.48 ns/iter (+/- 0.01)
test add3 ... bench:           0.72 ns/iter (+/- 0.01)
test add4 ... bench:           0.96 ns/iter (+/- 0.01)
test empty ... bench:           0.24 ns/iter (+/- 0.00)

[rustbot]

r? @jhpratt

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[rustbot]

Some changes occurred in run-make tests.

cc @jieyouxu

[jhpratt]

Do you know if there's a reason that it's always calculated in nanoseconds? Being most familiar with criterion (as are many others), I suspect that displaying ms, µs, ns, and ps as appropriate is reasonable.

[the8472]

Switching units when doing a before/after comparison would makes thing more difficult to eyeball.

[jhpratt]

@bors r+

[bors]

📌 Commit 2a7c42f has been approved by jhpratt

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2a7c42f with merge e93f342...

[bors]

☀️ Test successful - checks-actions
Approved by: jhpratt
Pushing e93f342 to master...

[rust-timer]

Finished benchmarking commit (e93f342): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.7%	[-2.7%, -2.7%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 674.951s -> 674.435s (-0.08%)
Artifact size: 315.92 MiB -> 315.80 MiB (-0.04%)