Run-time configuration options for garbage collection

This page contains information about garbage collector (GC) settings that can be changed at run time. If you're trying to achieve peak performance of a running app, consider using these settings. However, the defaults provide optimum performance for most applications in typical situations.

Settings are arranged into groups on this page. The settings within each group are commonly used in conjunction with each other to achieve a specific result.

Note

  • These settings can also be changed dynamically by the app as it's running, so any run-time settings you set may be overridden.
  • Some settings, such as latency level, are typically set only through the API at design time. Such settings are omitted from this page.
  • For number values, use decimal notation for settings in the runtimeconfig.json file and hexadecimal notation for environment variable settings. For hexadecimal values, you can specify them with or without the "0x" prefix.

Flavors of garbage collection

The two main flavors of garbage collection are workstation GC and server GC. For more information about differences between the two, see Workstation and server garbage collection.

The subflavors of garbage collection are background and non-concurrent.

Use the following settings to select flavors of garbage collection:

System.GC.Server/COMPlus_gcServer

  • Configures whether the application uses workstation garbage collection or server garbage collection.
  • Default: Workstation garbage collection. This is equivalent to setting the value to false.
Setting name Values Version introduced
runtimeconfig.json System.GC.Server false - workstation
true - server
.NET Core 1.0
MSBuild property ServerGarbageCollection false - workstation
true - server
.NET Core 1.0
Environment variable COMPlus_gcServer 0 - workstation
1 - server
.NET Core 1.0
app.config for .NET Framework GCServer false - workstation
true - server

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.Server": true
      }
   }
}

Project file:

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

  <PropertyGroup>
    <ServerGarbageCollection>true</ServerGarbageCollection>
  </PropertyGroup>

</Project>

System.GC.Concurrent/COMPlus_gcConcurrent

  • Configures whether background (concurrent) garbage collection is enabled.
  • Default: Use background GC. This is equivalent to setting the value to true.
  • For more information, see Background garbage collection.
Setting name Values Version introduced
runtimeconfig.json System.GC.Concurrent true - background GC
false - non-concurrent GC
.NET Core 1.0
MSBuild property ConcurrentGarbageCollection true - background GC
false - non-concurrent GC
.NET Core 1.0
Environment variable COMPlus_gcConcurrent 1 - background GC
0 - non-concurrent GC
.NET Core 1.0
app.config for .NET Framework gcConcurrent true - background GC
false - non-concurrent GC

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.Concurrent": false
      }
   }
}

Project file:

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

  <PropertyGroup>
    <ConcurrentGarbageCollection>false</ConcurrentGarbageCollection>
  </PropertyGroup>

</Project>

Manage resource usage

Use the settings described in this section to manage the garbage collector's memory and processor usage.

For more information about some of these settings, see the Middle ground between workstation and server GC blog entry.

System.GC.HeapCount/COMPlus_GCHeapCount

  • Limits the number of heaps created by the garbage collector.
  • Applies to server garbage collection only.
  • If GC processor affinity is enabled, which is the default, the heap count setting affinitizes n GC heaps/threads to the first n processors. (Use the affinitize mask or affinitize ranges settings to specify exactly which processors to affinitize.)
  • If GC processor affinity is disabled, this setting limits the number of GC heaps.
  • For more information, see the GCHeapCount remarks.
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapCount decimal value .NET Core 3.0
Environment variable COMPlus_GCHeapCount hexadecimal value .NET Core 3.0
app.config for .NET Framework GCHeapCount decimal value .NET Framework 4.6.2

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.HeapCount": 16
      }
   }
}

Tip

If you're setting the option in runtimeconfig.json, specify a decimal value. If you're setting the option as an environment variable, specify a hexadecimal value. For example, to limit the number of heaps to 16, the values would be 16 for the JSON file and 0x10 or 10 for the environment variable.

System.GC.HeapAffinitizeMask/COMPlus_GCHeapAffinitizeMask

  • Specifies the exact processors that garbage collector threads should use.
  • If GC processor affinity is disabled, this setting is ignored.
  • Applies to server garbage collection only.
  • The value is a bit mask that defines the processors that are available to the process. For example, a decimal value of 1023 (or a hexadecimal value of 0x3FF or 3FF if you're using the environment variable) is 0011 1111 1111 in binary notation. This specifies that the first 10 processors are to be used. To specify the next 10 processors, that is, processors 10-19, specify a decimal value of 1047552 (or a hexadecimal value of 0xFFC00 or FFC00), which is equivalent to a binary value of 1111 1111 1100 0000 0000.
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapAffinitizeMask decimal value .NET Core 3.0
Environment variable COMPlus_GCHeapAffinitizeMask hexadecimal value .NET Core 3.0
app.config for .NET Framework GCHeapAffinitizeMask decimal value .NET Framework 4.6.2

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.HeapAffinitizeMask": 1023
      }
   }
}

System.GC.GCHeapAffinitizeRanges/COMPlus_GCHeapAffinitizeRanges

Setting name Values Version introduced
runtimeconfig.json System.GC.GCHeapAffinitizeRanges Comma-separated list of processor numbers or ranges of processor numbers.
Unix example: "1-10,12,50-52,70"
Windows example: "0:1-10,0:12,1:50-52,1:70"
.NET Core 3.0
Environment variable COMPlus_GCHeapAffinitizeRanges Comma-separated list of processor numbers or ranges of processor numbers.
Unix example: "1-10,12,50-52,70"
Windows example: "0:1-10,0:12,1:50-52,1:70"
.NET Core 3.0

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.GCHeapAffinitizeRanges": "0:1-10,0:12,1:50-52,1:70"
      }
   }
}

COMPlus_GCCpuGroup

  • Configures whether the garbage collector uses CPU groups or not.

    When a 64-bit Windows computer has multiple CPU groups, that is, there are more than 64 processors, enabling this element extends garbage collection across all CPU groups. The garbage collector uses all cores to create and balance heaps.

  • Applies to server garbage collection on 64-bit Windows operation systems only.

  • Default: GC does not extend across CPU groups. This is equivalent to setting the value to 0.

  • For more information, see Making CPU configuration better for GC on machines with > 64 CPUs on Maoni Stephens' blog.

Setting name Values Version introduced
runtimeconfig.json N/A N/A N/A
Environment variable COMPlus_GCCpuGroup 0 - disabled
1 - enabled
.NET Core 1.0
app.config for .NET Framework GCCpuGroup false - disabled
true - enabled

Note

To configure the common language runtime (CLR) to also distribute threads from the thread pool across all CPU groups, enable the Thread_UseAllCpuGroups element option. For .NET Core apps, you can enable this option by setting the value of the COMPlus_Thread_UseAllCpuGroups environment variable to 1.

System.GC.NoAffinitize/COMPlus_GCNoAffinitize

  • Specifies whether to affinitize garbage collection threads with processors. To affinitize a GC thread means that it can only run on its specific CPU. A heap is created for each GC thread.
  • Applies to server garbage collection only.
  • Default: Affinitize garbage collection threads with processors. This is equivalent to setting the value to false.
Setting name Values Version introduced
runtimeconfig.json System.GC.NoAffinitize false - affinitize
true - don't affinitize
.NET Core 3.0
Environment variable COMPlus_GCNoAffinitize 0 - affinitize
1 - don't affinitize
.NET Core 3.0
app.config for .NET Framework GCNoAffinitize false - affinitize
true - don't affinitize
.NET Framework 4.6.2

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.NoAffinitize": true
      }
   }
}

System.GC.HeapHardLimit/COMPlus_GCHeapHardLimit

  • Specifies the maximum commit size, in bytes, for the GC heap and GC bookkeeping.

  • This setting only applies to 64-bit computers.

  • The default value, which only applies in certain cases, is the greater of 20 MB or 75% of the memory limit on the container. The default value applies if:

Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimit decimal value .NET Core 3.0
Environment variable COMPlus_GCHeapHardLimit hexadecimal value .NET Core 3.0

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.HeapHardLimit": 209715200
      }
   }
}

Tip

If you're setting the option in runtimeconfig.json, specify a decimal value. If you're setting the option as an environment variable, specify a hexadecimal value. For example, to specify a heap hard limit of 200 mebibytes (MiB), the values would be 209715200 for the JSON file and 0xC800000 or C800000 for the environment variable.

System.GC.HeapHardLimitPercent/COMPlus_GCHeapHardLimitPercent

  • Specifies the allowable GC heap usage as a percentage of the total physical memory.

  • If System.GC.HeapHardLimit is also set, this setting is ignored.

  • This setting only applies to 64-bit computers.

  • If the process is running inside a container that has a specified memory limit, the percentage is calculated as a percentage of that memory limit.

  • The default value, which only applies in certain cases, is the lesser of 20 MB or 75% of the memory limit on the container. The default value applies if:

Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitPercent decimal value .NET Core 3.0
Environment variable COMPlus_GCHeapHardLimitPercent hexadecimal value .NET Core 3.0

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.HeapHardLimitPercent": 30
      }
   }
}

Tip

If you're setting the option in runtimeconfig.json, specify a decimal value. If you're setting the option as an environment variable, specify a hexadecimal value. For example, to limit the heap usage to 30%, the values would be 30 for the JSON file and 0x1E or 1E for the environment variable.

System.GC.RetainVM/COMPlus_GCRetainVM

  • Configures whether segments that should be deleted are put on a standby list for future use or are released back to the operating system (OS).
  • Default: Release segments back to the operating system. This is equivalent to setting the value to false.
Setting name Values Version introduced
runtimeconfig.json System.GC.RetainVM false - release to OS
true - put on standby
.NET Core 1.0
MSBuild property RetainVMGarbageCollection false - release to OS
true - put on standby
.NET Core 1.0
Environment variable COMPlus_GCRetainVM 0 - release to OS
1 - put on standby
.NET Core 1.0

Examples

runtimeconfig.json file:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.RetainVM": true
      }
   }
}

Project file:

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

  <PropertyGroup>
    <RetainVMGarbageCollection>true</RetainVMGarbageCollection>
  </PropertyGroup>

</Project>

Large pages

COMPlus_GCLargePages

  • Specifies whether large pages should be used when a heap hard limit is set.
  • Default: Don't use large pages when a heap hard limit is set. This is equivalent to setting the value to 0.
  • This is an experimental setting.
Setting name Values Version introduced
runtimeconfig.json N/A N/A N/A
Environment variable COMPlus_GCLargePages 0 - disabled
1 - enabled
.NET Core 3.0

Large objects

COMPlus_gcAllowVeryLargeObjects

  • Configures garbage collector support on 64-bit platforms for arrays that are greater than 2 gigabytes (GB) in total size.
  • Default: GC supports arrays greater than 2-GB. This is equivalent to setting the value to 1.
  • This option may become obsolete in a future version of .NET.
Setting name Values Version introduced
runtimeconfig.json N/A N/A N/A
Environment variable COMPlus_gcAllowVeryLargeObjects 1 - enabled
0 - disabled
.NET Core 1.0
app.config for .NET Framework gcAllowVeryLargeObjects 1 - enabled
0 - disabled
.NET Framework 4.5

Large object heap threshold

System.GC.LOHThreshold/COMPlus_GCLOHThreshold

  • Specifies the threshold size, in bytes, that causes objects to go on the large object heap (LOH).
  • The default threshold is 85,000 bytes.
  • The value you specify must be larger than the default threshold.
Setting name Values Version introduced
runtimeconfig.json System.GC.LOHThreshold decimal value .NET Core 1.0
Environment variable COMPlus_GCLOHThreshold hexadecimal value .NET Core 1.0
app.config for .NET Framework GCLOHThreshold decimal value .NET Framework 4.8

Example:

{
   "runtimeOptions": {
      "configProperties": {
         "System.GC.LOHThreshold": 120000
      }
   }
}

Tip

If you're setting the option in runtimeconfig.json, specify a decimal value. If you're setting the option as an environment variable, specify a hexadecimal value. For example, to set a threshold size of 120,000 bytes, the values would be 120000 for the JSON file and 0x1D4C0 or 1D4C0 for the environment variable.

Standalone GC

COMPlus_GCName

  • Specifies a path to the library containing the garbage collector that the runtime intends to load.
  • For more information, see Standalone GC loader design.
Setting name Values Version introduced
runtimeconfig.json N/A N/A N/A
Environment variable COMPlus_GCName string_path .NET Core 2.0