dotnet/roslyn

Honor BuildInParallel

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#82173 opened on Dec 6, 2025

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Area-Compilershelp wanted

Description

Commit 58047eaec542dec6081cd0393dc710f3f5d739be

My old system is very resource-constrained so I normally need to compile with the equivalent of --jobs 1 --load 2.0.

When building Roslyn, I had htop open because my system hung in a previous attempt (needed magic+sysrq+f (oom)), and saw all 4 cores were maxed and memory quickly climbed for the /opt/dotnet/dotnet exec /opt/dotnet/sdk/9.0.306/Roslyn/bincore/VBCSCompiler.dll -pipename:<pipeName> process.

Yes, "9.0". I made some small alterations to try to build with what I have (including rolling-back the arcade version). I've successfully done this in the past with omnisharp. It just takes time for me to figure out. But the "parallel" aspect is unrelated.

I looked anywhere I could for BuildInParallel that wasn't explicitly set to false, and forced it (eg src/Features/CSharpTest/Microsoft.CodeAnalysis.CSharp.Features.UnitTests.csproj and src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/PackAllRids.targets).

But even then, my command:

$ dotnet build \
--no-restore \
-verbosity:diag \
-maxCpuCount:1 \
-property:RestoreBuildInParallel=false \
-property:BuildInParallel=false \
-property:Configuration=Release \
-property:Platform="Any CPU" \
-property:TargetFramework=net9.0 \
-property:TargetRuntime=linux-x64 \
-property:DotNetBuildSourceOnly=true \
-property:DotNetBuildFromVMR=true \
-property:TargetRid=linux-x64 \
-property:RepoRoot=${PWD}/ \
-property:TreatWarningsAsErrors=false \
Compilers.slnf \
2>&1 | tee build.log

still resulted in:

<...>
/<pathTo>/roslyn/src/NuGet/Microsoft.CodeAnalysis.BuildClient.Package/Microsoft.CodeAnalysis.BuildClient.Package.csproj
                  AdditionalProperties=Configuration=Release; Platform=AnyCPU
                  Configuration=Release
                  Platform=AnyCPU
                  SkipNonexistentProjects=False
                  ToolsVersion= (TaskId:3)
  Task Parameter:BuildInParallel=True (TaskId:3)
  Global Properties: (TaskId:3)
    BuildingSolutionFile=true (TaskId:3)
    CurrentSolutionConfigurationContents=<SolutionConfiguration>
    <ProjectConfiguration Project="{600AF682-E097-407B-AD85-EE3CED37E680}" AbsolutePath="/<pathTo>/roslyn/src/Deployment/RoslynDeployment.csproj" BuildProjectInSolution="True">Release|AnyCPU</ProjectConfiguration>
<...>

Note BuildingSolutionFile=true for that task.

While the system resources were climbing for the process, I froze it to see the last stuff outputted was:

<...other *.cs exceeding beyond the scrollback buffer>
/<pathTo>/src/Compilers/CSharp/CSharpAnalyzerDriver/CSharpDeclarationComputer.cs "/<pathTo>/artifacts/obj/Microsoft.CodeAnalysis.CSharp/Release/net9.0/.NETCoreApp,Version=v9.0.AssemblyAttributes.cs" /<pathTo>/artifacts/obj/Microsoft.CodeAnalysis.CSharp/Release/net9.0/Microsoft.CodeAnalysis.CSharp.InternalsVisibleTo.cs /<pathTo>/artifacts/obj/Microsoft.CodeAnalysis.CSharp/Release/net9.0/Microsoft.CodeAnalysis.CSharp.CSharpResources.cs /<pathTo>/artifacts/obj/Microsoft.CodeAnalysis.CSharp/Release/net9.0/Microsoft.CodeAnalysis.CSharp.AssemblyInfo.cs /warnaserror+:NU1605,SYSLIB0011 /warnaserror-:IDE0055 (TaskId:307)
  Assembly loaded during TaskRun (Microsoft.CodeAnalysis.BuildTasks.Csc): System.Security.Principal.Windows, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a (location: /opt/dotnet/shared/Microsoft.NETCore.App/9.0.10/System.Security.Principal.Windows.dll, MVID: d7ae0db5-c7a2-4ffb-94b9-b59a3271f1fc, AssemblyLoadContext: Default) (TaskId:307)
  Assembly loaded during TaskRun (Microsoft.CodeAnalysis.BuildTasks.Csc): System.Security.Claims, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a (location: /opt/dotnet/shared/Microsoft.NETCore.App/9.0.10/System.Security.Claims.dll, MVID: 57921f00-3884-44e5-a77c-29ab174756fa, AssemblyLoadContext: Default) (TaskId:307)
  Assembly loaded during TaskRun (Microsoft.CodeAnalysis.BuildTasks.Csc): System.IO.Pipes, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a (location: /opt/dotnet/shared/Microsoft.NETCore.App/9.0.10/System.IO.Pipes.dll, MVID: b7b52840-70ad-48cd-b397-7a554b891b4f, AssemblyLoadContext: Default) (TaskId:307)
  Assembly loaded during TaskRun (Microsoft.CodeAnalysis.BuildTasks.Csc): System.Diagnostics.StackTrace, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a (location: /opt/dotnet/shared/Microsoft.NETCore.App/9.0.10/System.Diagnostics.StackTrace.dll, MVID: 6bcf773e-a3c7-4849-924e-d43e57ae6a0e, AssemblyLoadContext: Default) (TaskId:307)

So is the issue with VBCSCompiler.dll not receiving any parallel option? A quick ildasm glance didn't show any such capability.

Contributor guide

Tech stack
csharp
Domain
build systemperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Basic C# knowledgeUnderstanding of MSBuildFamiliarity with Roslyn build process
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate how the BuildInParallel property is propagated in MSBuild tasks during the Roslyn build. The issue shows that even when set to false via property:BuildInParallel=false, some tasks still have BuildInParallel=True, particularly when BuildingSolutionFile=true. Look at the custom MSBuild tasks in the Roslyn repository, especially those in 'src/NuGet/Microsoft.CodeAnalysis.BuildClient.Package' and related targets. The user suspects VBCSCompiler.dll does not receive parallel options. Check the MSBuild logs and property passing mechanisms.