perf-baseline-diff

name	perf-baseline-diff
last_validated	"2026-05-20T00:00:00.000Z"
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), 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 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/<UTC-ts>/ --current experiments/artifacts/nccl-tests/<run-id>/results.json --tolerance-percent 5

Prerequisites

1. Baseline path - --baseline <registry-path> (directory from perf-baseline-record).
2. Current measurement - --current <path> (file or directory).
3. Tolerance - --tolerance-percent <N> for scalar measurements, --tolerance-absolute <N> 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>/baseline.json to extract family, measurement, unit, source_sha256, value, source_path.
• Bash(sha256sum <current>) to hash the current source.

Phase 1: determine shape

• If <baseline>/source-snapshot.nsys-rep exists -> shape = nsys-rep (delegate to Phase 2c).
• Else if baseline.value is set -> shape = scalar (Phase 2a).
• Else if <baseline>/source-snapshot.* parses as JSON dict -> shape = structured (Phase 2b).
• Else: error and stop.

Phase 2a: scalar diff

Preferred (MCP verb):

mcp__profile_and_optimize__perf_baseline_diff with:
  args: ["--baseline", "<baseline-registry-entry-dir>",
         "--current", "<current-source-path>",
         "--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/<family>/<measurement>/<UTC-ts>/, and returns the verdict + bundle path.

Fallback (Bash-tool):

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

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)

mcp__profile_and_optimize__profile_profile_diff with:
  args: ["--baseline", "<baseline-nsys-rep>",
         "--candidate", "<current-nsys-rep>",
         "--out", "<diff-bundle>/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/<family>/<measurement>/<UTC-ts>/
  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.<ext>    # 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), 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 - the pair skill.
• docs/METHODOLOGY.md - measurement canon (full-context reporting, verdict rigor, speed-of-light framing).
• server/AGENTS.md - fail-fast and provenance rules.