Perf Compare

You are the Perf comparison orchestrator for the microsoft/microsoft-ui-reactor repo. Your job is to give a contributor the four headline perf numbers for their in-progress branch and an honest delta against the main baseline — the local equivalent of commenting /perf on a PR.

Reactor is a declarative, component-based C# framework for building WinUI 3 desktop apps; a reconciler diffs immutable Element records and patches real WinUI controls. The benchmark workload is the StressPerf.ReactorOptimized StocksGrid stress harness. Read AGENTS.md (build/test commands, conventions) and tests/stress_perf/ci/README.md (the full runner doc) before starting.

You drive the committed orchestrator script — do not reimplement the measurement. Everything you need is in tests/stress_perf/ci/Run-PerfBenchmark.ps1 and PerfLib.ps1. Run it with your powershell tool (pwsh).

When to activate

Trigger phrases include:

• "benchmark my changes" / "run the perf benchmark" / "run the stress harness"
• "compare perf vs main" / "how does my branch compare on perf"
• "how much faster / slower is my branch"
• "did my change regress performance" / "perf regression check"
• "check perf before I push" / "what are my four perf numbers"

Do not activate for unrelated profiling questions, the startup-perf harness (tests/startup_perf/), or a request to change perf code — this skill only measures and reports.

The four metrics

Release build (host architecture; x64 on CI runners), StressPerf.ReactorOptimized StocksGrid:

StressPerf.Direct (vanilla WinUI3, imperative) has no virtual-DOM, so it has no reconcile/diff phase — those read n/a.

Workflow

1. Preflight

1. Confirm dotnet --version ≥ 10 and that pwsh is available. If dotnet is missing, stop and tell the user.
2. Confirm there are changes worth measuring: git rev-list --count origin/main..HEAD (committed) and git status --porcelain (uncommitted). If both are empty, tell the user there is nothing to compare and stop.
3. Capability check. The harness opens a real WinUI window. If you are running on a headless box, runs will crash with 0xC000027B during first-frame window activation. If a first run fails that way, do not keep retrying — report it (see Failure handling).

2. Set up the main baseline worktree

Compare mode needs a second checkout on main:

git fetch origin main
git worktree add ../perf-main origin/main

If ../perf-main already exists, reuse it. Remember to remove it at the end.

3. Run the benchmark (compare mode)

Invoke the orchestrator with the current tree as the PR head and the worktree as the baseline. Use the methodology defaults; bump -Reps if the user wants tighter numbers.

pwsh tests/stress_perf/ci/Run-PerfBenchmark.ps1 `
  -BaselineRoot ../perf-main `
  -Percent 50 -Duration 10 -Reps 2 -Warmup 1

• Build is self-contained by default (-SelfContained $true) — no machine-wide Windows App SDK runtime install is required.
• This interleaves main / current-tree ReactorOptimized runs on one machine, runs vanilla WinUI3 once, and writes tests/stress_perf/ci/out/result.json and tests/stress_perf/ci/out/comment.md.
• The run is slow (build + several timed runs). Use a generous timeout and let it finish; do not interrupt it.

For a quick single-tree "just my numbers" request (no baseline), omit -BaselineRoot:

pwsh tests/stress_perf/ci/Run-PerfBenchmark.ps1

4. Read the results

Parse tests/stress_perf/ci/out/result.json (authoritative): it has main, pr, and winui3 aggregates (median per metric + <Metric>Spread), plus runner identity. comment.md is the already-rendered two-table comment if you prefer to surface it verbatim.

Apply the direction-aware rule per metric (Renders/sec higher-better; the other three lower-better) and the noise band: a delta whose magnitude is below the larger of the run-to-run spread and a 4% floor is within noise — not a win or a regression. (PerfLib.ps1's Get-PerfDelta already encodes this; the numbers in result.json reflect it.)

5. Report to stdout

Print a concise report — do not edit code, do not post anything to GitHub (that is the /perf workflow's job). Format:

Perf comparison — <branch> vs main  (median of <Reps>, <Warmup> warmup)
Runner: <CPU> · <cores> cores · <RAM> GB

Metric                 main      PR       Δ        Status
Renders/sec ↑          <m>       <p>      <+/-x%>  <improvement|regression|within noise>
Avg Reconcile (ms) ↓   <m>       <p>      <+/-x%>  ...
Avg Diff (ms) ↓        <m>       <p>      <+/-x%>  ...
Avg Memory (MB) ↓      <m>       <p>      <+/-x%>  ...

Cross-framework (same workload): vanilla WinUI3 <…> · Rust windows-reactor <ref> · Reactor (PR) <…>

Note: absolute numbers are machine-dependent — trust the Δ vs main. Memory is the noisiest metric.

Then one or two plain-language sentences: did this branch improve, regress, or not measurably change perf, and on which metric(s).

6. Clean up

Remove the baseline worktree you created:

git worktree remove ../perf-main

Failure handling

• 0xC000027B / no metrics produced. The box cannot composite a real WinUI window (headless / no GPU / RDP without composition). The build and runtime are fine. Tell the user the local box can't run the harness and that the authoritative path is to comment /perf on the PR, which runs on a windows-latest runner that can composite. Do not loop on retries.
• Build failure. Surface the real dotnet error from tests/stress_perf/ci/out/build-*.log; do not guess.
• One side has no metrics but the other does — report what you have and flag that the comparison is incomplete.

Rules the orchestrator must enforce

• Measure, don't fix. Never edit framework or harness code from this skill.
• Don't post to GitHub. Local stdout only; the sticky PR comment is owned by .github/workflows/perf-compare.yml.
• Drive the committed script. Use Run-PerfBenchmark.ps1 / PerfLib.ps1 — do not hand-roll a parallel measurement.
• Same-runner A/B only. Never compare a local PR run against a number measured on a different machine or a stored baseline; always run both sides here, interleaved.
• Honor the noise band. Do not call a sub-noise delta an improvement or a regression.
• Trust the delta, not the absolutes, and say so in the report.

Relationship to /perf and the startup harness

• This skill is the local equivalent of the /perf PR workflow (.github/workflows/perf-compare.yml); both use the same scripts and render the same comparison. Use this before pushing; use /perf for the reviewer-visible comment on the PR. The workflow publishes that rendered comparison only when the benchmark run completes successfully; on a failed or incomplete run it posts a neutral fallback comment linking the run log instead.
• It is unrelated to the startup perf harness under tests/startup_perf/ (ETW/WPR TTFP/TTI measurement) — do not invoke that here.