IBM Pattern Modeling and Analysis Tool for Java Garbage Collector : Overview

IBM Pattern Modeling and Analysis Tool for Java Garbage Collector (PMAT) parses IBM verbose GC trace, analyzes Java heap usage, and recommends key configurations based on pattern modeling of Java heap usage.
When the JVM cannot allocate an object from the current heap because of lack of space, a memory allocation fault occurs, and the Garbage Collector is invoked. The first task of the Garbage Collector is to collect all the garbage that is in the heap. This process starts when any thread calls the Garbage Collector either indirectly as a result of allocation failure or directly by a specific call to System.gc(). The first step is to get all the locks needed by the garbage collection process. This step ensures that other threads are not suspended while they are holding critical locks. All other threads are then suspended. Garbage collection can then begin. It occurs in three phases: Mark, Sweep, and Compaction (optional).
Verbose GC is a command-line option that one can supply to the JVM at start-up time. The format is: -verbose:gc or -verbosegc. This option switches on a substantial trace of every garbage collection cycle. The format for the generated information is not designed and therefore varies among various platforms and releases.
This trace should allow one to see the gross heap usage in every garbage collection cycle. For example, one could monitor the output to see the changes in the free heap space and the total heap space. This information can be used to determine whether garbage collections are taking too long to run; whether too many garbage collections are occurring; and whether the JVM crashed during garbage collection. "