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.

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.

	Setting name	Values
runtimeconfig.json	N/A	N/A
Environment variable	COMPlus_Thread_UseAllCpuGroups or DOTNET_Thread_UseAllCpuGroups	0 - disabled 1 - enabled

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.

	Setting name	Values
runtimeconfig.json	N/A	N/A
Environment variable	COMPlus_Thread_AssignCpuGroups or DOTNET_Thread_AssignCpuGroups	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
      }
   }
}

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.

	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
      }
   }
}

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.

	Setting name	Values	Version introduced
runtimeconfig.json	System.Threading.ThreadPool.UseWindowsThreadPool	true - enabled false - disabled	.NET 8
MSBuild property	UseWindowsThreadPool	true - enabled false - disabled	.NET 8
Environment variable	DOTNET_ThreadPool_UseWindowsThreadPool	1 - enabled 0 - disabled	.NET 8

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.

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
System.Threading.ThreadPool.Blocking.IgnoreMemoryUsage	By default, the rate of thread injection in response to blocking is limited by heuristics that determine whether there is sufficient physical memory available. In some situations, it may be preferable to inject threads more quickly even in low-memory situations. You can disable the memory usage heuristics by turning off this switch.	.NET 7

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.

	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
      }
   }
}

runtimeconfig.template.json file:

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

Project file:

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

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

</Project>