build-parallelism

name	build-parallelism
description	Guide for optimizing MSBuild build parallelism and multi-project scheduling. Only activate in MSBuild/.NET build context. USE FOR: builds not utilizing all CPU cores, speeding up multi-project solutions, evaluating graph build mode (/graph), build time not improving with -m flag, understanding project dependency topology. Note: /maxcpucount default is 1 (sequential) — always use -m for parallel builds. Covers /maxcpucount, graph build for better scheduling and isolation, BuildInParallel on MSBuild task, reducing unnecessary ProjectReferences, solution filters (.slnf) for building subsets. DO NOT USE FOR: single-project builds, incremental build issues (use incremental-build), compilation slowness within a project (use build-perf-diagnostics), non-MSBuild build systems. INVOKES: binlog MCP server tools (expensive_projects, expensive_targets, project_target_times); falls back to dotnet build -m, dotnet build /graph, binlog replay + grep.
license	MIT

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