jvm - Java heap Xms and linux free memory different

Question

I have a java program running in centos Box. My -Xmx and -Xms set to 4000 Mb.

The program works fine.

But when i do free -m , the used memory is showing as 506 MB. As per my understanding , XMS memory should be reserved for JVM.Why does free command not showing the java used memory ?

I have also done jstat -gccapacity $(pidof java) and there NGCMN and NGCMX updated and have the same value ? Any support would be helpful. I'm running my program as java -Xms41000m -Xmx42000m -jar

Answer 1

Even when -Xmx and -Xms set to the same value, the space reserved for Java Heap is not immediately allocated in RAM.

Operating System typically allocates physical memory lazily, only on the first access to a virtual page. So, while unused part of Java Heap is not touched, it won't really consume memory.

You may use -XX:+AlwaysPreTouch option to forcibly touch all heap pages on JVM start.