Using Specialized Garbage Collection Displays

Heap Usage After GC

This visualizer shows heap usage after each garbage collection. When viewed with consideration of the type of each event, this graph provides a general picture of the heap resources minimally required by an application.

Figure 8-19 GC Metric: Heap Usage After GC

Duration (Stop the World)

This visualizer shows the amount of stop-the-world time spent for each garbage collection event.

A well-tuned application shows a relatively large number of short scavenges with less frequent, but more expensive, full garbage collections.

This graph is also useful for exposing the cost of explicit System.gc() method calls within an application, which often result in expensive and unneeded garbage collection overhead.

Figure 8-20 GC Metric: Duration

Cumulative Allocation

This visualizer shows the cumulative amount of storage for new objects requested by the application over time, which can indicate the overall progress of the application. Because the rate at which an application creates new objects is often an indicator of its performance, changes in the slope of this curve may indicate opportunities for tuning garbage collection.

To see a more dramatic presentation of incremental changes in object creation, see the Creation Rate visualizer.

Note that storage recovered by the garbage collector for unused objects is not deducted from the cumulative amount.

Figure 8-21 GC Metric: Cumulative Allocation

Creation Rate

This panel shows the incremental object creation rate for the application as observed at each garbage collection event. The increment at each data point is the amount of elapsed time since the previous GC event.

The rate of object creation can be a good measure of your application's overall performance. By correlating GC events with changes in the object creation rate, you can discover opportunities for improving this performance through heap parameter tuning.

Figure 8-22 GC Metric: Creation Rate

User-Defined X-Y Axes

This visualizer is supplied to help you to experiment with your own combinations of data gathered by -Xverbosegc or -Xloggc that you might find useful.

Select variables for the X and Y axes from the pull-down lists. HPjmeter plots the X-Y relationships for your captured data file. For the list of available metrics, see –Xverbosegc and –Xloggc Options and Their Corresponding Metrics

Figure 8-23 User-Defined Comparisons of GC Metrics