Runtime configuration options for garbage collection

This page contains information about settings for the .NET runtime garbage collector (GC). 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 configuration options 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.
• If you're using the environment variables, .NET 6 standardizes on the prefix DOTNET_ instead of COMPlus_. 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 example, COMPlus_gcServer.

Ways to specify the configuration

For different versions of the .NET runtime, there are different ways to specify the configuration values. The following table shows a summary.

Config location .NET versions this location applies to Formats How it's interpreted
runtimeconfig.json file .NET Core n n is interpreted as a decimal value.
Environment variable .NET Framework, .NET Core 0xn or n n is interpreted as a hex value in either format
app.config file .NET Framework 0xn n is interpreted as a hex value1

1 You can specify a value without the 0x prefix for an app.config file setting, but it's not recommended. On .NET Framework 4.8+, due to a bug, a value specified without the 0x prefix is interpreted as hexadecimal, but on previous versions of .NET Framework, it's interpreted as decimal. To avoid having to change your config, use the 0x prefix when specifying a value in your app.config file.

For example, to specify 12 heaps for GCHeapCount for a .NET Framework app named A.exe, add the following XML to the A.exe.config file.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
...
<runtime>
<gcServer enabled="true"/>
<GCHeapCount>0xc</GCHeapCount>
</runtime>
</configuration>


For both .NET Core and .NET Framework, you can use environment variables.

On Windows using .NET 5 or a later version:

SET DOTNET_gcServer=1
SET DOTNET_GCHeapCount=c


On Windows using .NET Core 3.1 or earlier:

SET COMPlus_gcServer=1
SET COMPlus_GCHeapCount=c


On other operating systems:

For .NET 5 or later versions:

export DOTNET_gcServer=1
export DOTNET_GCHeapCount=c


For .NET Core 3.1 and earlier versions:

export COMPlus_gcServer=1
export COMPlus_GCHeapCount=c


For .NET Core only, you can set the value in the runtimeconfig.json file.

{
"runtimeOptions": {
"configProperties": {
"System.GC.Server": true,
"System.GC.HeapCount": 12
}
}
}


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:

Workstation vs. server

• 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
Environment variable DOTNET_gcServer 0 - workstation
1 - server
.NET 6
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>


Background GC

• Configures whether background (concurrent) garbage collection is enabled.
• Default: Use background GC. This is equivalent to setting the value to true.
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
Environment variable DOTNET_gcConcurrent 1 - background GC
0 - non-concurrent GC
.NET 6
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 following settings 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.

Heap count

• 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.
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
Environment variable DOTNET_GCHeapCount hexadecimal value .NET 6
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.

• 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
Environment variable DOTNET_GCHeapAffinitizeMask hexadecimal value .NET 6
app.config for .NET Framework GCHeapAffinitizeMask decimal value .NET Framework 4.6.2

Example:

{
"runtimeOptions": {
"configProperties": {
}
}
}


Affinitize ranges

Setting name Values Version introduced
runtimeconfig.json System.GC.HeapAffinitizeRanges 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
Environment variable DOTNET_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 6

Example:

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


CPU groups

• 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 operating 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 System.GC.CpuGroup false - disabled
true - enabled
.NET 5
Environment variable COMPlus_GCCpuGroup 0 - disabled
1 - enabled
.NET Core 1.0
Environment variable DOTNET_GCCpuGroup 0 - disabled
1 - enabled
.NET 6
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 DOTNET_Thread_UseAllCpuGroups environment variable to 1.

Affinitize

• 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
Environment variable DOTNET_GCNoAffinitize 0 - affinitize
1 - don't affinitize
.NET 6
app.config for .NET Framework GCNoAffinitize false - affinitize
true - don't affinitize
.NET Framework 4.6.2

Example:

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


Heap limit

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

• This setting only applies to 64-bit computers.

• This setting is ignored if the Per-object-heap limits are configured.

• 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
Environment variable DOTNET_GCHeapHardLimit hexadecimal value .NET 6

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.

Heap limit percent

• 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.

• This setting is ignored if the Per-object-heap limits are configured.

• 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.HeapHardLimitPercent decimal value .NET Core 3.0
Environment variable COMPlus_GCHeapHardLimitPercent hexadecimal value .NET Core 3.0
Environment variable DOTNET_GCHeapHardLimitPercent hexadecimal value .NET 6

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.

Per-object-heap limits

You can specify the GC's allowable heap usage on a per-object-heap basis. The different heaps are the large object heap (LOH), small object heap (SOH), and pinned object heap (POH).

• If you specify a value for any of the DOTNET_GCHeapHardLimitSOH, DOTNET_GCHeapHardLimitLOH, or DOTNET_GCHeapHardLimitPOH settings, you must also specify a value for DOTNET_GCHeapHardLimitSOH and DOTNET_GCHeapHardLimitLOH. If you don't, the runtime will fail to initialize.
• The default value for DOTNET_GCHeapHardLimitPOH is 0. DOTNET_GCHeapHardLimitSOH and DOTNET_GCHeapHardLimitLOH don't have default values.
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitSOH decimal value .NET 5
Environment variable COMPlus_GCHeapHardLimitSOH hexadecimal value .NET 5
Environment variable DOTNET_GCHeapHardLimitSOH hexadecimal value .NET 6
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitLOH decimal value .NET 5
Environment variable COMPlus_GCHeapHardLimitLOH hexadecimal value .NET 5
Environment variable DOTNET_GCHeapHardLimitLOH hexadecimal value .NET 6
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitPOH decimal value .NET 5
Environment variable COMPlus_GCHeapHardLimitPOH hexadecimal value .NET 5
Environment variable DOTNET_GCHeapHardLimitPOH hexadecimal value .NET 6

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.

Per-object-heap limit percents

You can specify the GC's allowable heap usage on a per-object-heap basis. The different heaps are the large object heap (LOH), small object heap (SOH), and pinned object heap (POH).

• If you specify a value for any of the DOTNET_GCHeapHardLimitSOHPercent, DOTNET_GCHeapHardLimitLOHPercent, or DOTNET_GCHeapHardLimitPOHPercent settings, you must also specify a value for DOTNET_GCHeapHardLimitSOHPercent and DOTNET_GCHeapHardLimitLOHPercent. If you don't, the runtime will fail to initialize.
• These settings are ignored if DOTNET_GCHeapHardLimitSOH, DOTNET_GCHeapHardLimitLOH, and DOTNET_GCHeapHardLimitPOH are specified.
• A value of 1 means that GC uses 1% of total physical memory for that object heap.
• Each value must be greater than zero and less than 100. Additionally, the sum of the three percentage values must be less than 100. Otherwise, the runtime will fail to initialize.
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitSOHPercent decimal value .NET 5
Environment variable COMPlus_GCHeapHardLimitSOHPercent hexadecimal value .NET 5
Environment variable DOTNET_GCHeapHardLimitSOHPercent hexadecimal value .NET 6
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitLOHPercent decimal value .NET 5
Environment variable COMPlus_GCHeapHardLimitLOHPercent hexadecimal value .NET 5
Environment variable DOTNET_GCHeapHardLimitLOHPercent hexadecimal value .NET 6
Setting name Values Version introduced
runtimeconfig.json System.GC.HeapHardLimitPOHPercent decimal value .NET 5
Environment variable COMPlus_GCHeapHardLimitPOHPercent hexadecimal value .NET 5
Environment variable DOTNET_GCHeapHardLimitPOHPercent hexadecimal value .NET 6

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.

High memory percent

Memory load is indicated by the percentage of physical memory in use. By default, when the physical memory load reaches 90%, garbage collection becomes more aggressive about doing full, compacting garbage collections to avoid paging. When memory load is below 90%, GC favors background collections for full garbage collections, which have shorter pauses but don't reduce the total heap size by much. On machines with a significant amount of memory (80GB or more), the default load threshold is between 90% and 97%.

The high memory load threshold can be adjusted by the DOTNET_GCHighMemPercent environment variable or System.GC.HighMemoryPercent JSON configuration setting. Consider adjusting the threshold if you want to control heap size. For example, for the dominant process on a machine with 64GB of memory, it's reasonable for GC to start reacting when there's 10% of memory available. But for smaller processes, for example, a process that only consumes 1GB of memory, GC can comfortably run with less than 10% of memory available. For these smaller processes, consider setting the threshold higher. On the other hand, if you want larger processes to have smaller heap sizes (even when there's plenty of physical memory available), lowering this threshold is an effective way for GC to react sooner to compact the heap down.

Note

For processes running in a container, GC considers the physical memory based on the container limit.

Setting name Values Version introduced
runtimeconfig.json System.GC.HighMemoryPercent decimal value .NET 5
Environment variable COMPlus_GCHighMemPercent hexadecimal value .NET Core 3.0
.NET Framework 4.7.2
Environment variable DOTNET_GCHighMemPercent hexadecimal value .NET 6

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 the high memory threshold to 75%, the values would be 75 for the JSON file and 0x4B or 4B for the environment variable.

Retain VM

• 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
Environment variable DOTNET_GCRetainVM 0 - release to OS
1 - put on standby
.NET 6

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

• 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
Environment variable DOTNET_GCLargePages 0 - disabled
1 - enabled
.NET 6

Allow large objects

• 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
Environment variable DOTNET_gcAllowVeryLargeObjects 1 - enabled
0 - disabled
.NET 6
app.config for .NET Framework gcAllowVeryLargeObjects 1 - enabled
0 - disabled
.NET Framework 4.5

Large object heap threshold

• 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
Environment variable DOTNET_GCLOHThreshold hexadecimal value .NET 6
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

• Specifies the name of a standalone GC DLL that the runtime loads in place of the one it has in the main runtime DLL (coreclr.dll). This DLL needs to reside in the same directory as coreclr.dll.
Setting name Values Version introduced
runtimeconfig.json N/A N/A N/A
Environment variable COMPlus_GCName string_path .NET Core 2.0
Environment variable DOTNET_GCName string_path .NET 6

Conserve memory

• Configures the garbage collector to conserve memory at the expense of more frequent garbage collections and possibly longer pause times.
• Default value is 0 - this implies no change.
• Besides the default value 0, values between 1 and 9 (inclusive) are valid. The higher the value, the more the garbage collector tries to conserve memory and thus to keep the heap small.
• If the value is non-zero, the large object heap will be compacted automatically if it has too much fragmentation.
Setting name Values Version introduced
runtimeconfig.json N/A N/A N/A
Environment variable COMPlus_GCConserveMemory 0 -9 .NET Framework 4.8
Environment variable DOTNET_GCConserveMemory 0 -9 .NET 6
app.config for .NET Framework GCConserveMemory 0 -9 .NET Framework 4.8

Example app.config file:


<configuration>
<runtime>
<GCConserveMemory enabled="5"/>
</runtime>
</configuration>


Tip

Experiment with different numbers to see which value works best for you. Start with a value between 5 and 7.