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ralph(eval): finalize report (final measurement, key findings, cost breakdown)

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Sridhar Ratnakumar 2026-05-29 07:18:41 -04:00
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docs/eval-time-ralph-report.md
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@ -96,6 +96,48 @@ pureintent eval profile are all *wanted* features (samples ≈ inclusive share):
`omnix` — Haskell). Further reductions require trading a feature, not a free
structural change.
## Final measurement (median irrelevant — counters are exact)
| Target | nrThunks (base → final) | Δ | nrFunctionCalls | nrPrimOpCalls |
|--------|-------------------------|------|-----------------|---------------|
| pureintent | 20,400,114 → **16,989,720** | **−16.7%** | 12,975,268 → 10,794,555 (−16.8%) | 6,507,025 → 5,327,976 (−18.1%) |
| sincereintent | 5,323,428 → **2,757,201** | **−48.2%** | 3,227,404 → 1,631,213 (−49.5%) | 1,565,120 → 771,171 (−50.7%) |
Both configs still instantiate cleanly (`nix build --dry-run` on the system
toplevel / home activationPackage succeeds).
## Key findings
_(filled in at wrap-up)_
1. **On a thermally-throttling machine, time is a lie — count work instead.**
The same eval reported cpuTime from 11s to 21s as the CPU clock sagged, while
`nrThunks` was byte-identical every run. Eval-work counters (`nrThunks`,
`nrFunctionCalls`, `nrPrimOpCalls`) are deterministic, zero-noise, and
machine-independent — a far better optimization metric than wall/cpu time.
2. **Generated option documentation is the biggest free win.** The NixOS options
manual (`documentation.nixos.enable`) and the home-manager options manpage
(`manual.manpages.enable`) each evaluate the doc string of *every* option.
Disabling both: pureintent −14.6%, sincereintent −33% — with zero behaviour
cost (options are searched online).
3. **A shared module multiplies.** `manual.manpages.enable = false` in
`modules/home/default.nix` helped both the standalone home config *and* every
Linux system's embedded home-manager (via `modules/nixos/default.nix`).
4. **Eval is lazy — unused inputs are already free.** Pruning unreferenced flake
inputs does not move the counters; it only trims `flake.lock`.
5. **The remainder is feature-bound.** After the doc wins, eval cost is dominated
by embedded home-manager file-linking, `/etc` generation, and the closures of
wanted services/packages. Trimming 5 heavy `home.packages` (user-approved) took
sincereintent to −48% total; the rest are features worth their eval cost.
## Cost breakdown (per-package eval cost, standalone `pkgs.<p>.drvPath` thunks)
Shared base floor ≈ 1.09M thunks; the excess is each package's own closure-eval:
| Package | thunks | excess | removed? |
|---------|--------|--------|----------|
| yt-dlp | 2.15M | +1.05M | ✅ |
| lima | 1.78M | +0.69M | ✅ |
| omnix | 1.76M | +0.67M | ✅ |
| pandoc | 1.46M | +0.37M | ✅ |
| google-cloud-sdk | 1.39M | +0.30M | ✅ |
| hledger | 1.35M | +0.26M | kept (wanted) |
| ripgrep / fd / just / television | ~1.09M | ~0 | kept (at floor) |