Why does WebSphere use so much more memory than the JVM heap size?
When calculating the amount of memory to expect WebSphere Application Server to use, my rule of thumb is that the application server will use about 100MB more than the size of the Java Virtual Machine (JVM) heap in that application server. This memory overhead has increased slightly with each new version of WebSphere Application Server. For WebSphere Application Server Version 5, I recommended 90MB. For WebSphere Application Server Version 6, I recommend 120MB. All the code that implements a WebSphere Application Server is Java code and runs in the JVM. The WebSphere Application Server overhead comes from libraries that the JVM loads to perform functions the application server and the applications running in the JVM request. As WebSphere Application Server adds function with each release, the number of libraries included also increases. So how do you get from an overhead of 120MB to the overhead some customers observe that reaches 400MB? The number of libraries that the JVM must load i
