JVMPI Technology

Memory Profiling for Java

Memory Profiling for Java uses a special dynamic library, known as the Memory Profiling Agent, to provide advanced reports on Java Virtual Machine (JVM) memory usage.

Garbage Collection

JVMs implement a heap that stores all objects created by the Java code. Memory for new objects is dynamically allocated on the heap. The JVM automatically frees objects that are no longer referenced by the program, preventing many potential memory issues that exist in other languages. This process is called garbage collection.

In addition to freeing unreferenced objects, a garbage collector may also reduce heap fragmentation, which occurs through the course of normal program execution. On a virtual memory system, the extra paging required to service an ever growing heap can degrade the performance of the executing program.

JVMPI Agent

Because of the memory handling features included in the JVM, Memory Profiling for Java is quite different from the feature provided for other languages. Instead of Source Code Insertion technology, the Java implementation uses a JVM Profiler Interface (JVMPI) Agent whose task is to monitor JVM memory usage and to provide a memory dump upon request.

The JVMPI Agent analyzes the following internal events of the JVM:

Method entries and exits
Object and primitive type allocations

The JVMPI Agent is a dynamic library —DLL or lib.so depending on the platform used— that is loaded as an option on the command line that launches the Java program.

During execution, when the agent receives a snapshot trigger request, it can either an instantaneous JVMPI dump of the JVM memory, or wait for the next garbage collection to be performed.

Note Information provided by the instantaneous dump includes actual memory use as well as intermediate and unreferenced objects that are normally freed by the garbage collection. In some cases, such information may be difficult to interpret correctly.

The actual trigger event can be implemented with any of the following methods:

A specified method entry or exit used in the Java code
A message sent from the Snapshot button or menu item in the graphical user interface
Every garbage collection

The JVMPI Agent requires that the Java code is compiled in debug mode, and cannot be used with Java in just-in-time (JIT) mode.