--- name: perf-baseline-diff last_validated: 2026-05-20 description: >- Diff a current performance measurement against a registered baseline from the perf-baselines registry. Works for any measurement type (NCCL BW, MFU, step-time, latency, throughput, per-kernel times, structured heatmaps, nsys-rep profiles). Honors the operator-supplied tolerance, reports per-dimension deltas, and classifies the overall verdict as GREEN / YELLOW / RED. Pairs with perf-baseline-record. Triggers on "diff baseline", "diff against baseline", "perf regression check", "is this regression real", "compare to baseline", "perf-baseline-diff", "regression diff", or any combination of "diff / compare / regression / check" with "baseline / golden / reference / perf-of-record". allowed-tools: - mcp__profile_and_optimize__perf_baseline_diff - mcp__profile_and_optimize__profile_profile_diff - mcp__profile_and_optimize__search_evidence - Bash(sha256sum:*) - Bash(jq:*) - Bash(diff:*) - Read - Write --- # perf-baseline-diff ## Purpose Take a current perf measurement (any kind) and a registered baseline (from [`perf-baseline-record`](/plugins/profile-and-optimize/skills/perf-baseline-record/SKILL.md)), produce a structured diff with per-dimension deltas, and classify the overall verdict. Works for scalar, structured, and nsys-rep measurements. > This skill is backed by a native MCP verb: `mcp__profile_and_optimize__perf_baseline_diff`. The verb handles scalar + structured-JSON shapes natively and delegates nsys-rep diffs to `mcp__profile_and_optimize__profile_profile_diff`. The Bash-tool path documented below remains supported as a fallback. Output: - A **per-dimension delta table** (every (key, baseline-value, current-value, delta, delta-%) row). - An **overall verdict**: GREEN (within tolerance), YELLOW (one or two dimensions outside tolerance), RED (multiple dimensions outside tolerance OR the headline dimension regressed beyond tolerance). - A **next-action recommendation** (bisect, ignore, file regression, etc.). Handles three measurement shapes: 1. **Scalar** - single number (e.g. step-time-ms). Delta is `current - baseline`, delta-% is `100 * (current - baseline) / baseline`. 2. **Structured key->value** (e.g. per-pair NCCL BW). Computes delta per key. Reports worst-N regressions. 3. **Nsys-rep** - delegates to `mcp__profile_and_optimize__profile_profile_diff` (the profile-diff harness shipped with the `profile_and_optimize` MCP server). ### Inference-perf use For inference perf-bench output specifically, prefer the [`inference-perf-baseline-bridge`](/plugins/profile-and-optimize/skills/inference-perf-baseline-bridge/SKILL.md) skill. It is a structured-shape diff (Phase 2b below) keyed on the `inference_perfbench_v1` schema, with per-metric tolerances tuned for inference workloads (TTFT 5%, throughput 3%, cache-hit-rate 2 absolute pts) and a verdict-classification step that knows the headline metric is `throughput_toks_per_s` at concurrency 16. No new MCP verb is introduced. The bridge wraps `perf_baseline_diff` directly. ## When to use - After re-running a fabric benchmark (e.g. upstream `nccl-tests`) post-driver-upgrade, diff against the pre-upgrade baseline. - After re-running an `nvbandwidth` link sweep after a hardware swap, diff against the prior heatmap. - After a candidate training run, diff step-time / MFU against the registered best run. - Periodic regression sweep: weekly cron, diff each measurement against the most recent registered baseline. Do **not** use this skill for: - Comparing two arbitrary measurements neither of which is in the registry - that's `diff` or `jq` directly. Register one of them as a baseline first. - One-shot debugging - too much ceremony for a quick eyeball check. ## Example prompts - "Diff this morning's nccl-tests bundle against the baseline from last Tuesday." - "Is this MFU regression real? Diff against the registered baseline." - "Compare this nvbandwidth heatmap to the one we registered before the firmware upgrade." - "Perf-baseline-diff family=llama31_8b measurement=nccl_busbw current= tolerance=5%" - `/perf-baseline-diff --baseline experiments/artifacts/perf-baselines/llama31_8b/nccl_busbw// --current experiments/artifacts/nccl-tests//results.json --tolerance-percent 5` ## Prerequisites 1. **Baseline path** - `--baseline ` (directory from `perf-baseline-record`). 2. **Current measurement** - `--current ` (file or directory). 3. **Tolerance** - `--tolerance-percent ` for scalar measurements, `--tolerance-absolute ` for structured. Default: `5%`. 4. **`PROFILE_AND_OPTIMIZE_REPO_ROOT`** for writing the diff bundle. ## Interaction style Fast and autonomous. One pause: confirm the baseline + current paths + tolerance before writing the diff bundle. The verdict + recommendation is the last step. ## Workflow ### Phase 0: resolve paths - `Read` `/baseline.json` to extract `family`, `measurement`, `unit`, `source_sha256`, `value`, `source_path`. - `Bash(sha256sum )` to hash the current source. ### Phase 1: determine shape - If `/source-snapshot.nsys-rep` exists -> shape = `nsys-rep` (delegate to Phase 2c). - Else if `baseline.value` is set -> shape = `scalar` (Phase 2a). - Else if `/source-snapshot.*` parses as JSON dict -> shape = `structured` (Phase 2b). - Else: error and stop. ### Phase 2a: scalar diff **Preferred (MCP verb):** ```text mcp__profile_and_optimize__perf_baseline_diff with: args: ["--baseline", "", "--current", "", "--tolerance-percent", "5", "--json"] ``` The verb auto-detects shape (scalar / structured JSON / nsys-rep), runs the diff, writes the diff bundle to `experiments/artifacts/perf-baseline-diffs////`, and returns the verdict + bundle path. **Fallback (Bash-tool):** ```text delta = current_value - baseline.value delta_pct = 100 * delta / baseline.value verdict = "GREEN" if |delta_pct| <= tolerance_percent else "RED" ``` ### Phase 2b: structured (key->value) diff ```text For each key in baseline AND current: baseline_v = baseline[key] current_v = current[key] delta = current_v - baseline_v delta_pct = 100 * delta / baseline_v Sort by |delta_pct| descending; top 20 in the report. verdict: GREEN if no key exceeds tolerance YELLOW if 1-2 keys exceed tolerance RED if 3+ keys exceed OR headline key (operator-flagged) exceeds ``` ### Phase 2c: nsys-rep diff (delegate) ```text mcp__profile_and_optimize__profile_profile_diff with: args: ["--baseline", "", "--candidate", "", "--out", "/profile-diff.md", "--json"] ``` Then translate the harness output into the same verdict shape (GREEN/YELLOW/RED) for consistency. ### Phase 3: write diff bundle ``` ${PROFILE_AND_OPTIMIZE_REPO_ROOT}/experiments/artifacts/perf-baseline-diffs//// diff.json # structured diff: baseline_sha, current_sha, per-key delta, verdict diff.md # human-readable: top 20 deltas, verdict, recommendation baseline-ref.txt # absolute path to the baseline directory current-snapshot. # copy of the current source for audit ``` ### Phase 4: recommend Based on verdict: - **GREEN**: "no regression detected. No action needed." - **YELLOW**: "spot regression on N keys. Recommend re-running the measurement on the same cohort to rule out noise, then re-diff." - **RED**: - If shape == `nsys-rep`: drill into the `profile_profile_diff` per-kernel bucket output to name the regressed bucket. - If shape == `structured` and the regressions cluster on specific keys (e.g. all on one node-pair, or all on one collective): name the suspect axis. - If shape == `scalar`: name the absolute + percent delta and ask the operator whether to bisect. ## Safety - **Read-only on the registry.** This skill never writes to `experiments/artifacts/perf-baselines/`. It writes only to `experiments/artifacts/perf-baseline-diffs/`. - **Provenance preserved.** The diff bundle records `baseline_sha256` and `current_sha256` so the comparison is auditable. - **No silent shape conversion.** If the baseline is structured and the current is scalar (or vice versa), the skill stops and asks. No fuzzy diffing. - **Tolerance is explicit.** Default 5% but operator must agree. The skill prints the resolved tolerance in Phase 0 and writes it into the diff bundle. ## Full-context reporting (no bare numbers) Per `docs/METHODOLOGY.md` "Full-context reporting": every number this skill emits MUST carry its full measurement-context descriptor, and every comparison MUST be matched on it. A bare `tok/s` / TPOT / BW / %SoL / speedup is a defect - it cannot set a default, ship a config, or appear in a report. - **Identity:** model (+HF path), hardware (exact ceiling token `GB300`/`B200`), quant, kv-cache dtype. - **Parallelism:** TP, DP (replicas), PP, EP, parallel_strategy. - **Serving cfg:** max-num-seqs, max-num-batched-tokens, gpu-memory-utilization, max-model-len, cudagraph_mode/enforce_eager, async_scheduling, prefix-caching. - **Workload:** dataset, ISL/OSL (or mean in/out tokens), concurrency, num-prompts. - **Regime:** warm vs cold. Latency vs throughput tier. - **Stack:** image/vllm commit, bench backend, serving engine. - **Grounding:** `%SoL` (+ ceiling key from `configs/sol-ceilings.yaml` - never inline a peak), sol_rigor (L1-L4), trials n (mean±std), same-node, baseline named. - **Per-number exact shape (no smoothing):** when reporting more than one number, keep EACH with its own exact shape (ISL/OSL, concurrency, dataset, regime) - never normalize a set to one uniform descriptor that hides per-point variation (e.g. `c=1 @ ISL1024/OSL256` + `c=64 @ ISL4096/OSL512`, NOT one shared "random"). Per `docs/METHODOLOGY.md` "Speed-of-light framing", when the baseline carries `sol_pct` / `sol_ceiling_key` / `sol_ceiling_value` (the optional fields added by [`perf-baseline-record`](/plugins/profile-and-optimize/skills/perf-baseline-record/SKILL.md)), the diff output MUST also report **SoL delta** alongside the absolute delta. Format: ``` measurement: nccl_all_reduce_busbw @ 1GB baseline: 310 GB/s (17.2% of nvlink5_tbps SoL) current: 295 GB/s (16.4% of nvlink5_tbps SoL) delta: -15 GB/s (-4.8% absolute, -0.8 pp SoL) verdict: YELLOW (within 5% tolerance, but trending away from peak) ``` The SoL delta surfaces ceiling-distance shifts that absolute deltas hide: a 4.8% drop on a measurement already at 90% SoL is structurally different from the same drop at 30% SoL (former leaves no recovery room. Latter is well within recoverable headroom). When the baseline lacks `sol_*` fields (older baselines, or non-roofline-bound measurements), the diff skips the SoL line silently - no synthesised retrofit. ## Next lever / BREAKTHROUGH (Grind Mandate) If this skill emits a measured result, its output MUST end by naming the **next perf lever**, its **expected unlock** (direction + rough magnitude), and the **gate** that proves/refutes it, per `docs/METHODOLOGY.md` "Always be grinding". A measured win is the new floor, not the finish -- so **do everything we can to find the next BREAKTHROUGH**: the highest-EV unlock toward Speed-of-Light (a new champion / kernel / router / quant / parallelism / spec-decode win, or an unblocked stack), not just the next micro-lever. Rank the candidate breakthrough levers by value x cost (the GRIND FRONTIER, `perftunereport value_view`), pursue the top, bank the rest with evidence. Record WHY a refuted lever loses, update the standing frontier in the active bundle's `HANDOFF.md`. Never conclude "exhausted/optimal/done" without an explicit next-lever frontier (an empty frontier AND a documented SoL wall only). Delete this section ONLY if the skill produces no measurements. ## Verdict rigor (DRAFT vs VERDICT) Per `docs/METHODOLOGY.md` "Verdict rigor: DRAFT vs VERDICT", tier the diff verdict. A GREEN/YELLOW/RED call is a **DRAFT** unless it is variance-controlled (same-node, >=3 trials per arm, mean +/- std - a single-trial or cross-node delta is provisional), metric-isolated (median TPOT/ITL for decode-latency claims, not output tok/s at small num_prompts), and against a production-representative baseline. A "which kernel/path regressed" claim additionally needs nsys/ncu per-kernel data, not a DCGM regime %. Supersede a DRAFT everywhere once a controlled VERDICT overturns it. ## Source-of-truth references - [`perf-baseline-record`](/plugins/profile-and-optimize/skills/perf-baseline-record/SKILL.md) - the pair skill. - [`docs/METHODOLOGY.md`](/docs/METHODOLOGY.md) - measurement canon (full-context reporting, verdict rigor, speed-of-light framing). - [`server/AGENTS.md`](/plugins/profile-and-optimize/server/AGENTS.md) - fail-fast and provenance rules.