JVM Parameters

Modify the java heap memory based on container memory using one of the following JVM parameters:

Parameter Description
-XX:InitialRAMPercentage The initial size of the heap based on the total container memory.
-XX:MinRAMPercentage The maximum heap size based on the size of the JVM running on small heap. The small is heap is of approximately 125MB.
-XX:MaxRAMPercentage The maximum heap size based on the size of the JVM running on greater than small heap.

Displaying the Current Value

Use the following command the display the current value of the container aware JVM Parameters using the following:

docker container run -it --rm openjdk:17.0.2-slim java -XX:+PrintFlagsFinal -version | grep -E ".*RAMPercentage"

Expect to see something like the following:

   double InitialRAMPercentage                     = 1.562500
                   {product} {default}
   double MaxRAMPercentage                         = 25.000000
                   {product} {default}
   double MinRAMPercentage                         = 50.000000
                   {product} {default}

Checking the Current Memory and CPU using JShell

  1. Open a jshell in the container using the following:

    docker container run -it --rm openjdk:17.0.2-slim jshell

  2. Paste the following command in jshell:

    var rt = Runtime.getRuntime();
System.out.printf("Heap size: %dMB%nMaximum size of heap: %dMB%nAvailable processors: %d%n", 
 rt.totalMemory()/1024/1024, rt.maxMemory()/1024/1024, rt.availableProcessors());

  3. Press enter.

    Expect to see something similar to the following:

    rt ==> java.lang.Runtime@1d81eb93
Heap size: 252MB
Maximum size of heap: 3966MB
Available processors: 16
$2 ==> java.io.PrintStream@34c45dca

  4. Type the following in jshell and press enter.

    /exit

Allocating a Memory and CPU to a Container

  1. Execute the following command to allocate 100mb of memory and 1 CPU to the container:

    docker container run -it --rm -m 100m --cpus=1 openjdk:17.0.2-slim jshell

  2. Paste the following command in jshell:

    var rt = Runtime.getRuntime();
System.out.printf("Heap size: %dMB%nMaximum size of heap: %dMB%nAvailable processors: %d%n", 
 rt.totalMemory()/1024/1024, rt.maxMemory()/1024/1024, rt.availableProcessors());

  3. Press enter.

    Expect to see something similar to the following:

    rt ==> java.lang.Runtime@6659c656
Heap size: 7MB
Maximum size of heap: 48MB
Available processors: 1
$2 ==> java.io.PrintStream@2d8e6db6

    Notice the following:

    Value
    Heap size 7MB
    Maximum size of heap 48MB
    Available processors 1

  4. Type the following in jshell and press enter.

    /exit

Modifying the Allocated Maximum Size of Heap Memory

  1. Execute the following command to allocate 100mb of memory and 1 CPU to the container:

    docker container run -it --rm -m 100m --cpus=1 openjdk:17.0.2-slim jshell -R-XX:MinRAMPercentage=80

  2. Paste the following command in jshell:

    var rt = Runtime.getRuntime();
System.out.printf("Heap size: %dMB%nMaximum size of heap: %dMB%nAvailable processors: %d%n", 
 rt.totalMemory()/1024/1024, rt.maxMemory()/1024/1024, rt.availableProcessors());

  3. Press enter.

    Expect to see something similar to the following:

    rt ==> java.lang.Runtime@6659c656
Heap size: 7MB
Maximum size of heap: 77MB
Available processors: 1
$2 ==> java.io.PrintStream@2d8e6db6

    Notice the Maximum size of heap become 77MB. This is because of the JVM argument -XX:MinRAMPercentage=80 passed in jshell as:

    -R-XX:MinRAMPercentage=80

    We use the --XX:MinRAMPercentage=80 because the memory allocated is a small heap.

  4. Type the following in jshell and press enter.

    /exit