perf(go): make the Go numbers reproducible, and say which ones are not - #1034
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`go/README.md` led with a table nobody could regenerate — including us. This adds the harness for our own row: `mise run perf:go` builds a binder against mise's committed spec, binds the argv the Rust shadows use, and reports the cost. The protocol is `benches/gate`'s with one change the language forces. The Rust harness differences `PARSE_N=1` against `PARSE_N=0` and calls that a cold parse, which works because Rust's startup is deterministic to within a few hundred instructions. Go's is not: repeated `PARSE_N=0` runs here differ by ±50,000 instructions, twenty times the thing being measured, so differencing a single bind gives a number that changes sign between runs. Amortized over a thousand binds the jitter cancels — 1,500-1,700 instructions, stable to a few percent across runs — and the floor is reported *beside* it rather than subtracted once and forgotten. It is worth seeing: 890,000 instructions before `main`, three orders of magnitude above the bind, which is the real ceiling on what a Go parser can win. The three framework rows stay, marked as hand-measured against programs that are not in the repository, with the shadow generator named as what would fix that. Leaving them unmarked was the part worth correcting: a number in a table reads as a measurement whether or not anyone can repeat it. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Included review availability: Your plan provides up to 4 included reviews per hour; 2 remain after this review. 📝 WalkthroughWalkthroughThe change adds a Go repeated-bind benchmark, a ChangesGo performance benchmark
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to This change adds reproducible Go benchmark tooling and clarifies which documentation numbers are hand-measured; no actionable merge-blocking risk remains. Sequence Diagram(s)sequenceDiagram
participant Developer
participant Mise
participant PerfGo
participant ParseN
participant Valgrind
Developer->>Mise: run perf:go
Mise->>PerfGo: execute benchmark harness
PerfGo->>ParseN: build and run repeated-bind benchmark
ParseN-->>PerfGo: return binding validation
PerfGo->>Valgrind: measure instructions when available
Valgrind-->>PerfGo: return instruction counts
PerfGo-->>Developer: write Markdown benchmark results
Poem
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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Inline comments:
In `@go/README.md`:
- Line 25: Update the usage benchmark table in README so the header uses
“instructions” instead of “cold instructions,” and label the usage-go row as
amortized while preserving its existing values.
In `@tasks/perf-go.sh`:
- Line 29: Update the cleanup trap in the perf-go script so Cachegrind output is
written under the temporary directory associated with bin, and the EXIT trap
removes only that directory rather than matching cachegrind.out.* in the
caller’s working directory.
- Around line 52-58: Update the timing logic around the start and end timestamp
calls in the performance loop to use a timer source portable across supported
platforms, including macOS, or add an early prerequisite check that clearly
reports the requirement before arithmetic expansion occurs. Preserve the
existing millisecond average calculation in the awk command.
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go/README.mdgo/internal/bench/parse-n/main.gomise.tomltasks/perf-go.sh
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Instruction counts
No instruction-count regression above 1%. Only instruction counts gate. Wall clock is shown for context — on identical hardware it moves 4-20% run to run. Measured by tak — instruction-counted CLI benchmarks, stored in this repository's git notes. Shadow comparisonParsing
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… not own Three things review found in the harness, all of them things it does to the machine it runs on rather than things it measures. The trap globbed `cachegrind.out.*` in the caller's working directory. Those are not this script's files: one could be a report someone was reading, or one a concurrent valgrind was still writing. cachegrind is told to write into the temporary directory that goes away instead. `date +%s%N` is GNU's. BSD `date` prints a literal `N`, and the arithmetic then fails under `set -u` — *before* the valgrind check, so the wall-clock-only path meant for machines without cachegrind was the one path that could not run on a Mac. There is a python3 fallback now, and the column reports itself unavailable rather than being guessed at. Which clock is used is overridable, because a fallback that only runs on a machine nobody here has is a fallback nobody has run: all three paths were exercised through `PERF_GO_CLOCK`. And the table in the README said `instructions, cold` over a column where one row is amortized over a thousand binds and three are a single cold parse. Two different measurements under one heading, which is the thing the rest of that section is at pains to explain. Labelled per row now. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Inline comments:
In `@tasks/perf-go.sh`:
- Around line 59-80: The clock-selection and timestamp logic must reject unknown
PERF_GO_CLOCK values, validate that every now_ns result is decimal nanoseconds,
and emit unavailable when the selected gnu or py clock cannot produce a valid
reading instead of allowing an empty result or set -e failure. Update now_ns and
the override-selection block while preserving the existing gnu, py, and none
behaviors.
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…back The python fallback measured python. Reading the clock through two `python3` invocations put a whole interpreter startup — tens of milliseconds — inside an interval measuring ten runs of about one millisecond each, so the path written for machines without GNU `date` reported the interpreter rather than the program it was pointed at. It times the loop itself now, from one process, and the two paths agree where both are available: 1.25 ms through `date`, 1.03 ms through python, where before the second read 1.88. And nothing was checked. A `date` that answered with something other than digits made the arithmetic produce 0.00 ms, which reads as a measurement; an unknown `PERF_GO_CLOCK` did the same silently. Both reads are validated, an unknown override is refused by name, and a clock that cannot be read reports the column unavailable rather than fast. All four paths exercised: auto, gnu, py, none, plus the refusal. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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go/README.mdleads with a performance table, and none of it could be regenerated — including our own row. This adds the harness for the row we can measure, and marks the rows we cannot.Why it is not the Rust harness's protocol
tasks/perf-shadow.shdifferencesPARSE_N=1againstPARSE_N=0and calls the result a cold parse. That works in Rust, whose startup is deterministic to within a few hundred instructions. It does not work here:Go's runtime creates threads, starts the collector, and varies with what the linker kept; repeated
N=0runs differ by ±50,000 instructions, twenty times the thing being measured. So the per-bind figure is amortized over a thousand binds, where the jitter cancels — three consecutive runs give 1,733 / 1,667 / 1,669 — and the floor is reported beside it instead of being subtracted once and forgotten.That floor is worth seeing rather than hiding: 890,000 instructions before
mainis three orders of magnitude above the bind, and it is the real ceiling on what any Go parser can win.What is still not reproducible
cobra's, urfave's and kong's rows were measured by hand against programs that are not in this repository. They stay in the table, now marked as such, with the fix named: generate mise-sized programs for them from the same spec, as
xtask shadowalready does for clap, argh and bpaf. Until then they should be read as an order of magnitude rather than a measurement.Leaving them unmarked was the part worth correcting. A number in a table reads as a measurement whether or not anyone can repeat it.
Verified
shellcheckclean,mise run perf:goruns at a terminal and reports wall clock only where valgrind is absent,cargo test --all --all-features,go test ./..., clippy, fmt, prettier, actionlint.🤖 Generated with Claude Code
Note
Low Risk
Benchmark tooling and documentation only; no production CLI or parsing behavior changes.
Overview
Adds a reproducible Go performance path (
mise run perf:go) so the README’s usage-go row is no longer hand-waved: a newparse-nharness bindsmise use -g node@20against the committed mise shadow tables, andtasks/perf-go.shreports amortized instruction counts (when valgrind is present), runtime floor, wall time, and binary size.Because Go startup jitter swamps a single cold bind, the harness amortizes over 1,000 binds and reports the floor beside the per-bind figure instead of using the Rust
PARSE_N=1 − PARSE_N=0subtraction. The script also handles macOS/BSD (nanosecond clock fallbacks, valgrind-absent wall-only output) and validates that parsing actually reaches a subcommand.Docs in
go/README.mdrelabel the benchmark table (usage-go reproducible; cobra/urfave/kong still hand-measured until generated shadows exist) and note shadow programs for other frameworks as follow-up work.Reviewed by Cursor Bugbot for commit 6e3541f. Bugbot is set up for automated code reviews on this repo. Configure here.
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