Runtime configuration options for threading

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

Use all CPU groups on Windows

• On machines that have multiple CPU groups, this setting configures whether components such as the thread pool use all CPU groups or only the primary CPU group of the process. The setting also affects what Environment.ProcessorCount returns.
• When this setting is enabled, all CPU groups are used and threads are also automatically distributed across CPU groups by default.
• This setting is enabled by default on Windows 11 and later versions, and disabled by default on Windows 10 and earlier versions. For this setting to take effect when enabled, the GC must also be configured to use all CPU groups; for more information, see GC CPU groups.

Assign threads to CPU groups on Windows

• On machines that have multiple CPU groups and all CPU groups are being used, this setting configures whether threads are automatically distributed across CPU groups.
• When this setting is enabled, new threads are assigned to a CPU group in a way that tries to fully populate a CPU group that is already in use before utilizing a new CPU group.
• This setting is enabled by default.

Minimum threads

• Specifies the minimum number of threads for the worker thread pool.
• Corresponds to the ThreadPool.SetMinThreads method.

Examples

runtimeconfig.json file:

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

runtimeconfig.template.json file:

{
   "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.

Examples

runtimeconfig.json file:

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

runtimeconfig.template.json file:

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

Project file:

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

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

</Project>

Windows thread pool

• For projects on Windows, configures whether thread pool thread management is delegated to the Windows thread pool.
• If you omit this setting or the platform is not Windows, the .NET thread pool is used instead.
• Only applications published with Native AOT on Windows use the Windows thread pool by default, for which you can opt to use the .NET thread pool instead by disabling the config setting.
• The Windows thread pool may perform better in some cases, such as in cases where the minimum number of threads is configured to a high value, or when the Windows thread pool is already being heavily used by the app. There may also be cases where the .NET thread pool performs better, such as in heavy I/O handling on larger machines. It's advisable to check performance metrics when changing this config setting.
• Some APIs are not supported when using the Windows thread pool, such as ThreadPool.SetMinThreads, ThreadPool.SetMaxThreads, and ThreadPool.BindHandle(SafeHandle). Thread pool config settings for minimum and maximum threads are also not effective. An alternative to ThreadPool.BindHandle(SafeHandle) is the ThreadPoolBoundHandle class.

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.Threading.ThreadPool.UseWindowsThreadPool": true
      }
   }
}

runtimeconfig.template.json file:

{
   "configProperties": {
      "System.Threading.ThreadPool.UseWindowsThreadPool": true
   }
}

Project file:

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

  <PropertyGroup>
    <UseWindowsThreadPool>true</UseWindowsThreadPool>
  </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.

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
      }
   }
}

runtimeconfig.template.json file:

{
   "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.

Examples

runtimeconfig.json file:

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

runtimeconfig.template.json file:

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

Project file:

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

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

</Project>