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: 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 3: 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 4: 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" />

Disable Transitive Project References

For explicit-only dependency management (extreme measure for very large repos):

<PropertyGroup>
  <DisableTransitiveProjectReferences>true</DisableTransitiveProjectReferences>
</PropertyGroup>

Caution: This requires all dependencies to be listed explicitly. Only use in large repos where transitive closure is causing excessive rebuilds.

Step 5: Static Graph Builds (/graph)

Static graph mode evaluates the entire project graph before building, enabling better scheduling and isolation.

Enabling Graph Build

# Single invocation
dotnet build /graph

# With binary log for analysis
dotnet build /graph /bl:graph-build.binlog

Benefits

• Better parallelism: MSBuild knows the full graph upfront and can schedule optimally
• Build isolation: Each project builds in isolation (no cross-project state leakage)
• Caching potential: With isolation, individual project results can be cached

When to Use

Troubleshooting Graph Build

Graph build requires that all ProjectReference items are statically determinable (no dynamic references computed in targets). If graph build fails:

error MSB4260: Project reference "..." could not be resolved with static graph.

Fix: Ensure all ProjectReference items are declared in <ItemGroup> outside of targets (not dynamically computed inside <Target> blocks).

Step 6: Parallel Build Tuning

MaxCpuCount

# Use all available cores (default in dotnet build)
dotnet build -m

# Specify explicit core count (useful for CI with shared agents)
dotnet build -m:4

# MSBuild.exe syntax
msbuild /m:8 MySolution.sln

Identifying Parallelism Bottlenecks

In a binlog, look for:

• Long sequential chains: Projects that must build one after another due to dependencies
• Uneven load: Some build nodes idle while others are overloaded
• Single-project bottleneck: One large project on the critical path that blocks everything

Use grep 'Target Performance Summary' -A 30 full.log in binlog analysis to see build node utilization.

Reducing the Critical Path

The critical path is the longest chain of dependent projects. To shorten it:

1. Break large projects into smaller ones that can build in parallel
2. Remove unnecessary ProjectReferences (see Step 5)
3. Use ReferenceOutputAssembly="false" for build-order-only dependencies
4. Move shared code to a base library that builds first, then parallelize consumers

Step 7: Additional Quick Wins

Separate Restore from Build

# In CI, restore once then build without restore
dotnet restore
dotnet build --no-restore -m
dotnet test --no-build

Skip Unnecessary Targets

# Skip building documentation
dotnet build /p:GenerateDocumentationFile=false

# Skip analyzers during development (not for CI!)
dotnet build /p:RunAnalyzers=false

Use Project-Level Filtering

# Build only the project you're working on (and its dependencies)
dotnet build src/MyApp/MyApp.csproj

# Don't build the entire solution if you only need one project

Binary Log for All Investigations

Always start with a binlog:

dotnet build /bl:perf.binlog -m

Then use the build-perf-diagnostics skill and binlog tools for systematic bottleneck identification.

Optimization Decision Tree

Is your no-op build slow (> 10s per project)?
├── YES → See `incremental-build` skill (fix Inputs/Outputs)
└── NO
    Is your cold build slow?
    ├── YES
    │   Is restore slow?
    │   ├── YES → Optimize NuGet restore (use lock files, configure local cache)
    │   └── NO
    │       Is compilation slow?
    │       ├── YES
    │       │   Are analyzers/generators slow?
    │       │   ├── YES → See `build-perf-diagnostics` skill
    │       │   └── NO → Check parallelism, graph build, critical path (this skill + `build-parallelism`)
    │       └── NO → Check custom targets (binlog analysis via `build-perf-diagnostics`)
    └── NO
        Is your warm build slow?
        ├── YES → Projects rebuilding unnecessarily → check `incremental-build` skill
        └── NO → Build is healthy! Consider graph build or UseArtifactsOutput for further gains