7477e25b46
documentation.nixos.enable = false. Generating the NixOS manual evaluates every option's doc string. nrThunks 20,400,114 -> 19,201,996 (-5.9%), nrFunctionCalls -6.2%, nrPrimOpCalls -7.5%. Package man pages unaffected. Metric pivoted to deterministic NIX_SHOW_STATS counters: cpuTime is unusable on this thermally-throttling machine (same eval = 11s..21s).
3 KiB
Nix Eval-Time Optimization (Ralph)
Iterative measurement-driven reduction of Nix evaluation time for the two configs that get evaluated most often on this machine:
pureintent— the NixOS system (nixosConfigurations.pureintent)zest— the home-manager config (homeConfigurations."srid@zest")
Methodology
Primary metric: deterministic eval-work counters from NIX_SHOW_STATS=1 —
nrThunks, nrFunctionCalls, nrPrimOpCalls. These count the actual evaluation
work and are byte-identical across runs (independent of CPU clock, load, GC).
Why not
cpuTime/wall? Initially planned, but rejected after measuring: this is a live, thermally-throttling laptop. The same eval (identical nrThunks every run) reported cpuTime climbing 11s → 21s monotonically as the CPU clock dropped under sustained load. Time is meaningless here; counters are exact. A 1% drop in nrThunks is a real 1% less work the evaluator must do, on any machine. Single eval per target — no median needed.
# docs/eval-bench.sh [target] target = pureintent | zest | (both)
# - persistent $HOME=/tmp/ralph-evalhome keeps the fetcher/git cache warm
# - --option eval-cache false forces a full re-eval every run
# - reports nrThunks / nrFunctionCalls / nrPrimOpCalls per target (deterministic)
nix eval --raw .#nixosConfigurations.pureintent.config.system.build.toplevel.drvPath --option eval-cache false
nix eval --raw '.#homeConfigurations."srid@zest".activationPackage.drvPath' --option eval-cache false
No nix store gc between runs (per request). The eval cache is disabled rather
than cleared, so the warm fetcher cache removes input-fetch noise.
Commit rule: commit only if nrThunks improves >3% for a target with no
regression on the other. (Counters have ~zero noise, so any real reduction
counts; >3% is the bar for a "meaningful" cycle.) Behaviour may change only by
dropping inputs/features first confirmed unused.
Profiling tool: nix eval … --eval-profiler flamegraph --eval-profile-file f
then aggregate self/inclusive frames (collapsed-stack format).
Baseline (nrThunks / nrFunctionCalls / nrPrimOpCalls)
| Target | nrThunks | nrFunctionCalls | nrPrimOpCalls |
|---|---|---|---|
| pureintent | 20,400,114 | 12,975,268 | 6,507,025 |
| zest | 11,066,680 | 7,091,428 | 3,587,586 |
Env: nix 2.34.7, x86_64-linux.
Optimization log
(Δ% is on nrThunks vs baseline for the affected target.)
| Cycle | Change | pureintent thunks | zest thunks | Verdict |
|---|---|---|---|---|
| 0 | baseline | 20,400,114 | 11,066,680 | — |
| 1 | pureintent: documentation.nixos.enable = false (drop NixOS options manual; option doc-strings no longer evaluated) |
19,201,996 (−5.9%) | 11,066,680 | ✅ commit |
Dead ends
(investigated, no improvement — recorded so we don't retry)
Key findings
(filled in at wrap-up)