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1. Overview

In this tutorial, we'll discuss a few JVM parameters we can use to set the RAM percentage of the JVM.

Introduced in Java 8, the parameters InitialRAMPercentage, MinRAMPercentage, and MaxRAMPercentage help to configure the heap size of a Java application.

2. -XX:InitialRAMPercentage

The InitialRAMPercentage JVM parameter allows us to configure the initial heap size of the Java application. It's a percentage of the total memory of a physical server or container, passed as a double value.

For instance, if we set-XX:InitialRAMPercentage=50.0 for a physical server of 1 GB full memory, then the initial heap size will be around 500 MB (50% of 1 GB).

To start with, let's check the default value of the IntialRAMPercentage in the JVM:

$ docker run openjdk:8 java -XX:+PrintFlagsFinal -version | grep -E "InitialRAMPercentage"
   double InitialRAMPercentage                      = 1.562500                            {product}

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

Then, let's set the initial heap size of 50% for a JVM:

$ docker run -m 1GB openjdk:8 java -XX:InitialRAMPercentage=50.0 -XX:+PrintFlagsFinal -version | grep -E "InitialRAMPercentage"
   double InitialRAMPercentage                     := 50.000000                           {product}

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

It's important to note that the JVM ignores InitialRAMPercentage when we configure the -Xms option.

3. -XX:MinRAMPercentage

The MinRAMPercentage parameter, unlike its name, allows setting the maximum heap size for a JVM running with a small amount of memory (less than 200MB).

First, we'll explore the default value of the MinRAMPercentage:

$ docker run openjdk:8 java -XX:+PrintFlagsFinal -version | grep -E "MinRAMPercentage"
   double MinRAMPercentage                      = 50.000000                            {product}

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

Then, let's use the parameter to set the maximum heap size for a JVM with a total memory of 100MB:

$ docker run -m 100MB openjdk:8 java -XX:MinRAMPercentage=80.0 -XshowSettings:VM -version

VM settings:
    Max. Heap Size (Estimated): 77.38M
    Ergonomics Machine Class: server
    Using VM: OpenJDK 64-Bit Server VM

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

Also, the JVM ignores the MaxRAMPercentage parameter while setting the maximum heap size for a small memory server/container:

$ docker run -m 100MB openjdk:8 java -XX:MinRAMPercentage=80.0 -XX:MaxRAMPercentage=50.0 -XshowSettings:vm -version
VM settings:
    Max. Heap Size (Estimated): 77.38M
    Ergonomics Machine Class: server
    Using VM: OpenJDK 64-Bit Server VM

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

4. -XX:MaxRAMPercentage

The MaxRAMPercentage parameter allows setting the maximum heap size for a JVM running with a large amount of memory (greater than 200 MB).

First, let's explore the default value of the MaxRAMPercentage:

$ docker run openjdk:8 java -XX:+PrintFlagsFinal -version | grep -E "MaxRAMPercentage"
   double MaxRAMPercentage                      = 25.000000                            {product}

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

Then, we can use the parameter to set the maximum heap size to 60% for a JVM with 500 MB total memory:

$ docker run -m 500MB openjdk:8 java -XX:MaxRAMPercentage=60.0 -XshowSettings:vm -version
VM settings:
    Max. Heap Size (Estimated): 290.00M
    Ergonomics Machine Class: server
    Using VM: OpenJDK 64-Bit Server VM

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

Similarly, the JVM ignores the MinRAMPercentage parameter for a large memory server/container:

$ docker run -m 500MB openjdk:8 java -XX:MaxRAMPercentage=60.0 -XX:MinRAMPercentage=30.0 -XshowSettings:vm -version
VM settings:
    Max. Heap Size (Estimated): 290.00M
    Ergonomics Machine Class: server
    Using VM: OpenJDK 64-Bit Server VM

openjdk version "1.8.0_292"
OpenJDK Runtime Environment (build 1.8.0_292-b10)

5. Conclusion

In this short article, we discussed the use of JVM parameters InitialRAMPercentageMinRAMPercentage, and MaxRAMPercentage for setting the RAM percentages that the JVM will use for the heap.

First, we checked the default values of the flags set on the JVM. Then, we used the JVM parameters to set the initial and maximum heap sizes.

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