Understanding Java Memory Settings

As part of the Best Practices, we know that we should be setting -Xms & -Xmx Java command line parameters. What are these settings and why is it required to set these.

As JAVA starts, it creates within the systems memory a Java Virtual Machine (JVM). JVM is where the complete processing of any Java program takes place. All JAVA applications (including IQv6) by default allocates & reserves up to 64 MB of memory resource pool from the system on which it is running.

The Xms is the initial / minimum Java memory (heap) size within the JVM. Setting the initial memory (heap) size higher can help in a couple of ways. First, it will allowgarbage collection (GC) to work less which is more efficient. The higher initial memory value will cause the size of the memory (heap) not to have to grow as fast as a lower initial memory (heap) size, thereby saving the overhead of the Java VM asking the OS for more memory.

The Xmx is the maximum Java memory (heap) size within the Java Virtual Memory (JVM). As the JVM gets closer to fully utilizing the initial memory, it checks the Xmxsettings to find out if it can draw more memory from the system resources. If can, it does so. For the JVM to allocate contiguous memory to itself is a very expensive operation. So as the JVM gets closer to the initial memory, the JVM will use aggressive garbage collection (to clean the memory and if possible avoid memory allocation), increasing the load on the system.

If JVM is in need of memory beyond the value set in Xmx, the JVM will not be able to draw more memory from system resource (even if available) and run out of memory. Hence, the -Xms and -Xmx memory parameters should be increased depending upon the demand estimation of the system. Ideally both same should be the same value (set at maximum possible as per demand estimation). This ensure that the maximum memory is allocated right at the start-up eliminating the need for extra memory allocation during program execution. We recommend aggressive maximum memory (heap) size of between 1/2 and 3/4 of physical memory.

java -Xms256m -Xmx256m