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benbrownandClaude Fable 5
856b04dbba
Relicensed to make sharing and consumption frictionless — the skill is meant to be dropped into arbitrary client repos, and MIT removes every question a legal department would ask about that. Clean to do unilaterally: git shortlog shows no third-party human contributors (Ben Brown, Claude co-authoring Ben's sessions, and two copilot-swe-agent[bot] workflow commits made at Ben's direction). All living license surfaces swapped in lockstep: LICENSE (MIT text, copyright 2026 Ben Brown), Cargo.toml workspace license, README, and the skill frontmatter in both copies (.claude + .agents mirror, regenerated byte-identical). Historical documents in results/ are records and keep whatever they said at the time. Prior releases/clones under GPL-3.0 remain validly GPL-licensed at those commits, as always. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
154 lines
8.4 KiB
Markdown
154 lines
8.4 KiB
Markdown
---
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name: peltier
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description: "Prove a performance change is real — or refuse to claim it. Use when optimizing code for speed, verifying a claimed speedup ('is this actually faster?', 'did my optimization work?'), checking for a performance regression ('did this PR make things slower?'), or comparing the performance of two implementations. Runs peltier's trust layer — equivalence gates, A/A environment calibration, interleaved A/B with bootstrap confidence intervals. Never reports a naked percentage and never claims a win that is not statistically significant. Do NOT use for correctness-only work."
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license: "MIT"
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compatibility: "Needs a peltier checkout (https://github.com/bdbrown4/peltier.io; set PELTIER_HOME) with cargo to build bench-runner. Measurement requires a Linux/POSIX host; on unsupported hosts the skill refuses to produce numbers rather than degrade."
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version: "1.1.0"
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author: "peltier.io (Ben Brown)"
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tags: ["performance", "benchmarking", "statistics", "verification"]
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metadata:
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homepage: "https://bdbrown4.github.io/peltier.io/"
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source: "https://github.com/bdbrown4/peltier.io"
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---
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# peltier — prove it, or claim nothing
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A performance change that has not passed equivalence gates **and** shown a
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statistically significant improvement **does not exist**. You may discard it
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or log it. You may not claim it.
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Most benchmarking is wrong in ways that flatter the author: run-to-run noise
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read as a win, a faster binary that computes something subtly different, one
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lucky run out of five, a percentage with no interval attached. This skill
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exists to make those failures impossible rather than unlikely.
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## Preflight — never skip, never work around
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Run `scripts/preflight.sh` **relative to this skill's directory** (resolve its
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absolute path first — the skill may be installed at `~/.claude/skills/`, as a
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plugin, or copied into another project):
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sh <skill-dir>/scripts/preflight.sh
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# no POSIX shell on this host (Windows-native harness)? same contract:
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powershell -NoProfile -ExecutionPolicy Bypass -File <skill-dir>/scripts/preflight.ps1
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The two entry points implement one contract and are byte-diffed against each
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other in CI — same checks, same output, same refusals. Dispatch on launcher
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availability, not OS: try `sh` first; if the **launcher itself** is missing
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(`sh: not found` — a launch failure, not a refusal), run the PowerShell entry
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point instead. A `STATUS=refuse` from either is **final**: never run the
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second entry point hoping for a different answer — they are kept
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byte-identical in CI precisely so there is no different answer.
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Some harnesses block script files inside skills, so your installation may
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carry no `scripts/` directory. The script still exists exactly once — in the
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peltier checkout this skill drives (which you need anyway; it is where
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`bench-runner` lives). Run it from there:
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sh "$PELTIER_HOME/.claude/skills/peltier/scripts/preflight.sh"
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Never transcribe or re-implement the preflight logic inline; if you cannot
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locate a checkout to run it from, that IS a preflight refusal — report it.
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It locates the trust layer, builds the real `bench-runner`, and checks this
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host can run it. `STATUS=ok` does **not** mean the host can measure your
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change — only the A/A self-test decides that.
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**If it refuses, that ends the claim, not the work.** You may still write the
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patch and explain why you think it is faster. You may not attach a number to
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it. Do not fall back to `time`, `hyperfine`, or a stopwatch loop — a number
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you cannot stand behind is worse than no number, because someone will act on
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it. The statistics live in `bench-runner` and are used from there; a second
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copy of the math is a second thing that can lie.
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## Two modes
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| Mode | Use when | Read |
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|---|---|---|
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| **verify** | You have a baseline and a candidate — any repo, any language. | [references/verify.md](references/verify.md) |
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| **attempt** | You are inside a peltier checkout and want the full loop: profile → hypothesize → patch → gated verdict → ledger row. | [references/attempt.md](references/attempt.md) |
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Default to **verify**. It is the portable one, and it is what almost every
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request actually wants.
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### No candidate yet? Profile first — do not guess.
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"Measure before optimizing" is not advice, it is a gate. If you have one
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program and a hunch, get profile data *before* writing a patch: `perf record`
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(native), `py-spy` / `cProfile` (Python), `node --prof` / `--cpu-prof` (Node),
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`cargo flamegraph` (Rust), or `just coz` for causal profiling inside a peltier
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checkout. Then form a hypothesis about a *named* hot path, and only then patch.
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Guessing where the time goes is how people spend a week on 0.3% of the runtime.
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## The order of the gates is the point
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Each gate can end the job, and ending early is a success, not a failure.
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1. **Equivalence — before you time anything.** A faster program that does
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something different is not an optimization, it is a bug with good
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benchmarks. Prove baseline and candidate produce identical output on a fixed
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workload. Comparing two *empty* outputs is a failed gate, not a passed one.
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2. **Calibration — can this machine measure this change at all?** The A/A
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self-test runs the same command against itself and must find **nothing**. A
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harness that "discovers" a speedup between a binary and itself is measuring
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noise, and every number it gives you afterwards is fiction.
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3. **Measurement — interleaved A/B, bootstrap CI.** Not "run A five times, then
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B five times" — that hands any thermal or cache drift to whichever side ran
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last.
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4. **Risk — what did the patch actually touch?** Concurrency primitives,
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`unsafe`/raw memory, floating-point reassociation: `needs-human-review`,
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**regardless of how good the numbers are**. In verify mode *you* are the
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classifier — `bench-runner` only ever emits `accepted` or `rejected-bench`,
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so it cannot make this call for you.
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5. **Verdict — the CI lower bound decides.** Accept only if the bootstrap 95%
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CI lower bound of the speedup clears the threshold (default 2%). The median
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is not the claim. The lower bound is the claim.
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## Non-negotiables
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- **Cheapest wins first:** build flags → LTO → PGO → allocator swap → *then*
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code changes. Do not hand-write SIMD before you have tried `-O3`.
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- **Every attempt is recorded, including failures.** A rejected hypothesis is a
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result — it stops the next person from re-running your dead end.
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- **A failed gate is a complete, valid outcome.** Write it up and move on. Do
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not iterate on a rejected patch more than twice without a new hypothesis.
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- **The thing being measured never grades itself.** The patch may not touch the
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harness, the corpus, or the tests.
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## Never
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- **Never a naked percentage.** Every number carries its confidence interval
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and the workload it was measured on. "12% faster" is not a result.
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- **Never re-run a benchmark until it passes.** That is p-hacking with extra
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steps. The first calibrated run is the answer.
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- **Never claim a win the CI lower bound does not support**, however good the
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median looks and however "obviously" faster the change is.
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- **Never present a measurement from a host that failed calibration.**
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## What good looks like
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**A claim, fully dressed** (`phase3-cjson-002`, in the ledger):
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> Accepted. Speedup 1.088 (candidate is 8.8% faster), 95% CI [1.074, 1.100] —
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> so the *defensible* claim is ≥7.4%, not 8.8%. Workload: 5.6 MB synthetic
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> JSON, 20k records, parsed + serialized 2×, single thread. Gates: upstream
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> suite green, golden replay byte-identical, ASan + UBSan clean.
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Note what that write-up does **not** say. It does not claim a differential-fuzz
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run, because that row records `fuzz_iters=0` — the fuzzing of that era ran
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out-of-band and the machine never witnessed it. Fuzz is a pipeline gate now, and
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new rows carry a real count. Say what the record supports, not what you
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remember doing.
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**Declining to claim what the data won't support** (`phase4-cjson-service`):
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p50 latency improved 6.2% [95% CI +5.8%, +7.2%] at 150 rps — claimed. The p99
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CI came back [0.07, 4.97] — uselessly wide, so **p99 was not claimed at all**,
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even though its median looked like a win.
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**Good numbers on a bad change** (`phase2-comrak-010`): 1.107 speedup, 95% CI
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[1.079, 1.121], on 11 MB of markdown. Tight interval, real improvement,
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**accepted — and wrong.** LeakSanitizer caught it afterwards and the row was
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overturned. It never shipped. The bench was never the problem; skipping a gate
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was. This is why the order above is not negotiable.
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