build-parallelism

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name: build-parallelism description: "Diagnose and fix under-parallelized MSBuild builds. USE WHEN a multi-project solution build is slower than expected, doesn't speed up when you add cores, pegs a single core while others idle, or you want to know why -m isn't helping. Note: /maxcpucount default is 1 (sequential) — always pass -m for parallel builds. Covers finding the critical path (longest serial ProjectReference chain), graph build (/graph), BuildInParallel, and solution filters (.slnf). DO NOT USE FOR: single-project builds, incremental issues (use incremental-build), compilation slowness inside one project (use build-perf-diagnostics), non-MSBuild build systems." license: MIT

Diagnose a slow parallel build (start here)

Work this checklist in order — it targets the usual root cause (a serial dependency chain that no number of cores can parallelize):

1. Confirm parallelism is even on. Rebuild with dotnet build -m /bl:{} (PowerShell: dotnet build -m -bl:{{}}). -m with no number uses all logical processors; without -m MSBuild runs a single node (sequential).
2. Find the critical path. From the binlog, read per-project timings and the node timeline. If total build time ≈ the sum of the projects on one dependency chain, that chain — not CPU count — is the bottleneck.
3. Name the chain explicitly, e.g. Core → Api → Web → Tests. A long serial chain stays serial no matter how large -m is, because each project waits on its predecessor.
4. Look for unnecessary ProjectReference edges that lengthen the chain — a reference that only needs build order (not the output assembly), or one that could be a PackageReference, forces serialization it doesn't need.
5. Recommend flattening: break false dependencies so independent projects build concurrently, and consider /graph for better scheduling.

MSBuild Parallelism Model

• /maxcpucount (or -m): number of worker nodes (processes)
• Default: 1 node (sequential!). Always use -m for parallel builds
• Recommended: -m without a number = use all logical processors
• Each node builds one project at a time
• Projects are scheduled based on dependency graph

Project Dependency Graph

• MSBuild builds projects in dependency order (topological sort)
• Critical path: longest chain of dependent projects determines minimum build time
• Bottleneck: if project A depends on B, C, D and B takes 60s while C and D take 5s, B is the bottleneck
• Diagnosis: replay binlog to diagnostic log with performancesummary and check Project Performance Summary — shows per-project time; grep for node.*assigned to check scheduling
• Wide graphs (many independent projects) parallelize well; deep graphs (long chains) don't

Graph Build Mode (/graph)

• dotnet build /graph or msbuild /graph
• What it changes: MSBuild constructs the full project dependency graph BEFORE building
• Benefits: better scheduling, avoids redundant evaluations, enables isolated builds
• Limitations: all projects must use <ProjectReference> (no programmatic MSBuild task references)
• When to use: large solutions with many projects, CI builds
• When NOT to use: projects that dynamically discover references at build time

Optimizing Project References

• Reduce unnecessary <ProjectReference> — each adds to the dependency chain
• Use <ProjectReference ... SkipGetTargetFrameworkProperties="true"> to avoid extra evaluations
• <ProjectReference ... ReferenceOutputAssembly="false"> for build-order-only dependencies
• Consider if a ProjectReference should be a PackageReference instead (pre-built NuGet)
• Use solution filters (.slnf) to build subsets of the solution

BuildInParallel

• <MSBuild Projects="@(ProjectsToBuild)" BuildInParallel="true" /> in custom targets
• Without BuildInParallel="true", MSBuild task batches projects sequentially
• Ensure /maxcpucount > 1 for this to have effect

Multi-threaded MSBuild Tasks

• Individual tasks can run multi-threaded within a single project build
• Tasks implementing IMultiThreadableTask can run on multiple threads
• Tasks must declare thread-safety via [MSBuildMultiThreadableTask]

Analyzing Parallelism with Binlog

Primary: binlog MCP (preferred)

Use the binlog MCP server (Microsoft.AITools.BinlogMcp, exposed under the binlog MCP namespace):

1. Use expensive_projects tool → find the slowest projects and compare individual vs total build time
2. Use expensive_targets tool → find bottleneck targets
3. Use project_target_times tool → drill into a specific project's target-level timing
4. Ideal: build time should be much less than sum of project times (parallelism)
5. If build time ≈ sum of project times: too many serial dependencies, or one slow project blocking others

Fallback: text-log replay (when MCP is unavailable)

Step-by-step:

1. Replay the binlog: dotnet msbuild build.binlog -noconlog -fl -flp:v=diag;logfile=full.log;performancesummary
2. Check Project Performance Summary at the end of full.log
3. Ideal: build time should be much less than sum of project times (parallelism)
4. If build time ≈ sum of project times: too many serial dependencies, or one slow project blocking others
5. grep 'Target Performance Summary' -A 30 full.log → find the bottleneck targets
6. Consider splitting large projects or optimizing the critical path

CI/CD Parallelism Tips

• Use -m in CI (many CI runners have multiple cores)
• Consider splitting solution into build stages for extreme parallelism
• Use build caching (NuGet lock files, deterministic builds) to avoid rebuilding unchanged projects
• dotnet build /graph works well with structured CI pipelines