What is the memory footprint of the JVM and how can I minimize it?

Question

I just wanted to know what the actual footprint of the JavaVM (Sun, Linux) is when one starts to start to spawn multiple processes of JVMs. When I remember well those should share the rt.jar (and maybe beyond?). Does those JVM share the JIT cache (all JVM feature the same Classpath)?

Is there anything I can do to reduce the overhead of multi-instance JVM? (beside of setting smaller limits for the heap)?

Anything I can do while programming the application?

Can I share memory areas? Maybe share mapped memory blocks?

Answer 1

To complement other answers, here is a couple of insightful articles listing typical JVM memory footprint values and ways to measure them:

https://spring.io/blog/2015/12/10/spring-boot-memory-performance

http://trustmeiamadeveloper.com/2016/03/18/where-is-my-memory-java/

Answer 2

Probably it will be only a partial answer.

What is the memory footprint of the JVM

The full footprint of a Java app consists of Heap space and non-heap space (let me call it this way). Just a couple of examples of what resides in non-heap space: PermGen or Metaspace, derect allocation from code (malloc), NIO also uses native memeory.

how can I minimize it?

The heap usually occupies the biggest part of you footprint. So, I would start with it.

As for the non-heap space, you can minimize: PermGen (If you max size is redundant), Thread stacks (They are quite large, especially in 64-bit JMMs) and Code cache (insted of performance, of course).

Does those JVM share the JIT cache

In normal conditions (and I'm not aware of others) every process has its own footprint. That's actually what differs a process from a thread. And back to multiple JVMs, each JVM is a separate process.

should share the rt.jar

If you start Java from the same directory (the same installation), of course, they share the same rt.jar, but only as a source of classes. For example, the String class will be loaded as many times as a number of running JVMs.