Tw Lift

Lift

Intent

Deliver aggressive, measurement-driven performance improvements (latency/throughput/memory/GC/tail) with correctness preserved and regressions guarded.

Double Diamond fit

Lift lives in Define -> Deliver:

• Define: write a performance contract and pick a proof workload.
• Deliver: measure baseline, profile, run tight experiments, then ship with a guard.

Hard Rules

• Measure before and after every optimization (numbers + environment + command).
• Optimize the bottleneck, not the loudest hunch (profile/trace/counters required).
• Avoid micro-optimizations until algorithmic wins are exhausted.
• Keep correctness and safety invariants intact.
• Require a correctness signal before and after; never accept a perf win with failing correctness.
• Do not change semantics without explicit user approval.
• If you cannot run a proof workload, label the output UNMEASURED and provide exact benchmark/profiling commands; treat all optimization ideas as hypotheses.
• Stop and ask before raising resource/cost ceilings (CPU cores, memory footprint, I/O bytes, external calls), unless explicitly requested.
• Stop when ROI is negative or risk exceeds benefit.
• When running /tw-lift on /tw-lift with /tw-ms, require a runnable proof bundle before done.

Default policy (non-interactive)

Goal: stay autonomous without inventing SLOs.

Mode selection (measured vs unmeasured)

If you can run a proof workload, operate in measured mode. Otherwise operate in unmeasured mode.

• Measured: run baseline + variant on the same workload; include numbers, bottleneck evidence, and a correctness signal.
• Unmeasured: start with UNMEASURED: <why>; do not claim wins; provide the exact commands you would run to produce baseline/after + profiling evidence.

Contract derivation

If the user did not provide a numeric target:

• Define the contract as: "Improve on vs baseline; report delta; do not regress ."
• Do not invent SLO numbers; treat the goal as "maximize improvement within constraints".

Metric defaults (pick one):

• Request-like: latency p95 (also report p50/p99).
• Batch/offline: throughput (also report CPU% and memory).
• Memory issues: peak RSS + alloc rate / GC pause (also report latency).

Workload selection (proof signal)

Pick the first runnable, representative workload you can find:

1. User-provided repro/command.
2. Existing repo benchmark/harness (README, scripts, Makefile/justfile/taskfile).
3. A minimal harness around the hot path (microbench) plus a correctness signal.

Stop and ask only if you cannot find or create any runnable proof workload without product ambiguity.

Experiment hygiene

• Change one variable at a time; keep diffs small and reversible.
• Reject wins smaller than the noise floor; re-run when variance is high.
• Track second-order regressions (memory, tail latency, CPU) even if the primary metric improves.

Workflow (Opinionated)

0. Preflight
   • Capture environment (hardware/OS/runtime flags).
   • Pick a correctness signal and a performance workload; run each once to verify they work.
1. Performance contract
   • Metric + percentile + workload + environment + constraints.
2. Baseline
   • Warm up; collect enough samples for stable percentiles (keep raw samples when possible).
3. Locate the bottleneck
   • Profile/trace; classify bound (CPU/memory/I/O/lock/tail).
4. Choose the highest-leverage lever
   • Follow the optimization ladder: delete work -> algorithm -> data/layout -> concurrency -> I/O -> micro-arch -> runtime/compiler.
5. Run tight experiments (loop)
   • Hypothesis -> patch -> measure -> accept/reject -> record.
6. Ship with guards
   • Add/extend a benchmark, budget, or alert; document trade-offs.
7. Report
   • Present baseline vs variant and the evidence trail.

Decision Gates

• If the baseline is noisy or unstable, fix measurement first.
• If the complexity class dominates, change the algorithm first.
• If tail latency dominates, treat variance reduction as the primary goal.
• If I/O dominates, reduce bytes, syscalls, or round trips before CPU tuning.
• If the only remaining wins require higher resource/cost ceilings, surface the trade-off and ask.
• Stop when ROI is negative or risk exceeds benefit.

Deliverable format (chat)

If unmeasured, prefix the response with UNMEASURED: <reason> and fill sections with a concrete measurement plan (no claimed deltas).

Output exactly these sections (short, numbers-first):

Performance contract

• Metric + percentile:
• Workload command:
• Dataset:
• Environment:
• Constraints:

Baseline

• Samples + warmup:
• Results (min/p50/p95/p99/max):
• Notes on variance/noise (or estimated noise floor):

Bottleneck evidence

• Tool + key finding:
• Hot paths / contention points:
• Bound classification:

Experiments

• <1-3 entries> Hypothesis -> change -> measurement delta -> decision

Result

• Variant results (min/p50/p95/p99/max):
• Delta vs baseline:
• Confidence (noise/variance):
• Trade-offs / regressions checked:

Regression guard

• Benchmark/budget added:
• Threshold (if any):

Validation

• Correctness command(s) -> pass/fail
• Performance command(s) -> numbers

Residual risks / next steps

lift_compliance: mode=<measured|unmeasured>; workload=<yes|no>; baseline=<yes|no>; after=<yes|no>; correctness=<yes|no>; bottleneck_evidence=<yes|no>

Core References (Load on Demand)

• Read references/playbook.md for the master flow and optimization ladder.
• Read references/measurement.md for benchmarking and statistical rigor.
• Read references/profiling-tools.md for a profiler/tool matrix and evidence artifacts.
• Read references/algorithms-and-data-structures.md for algorithmic levers.
• Read references/systems-and-architecture.md for CPU, memory, and OS tactics.
• Read references/latency-throughput-tail.md for queueing and tail behavior.
• Read references/optimization-tactics.md for a tactical catalog by layer.
• Read references/checklists.md for fast triage and validation checklists.
• Read references/anti-patterns.md to avoid common traps.

Assets

• Use assets/perf-report-template.md as a ready-to-edit report.
• Use assets/experiment-log-template.md to track experiments and results.

Output Expectations

• Deliver a baseline, bottleneck evidence, hypothesis, experiment plan, and measured result.
• Provide a minimal diff that preserves correctness and includes a regression guard.
• Explain trade-offs in plain language and record the measured delta.