Build Perf Baseline

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.

# 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.

# 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.

# Build once to populate outputs
dotnet build -m

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

What Good Looks Like

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

# 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

# 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

dotnet build-server shutdown
dotnet build

Step 3: Artifacts Output Layout

The UseArtifactsOutput feature (introduced in .NET 8) changes the output directory structure to avoid bin/obj clash issues and enable better caching.

Enabling Artifacts Output

<!-- Directory.Build.props -->
<PropertyGroup>
  <UseArtifactsOutput>true</UseArtifactsOutput>
</PropertyGroup>

Before vs After

# Traditional layout (before)
src/
  MyLib/
    bin/Debug/net8.0/MyLib.dll
    obj/Debug/net8.0/...
  MyApp/
    bin/Debug/net8.0/MyApp.dll

# Artifacts layout (after)
artifacts/
  bin/MyLib/debug/MyLib.dll
  bin/MyApp/debug/MyApp.dll
  obj/MyLib/debug/...
  obj/MyApp/debug/...

Benefits

• No bin/obj clash: Each project+configuration gets a unique path automatically
• Easier to cache: Single artifacts/ directory to cache/restore in CI
• Cleaner .gitignore: Just ignore artifacts/
• Multi-targeting safe: Each TFM gets its own subdirectory

Customizing

<!-- Change the artifacts root -->
<PropertyGroup>
  <ArtifactsPath>$(MSBuildThisFileDirectory)output</ArtifactsPath>
</PropertyGroup>

Step 4: Deterministic Builds

Deterministic builds produce byte-for-byte identical output given the same inputs. This is essential for build caching and reproducibility.

Enabling Deterministic Builds

<!-- Directory.Build.props -->
<PropertyGroup>
  <!-- Enabled by default in .NET SDK projects since SDK 2.0+ -->
  <Deterministic>true</Deterministic>

  <!-- For full reproducibility, also set: -->
  <ContinuousIntegrationBuild Condition="'$(CI)' == 'true'">true</ContinuousIntegrationBuild>
</PropertyGroup>

What Deterministic Affects

• Removes timestamps from PE headers
• Uses consistent file paths in PDBs
• Produces identical output for identical input

Why It Matters for Performance

• Build caching: If outputs are deterministic, you can cache and reuse them across builds and machines
• CI optimization: Skip rebuilding unchanged projects by comparing inputs
• Distributed builds: Safe to cache compilation results in shared storage

Step 5: Dependency Graph Trimming

Reducing unnecessary project references shortens the critical path and reduces what gets built.

Audit the Dependency Graph

# Visualize the dependency graph
dotnet build /bl:graph.binlog

# In the binlog, check project references and build times
# Look for projects that are referenced but could be trimmed

Techniques

Remove Redundant Transitive References

<!-- BAD: Utils is already referenced transitively via Core -->
<ItemGroup>
  <ProjectReference Include="..\Core\Core.csproj" />
  <ProjectReference Include="..\Utils\Utils.csproj" />
</ItemGroup>

<!-- GOOD: Let transitive references flow automatically -->
<ItemGroup>
  <ProjectReference Include="..\Core\Core.csproj" />
</ItemGroup>

Build-Order-Only References

When you need a project to build before yours but don't need its assembly output:

<!-- Only ensures build order, doesn't reference the output assembly -->
<ProjectReference Include="..\CodeGen\CodeGen.csproj"
                  ReferenceOutputAssembly="false" />

Prevent Transitive Flow

When a dependency is an internal implementation detail that shouldn't flow to consumers:

<!-- Don't expose this dependency transitively -->
<ProjectReference Include="..\InternalHelpers\InternalHelpers.csproj"
                  PrivateAssets="all" />