build-perf-baseline

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• SKILL.md

SKILL.md (first lines)

# Build Performance Baseline & Optimization

## Overview

Before optimizing a build, you need a **baseline**. Without measurements, optimization is guesswork. This skill covers how to establish baselines and apply systematic optimization techniques.

**Related skills:**
- `build-perf-diagnostics` — binlog-based bottleneck identification
- `incremental-build` — Inputs/Outputs and up-to-date checks
- `build-parallelism` — parallel and graph build tuning
- `eval-performance` — glob and import chain optimization

---

## Step 1: Establish a Performance Baseline

Measure three scenarios to understand where time is spent:

### Cold Build (First Build)

No previous build output exists. Measures the full end-to-end time including restore, compilation, and all targets.

```bash
# Clean everything first
dotnet clean
# Remove bin/obj to truly start fresh
Get-ChildItem -Recurse -Directory -Include bin,obj | Remove-Item -Recurse -Force
# OR on Linux/macOS:
# find . -type d \( -name bin -o -name obj \) -exec rm -rf {} +

# Measure cold build
dotnet build /bl:cold-build.binlog -m
```

### Warm Build (Incremental Build)

Build output exists, some files have changed. Measures how well incremental build works.

```bash
# Build once to populate outputs
dotnet build -m

# Make a small change (touch one .cs file)
# Then rebuild
dotnet build /bl:warm-build.binlog -m
```

### No-Op Build (Nothing Changed)

Build output exists, nothing has changed. This should be nearly instant. If it's slow, incremental build is broken.

```bash
# Build once to populate outputs
dotnet build -m

# Rebuild immediately without changes
dotnet build /bl:noop-build.binlog -m
```

### What Good Looks Like

| Scenario | Expected Behavior |
|----------|------------------|
| Cold build | Full compilation, all targets run. This is your absolute baseline |
| Warm build | Only changed projects recompile. Time proportional to change scope |
| No-op build | < 5 seconds for small repos, < 30 seconds for large repos. All compilation targets should report "Skipping target — all outputs up-to-date" |

**Red flags:**
- No-op build > 30 seconds → incremental build is broken (see `incremental-build` skill)
- Warm build recompiles everything → project dependency chain forces full rebuild
- Cold build has long restore → NuGet cache issues

### Recording Baselines

Record baselines in a structured way before and after optimization:

```
| Scenario    | Before  | After   | Improvement |
|-------------|---------|---------|-------------|
| Cold build  | 2m 15s  |         |             |
| Warm build  | 1m 40s  |         |             |
| No-op build | 45s     |         |             |
```

---

## Step 2: MSBuild Server (Persistent Build Process)

The MSBuild server keeps the build process alive between invocations, avoiding JIT compilation and assembly loading overhead on every build.

### Enabling MSBuild Server

```bash
# Enabled by default in .NET 8+ but can be forced
dotnet build /p:UseSharedCompilation=true
```

The MSBuild server is started automatically and reused across builds. The compiler server (VBCSCompiler / `dotnet build-server`) is separate but complementary.

### Managing the Build Server

```bash
# Check if the server is running
dotnet build-server status

# Shut down all build servers (useful when debugging)
dotnet build-server shutdown
```

### When to Restart the Build Server

Restart after:
- Updating the .NET SDK
- Changing MSBuild tooling (custom tasks, props, targets)
- Debugging build infrastructure issues
- Seeing stale behavior in repeated builds

```bash
dotnet build-server shutdown

From SteffenCarlsen/agent-skills, MIT.

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