Runtime configuration options for threading

This article details the settings you can use to configure threading in .NET.

Note

.NET 6 standardizes on the prefix DOTNET_ instead of COMPlus_ for environment variables that configure .NET run-time behavior. However, the COMPlus_ prefix will continue to work. If you're using a previous version of the .NET runtime, you should still use the COMPlus_ prefix for environment variables.

CPU groups

  • Configures whether threads are automatically distributed across CPU groups.
  • If you omit this setting, threads are not distributed across CPU groups. This is equivalent to setting the value to 0.
Setting name Values
runtimeconfig.json N/A N/A
Environment variable COMPlus_Thread_UseAllCpuGroups or DOTNET_Thread_UseAllCpuGroups 0 - disabled
1 - enabled

Minimum threads

  • Specifies the minimum number of threads for the worker thread pool.
  • Corresponds to the ThreadPool.SetMinThreads method.
Setting name Values
runtimeconfig.json System.Threading.ThreadPool.MinThreads An integer that represents the minimum number of threads
MSBuild property ThreadPoolMinThreads An integer that represents the minimum number of threads
Environment variable N/A N/A

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.Threading.ThreadPool.MinThreads": 4
      }
   }
}

Project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <ThreadPoolMinThreads>4</ThreadPoolMinThreads>
  </PropertyGroup>

</Project>

Maximum threads

  • Specifies the maximum number of threads for the worker thread pool.
  • Corresponds to the ThreadPool.SetMaxThreads method.
Setting name Values
runtimeconfig.json System.Threading.ThreadPool.MaxThreads An integer that represents the maximum number of threads
MSBuild property ThreadPoolMaxThreads An integer that represents the maximum number of threads
Environment variable N/A N/A

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.Threading.ThreadPool.MaxThreads": 20
      }
   }
}

Project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <ThreadPoolMaxThreads>20</ThreadPoolMaxThreads>
  </PropertyGroup>

</Project>

Thread injection in response to blocking work items

In some cases, the thread pool detects work items that block its threads. To compensate, it injects more threads. In .NET 6+, you can use the following runtime configuration settings to configure thread injection in response to blocking work items. Currently, these settings take effect only for work items that wait for another task to complete, such as in typical sync-over-async cases.

runtimeconfig.json setting name Description Version introduced
System.Threading.ThreadPool.Blocking.ThreadsToAddWithoutDelay_ProcCountFactor After the thread count based on MinThreads is reached, this value (after it is multiplied by the processor count) specifies how many additional threads may be created without a delay. .NET 6
System.Threading.ThreadPool.Blocking.ThreadsPerDelayStep_ProcCountFactor After the thread count based on ThreadsToAddWithoutDelay is reached, this value (after it is multiplied by the processor count) specifies after how many threads an additional DelayStepMs would be added to the delay before each new thread is created. .NET 6
System.Threading.ThreadPool.Blocking.DelayStepMs After the thread count based on ThreadsToAddWithoutDelay is reached, this value specifies how much additional delay to add per ThreadsPerDelayStep threads, which would be applied before each new thread is created. .NET 6
System.Threading.ThreadPool.Blocking.MaxDelayMs After the thread count based on ThreadsToAddWithoutDelay is reached, this value specifies the max delay to use before each new thread is created. .NET 6

How the configuration settings take effect

  • After the thread count based on MinThreads is reached, up to ThreadsToAddWithoutDelay additional threads may be created without a delay.
  • After that, before each additional thread is created, a delay is induced, starting with DelayStepMs.
  • For every ThreadsPerDelayStep threads that are added with a delay, an additional DelayStepMs is added to the delay.
  • The delay may not exceed MaxDelayMs.
  • Delays are only induced before creating threads. If threads are already available, they would be released without delay to compensate for blocking work items.
  • Physical memory usage and limits are also used and, beyond a threshold, the system switches to slower thread injection.

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.Threading.ThreadPool.Blocking.ThreadsToAddWithoutDelay_ProcCountFactor": 5
      }
   }
}

AutoreleasePool for managed threads

This option configures whether each managed thread receives an implicit NSAutoreleasePool when running on a supported macOS platform.

Setting name Values Version introduced
runtimeconfig.json System.Threading.Thread.EnableAutoreleasePool true or false .NET 6
MSBuild property AutoreleasePoolSupport true or false .NET 6
Environment variable N/A N/A N/A

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.Threading.Thread.EnableAutoreleasePool": true
      }
   }
}

Project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <AutoreleasePoolSupport>true</AutoreleasePoolSupport>
  </PropertyGroup>

</Project>