Kubernetes and JVM memory settings

Question

In a Kubernetes cluster with numerous microservices, one of them is used exclusively for a Java Virtual Machine (JVM) that runs a Java 1.8 data processing application.

Up to recently, jobs running in that JVM pod consumed less than 1 GB of RAM, so the pod has been setup with 4 GB of maximum memory, without any explicit heap size settings for the JVM.

Some new data now require about 2.5 GB for the entire pod, including the JVM (as reported by the kubernetes top command, after launching with an increased memory limit of 8 GB), but the pod crashes soon after starting with a limit of 4 GB.

Using a head size range like -Xms256m -Xmx3072m with a limit of 4 GB does not solve the problem. In fact, now the pod does not even start.

Is there any way to parameterize the JVM for accommodating the 2.5 GB needed, without increasing the 4 GB maximum memory for the pod?

Answer 1

The default "max heap" if you don't specify -Xmx is 1/4 (25%) of the host RAM. JDK 10 improved support for containers in that it uses container's RAM limits instead of underlying host. As pointed by @David Maze this has been backported to JDK 8.

Assuming you have a sufficiently recent version of JDK 8, you can use -XX:MaxRAMPercentage to modify the default percentage of total RAM used for Max heap. So instead of specifying -Xmx you can tell, e.g. -XX:MaxRAMPercentage=75.0. See also https://medium.com/adorsys/usecontainersupport-to-the-rescue-e77d6cfea712

Here's an example using alpine JDK docker image: https://hub.docker.com/_/openjdk (see section "Make JVM respect CPU and RAM limits" in particular).

# this is running on the host with 2 GB RAM
docker run --mount type=bind,source="$(pwd)",target=/pwd -it openjdk:8

# running with MaxRAMPercentage=50 => half of the available RAM is used as "max heap"
root@c9b0b4d9e85b:/# java -XX:+PrintFlagsFinal -XX:MaxRAMPercentage=50.0 -version | grep -i maxheap
    uintx MaxHeapFreeRatio                          = 100                                 {manageable}
    uintx MaxHeapSize                              := 1044381696                          {product}
openjdk version "1.8.0_265"
OpenJDK Runtime Environment (build 1.8.0_265-b01)
OpenJDK 64-Bit Server VM (build 25.265-b01, mixed mode)

# running without MaxRAMPercentage => default 25% of RAM is used
root@c9b0b4d9e85b:/# java -XX:+PrintFlagsFinal -version | grep -i maxheap
    uintx MaxHeapFreeRatio                          = 100                                 {manageable}
    uintx MaxHeapSize                              := 522190848                           {product}
openjdk version "1.8.0_265"