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Limit MaxDegreeOfParallelism in RoslynParallel#80963

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333fred merged 2 commits intodotnet:mainfrom
chiangvincent:dev/chiangvincent/parallellimit
Oct 31, 2025
Merged

Limit MaxDegreeOfParallelism in RoslynParallel#80963
333fred merged 2 commits intodotnet:mainfrom
chiangvincent:dev/chiangvincent/parallellimit

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@chiangvincent chiangvincent commented Oct 30, 2025 •
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Description

This bug was found by Agent Kean, the VS PerfRel AI Agent. Please review this PR with extra care! For more information, visit our wiki. Please share feedback with TIP Insights

  • Issue: RoslynParallel.For calls Parallel.For with ParallelOptions that do not set MaxDegreeOfParallelism, permitting unbounded concurrency and contributing to high CPU via runaway thread pool usage.
  • Issue type: Rule 1 — DO NOT call Parallel.For/Parallel.ForEach/Parallel.Invoke without specifying ParallelOptions.MaxDegreeOfParallelism.
  • Proposed fix: Set MaxDegreeOfParallelism = Environment.ProcessorCount in both DefaultParallelOptions and the cancellation-aware ParallelOptions used by RoslynParallel.For. This is a small, single-file change in a leaf-frame method present in the call stack.

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@chiangvincent chiangvincent requested a review from a team as a code owner October 30, 2025 16:45
@dotnet-policy-service dotnet-policy-service bot added the Community The pull request was submitted by a contributor who is not a Microsoft employee. label Oct 30, 2025
333fred
333fred reviewed Oct 30, 2025
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333fred commented Oct 31, 2025

@dotnet/roslyn-compiler for a second review

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RikkiGibson commented Oct 31, 2025

CI failure looks weird:

##[error](NETCORE_ENGINEERING_TELEMETRY=Test) Tests failed
PublicAccessNotPermittedPublic access is not permitted on this storage account.
RequestId:743eaea9-101e-005f-28f5-49ec70000000
Time:2025-10-30T23:34:38.8609499Z
System.Net.WebException: The remote server returned an error: (409) Conflict.
   at Microsoft.PowerShell.Commands.WebRequestPSCmdlet.GetResponse(WebRequest request)
   at Microsoft.PowerShell.Commands.WebRequestPSCmdlet.ProcessRecord()
at Ensure-ProcDump, D:\a\_work\1\s\eng\build.ps1: line 660
at TestUsingRunTests, D:\a\_work\1\s\eng\build.ps1: line 449
at <ScriptBlock>, D:\a\_work\1\s\eng\build.ps1: line 780
at <ScriptBlock>, D:\a\_work\_temp\fd46ff11-732c-48c5-8cf9-818bd9b63d11.ps1: line 4
at <ScriptBlock>, <No file>: line 1

We'll see how it goes on re-run

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chiangvincent commented Oct 31, 2025

Thanks @RikkiGibson and @333fred for the reviews -- looks like the CI is failing since there's a storage account permissions issue on the unit test suite. Tried requeuing but running into a Cannot read properties of undefined (reading 'replace') error. Could you guys requeue again, thanks!

Failing build run

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@333fred 333fred merged commit 72c44da into dotnet:main Oct 31, 2025
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@dotnet-policy-service dotnet-policy-service bot added this to the Next milestone Oct 31, 2025
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@davidwengier davidwengier modified the milestones: Next, 18.3 Jan 6, 2026
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@333fred 333fred 333fred approved these changes

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