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Claude
328aafae77
Commit the spec and scaffold the system it describes:
- SPEC.md: the full v0.1 engineering spec
- crates/: trust-layer Rust workspace, unit-tested
- bench-runner: interleaved ABAB scheduling with discarded warm-ups,
deterministic bootstrap CIs over the ratio of medians,
CI-lower-bound accept rule, env fingerprinting, and a CLI with
compare + A/A self-test modes (A/A verified passing end-to-end)
- diff-test: gate outcome model, sha256 corpus pin verifier that
refuses on mismatch, per-target equivalence policy parser
(byte-identical default, explicit fp-tolerance opt-in)
- ledger: append-only SQLite attempt schema with UPDATE/DELETE
refused by triggers, anti-double-attempt query
- report: cores-saved and dollars/yr ROI math over speedup CIs,
pricing.toml inputs
- playbook/: classes 1-7 with preconditions, procedure, verification
notes, and known failure modes per class
- agent/: Python package skeleton — six-tool contract and prompting
spine, harness IPC deferred to Phase 1
- config/: accept.toml (2% threshold, 30 runs/side), pricing.toml
- targets/, corpora/, results/calibration/: layout and policy docs
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2
38 lines
1.8 KiB
Markdown
38 lines
1.8 KiB
Markdown
# Class 4 — Data layout
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Make the memory hierarchy work for the access pattern.
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## Preconditions
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- Profile signature: high cache-miss rate in `perf stat` (LLC misses per
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instruction), hot loops whose flamegraph frames are memory-bound
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(`perf` cycles concentrated on loads), low IPC (<1) on wide cores.
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- For false sharing: multithreaded target where per-thread throughput
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degrades as threads are added; `perf c2c` confirms.
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## Procedure (cheapest first)
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1. Struct field reordering / packing: kill padding, group hot fields on
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one cache line, split hot/cold (`#[repr(C)]` review, cold-field
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box-out).
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2. AoS→SoA for loops that touch one field across many elements —
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restructure the container, not the algorithm.
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3. False-sharing fixes: pad or align per-thread state to 64 bytes
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(`#[repr(align(64))]`, `crossbeam_utils::CachePadded`).
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4. Cache blocking / loop tiling for nested loops over large matrices or
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images; tile to L1/L2, verified empirically, not by datasheet.
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## Verification notes
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- Layout changes to any type that crosses FFI, serialization, or hashing
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boundaries can change observable behavior (struct hash, serialized
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bytes, memcmp-based equality) — golden replay catches this; check the
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target for `#[repr]`-sensitive traits before patching.
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- Differential fuzz the functions whose types changed, not just the
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functions whose code changed.
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## Known failure modes
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- Reordering fields of a type that derives `Hash`/`Ord` used in ordered
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iteration → output order changes → replay divergence (correct reject).
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- AoS→SoA wins the target loop but loses every other access site; the
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end-to-end number is the verdict, not the microbench.
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- Alignment padding blowing up per-element size until the working set
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falls out of cache — the cure worse than the disease.
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