Java Memory commands

Dawid Kuder

Updated March 12, 2026 05:25

Java Heap space is used by the Java runtime to allocate memory to Objects and JRE classes. Whenever we create an object, it’s always created in the heap space. This post, we’ll explain to how to set the memory size on Upsun

Garbage Collection runs on the heap to free memory used by objects that don’t have a reference. Any object created in the heap space has global access and can be referenced from anywhere in the application.

To set the JVM memory size on Upsun we need to append the memory settings to the Java startup parameter.

Upsun has a command that returns available memory with -$(jq .info.limits.memory /run/config.json) that value is megabytes. As a recommendation, you can use the fine maximum JVM memory.

web:
    commands:
        start: java -jar -Xmx$(jq .info.limits.memory /run/config.json)m target/microprofile-microbundle.jar --port $PORT

Below, more arguments to set the memory:

`-Xmn size`

Sets the initial and maximum size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The young generation region of the heap is used for new objects. GC is performed in this region more often than others. If the size for the young generation is too small, then a lot of minor garbage collections are performed. If the size is too large, then only full garbage collections are performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation greater than 25% and less than 50% of the overall heap size.

`-Xms size`

Sets the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.

`-Xmx size`

Specifies the maximum size (in bytes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, and g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments,-Xms and -Xmx are often set to the same value.

As previously mentioned, use the $(jq .info.limits.memory /run/config.json) to create the Xmx command. E.g.: java -jar -Xmx$(jq .info.limits.memory /run/config.json)m

`-Xnoclassgc`

Disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions during the application run. When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always considered live.

`-Xss size`

Sets the thread stack size (in bytes). Append the letter k or K to indicate KB, m or M to indicate MB, and g or G to indicate GB.

`ExitOnOutOfMemoryError`

When you enable this option, the JVM exits on the first occurrence of an out-of-memory error. It can be used if you prefer restarting an instance of the JVM rather than handling out of memory errors.

Recommended Java memory command on Upsun

java -jar -Xmx$(jq .info.limits.memory /run/config.json)m -XX:+ExitOnOutOfMemoryError ...

GC Log activated

java -jar -Xmx$(jq .info.limits.memory /run/config.json)m -XX:+ExitOnOutOfMemoryError -XX:+PrintGCDetails ...