JVM Heap Analysis using GCViewer

JVM Heap Analysis using GCViewer

Basics

In an earlier article, I had listed the review of JVM memory parameters as one of the important checks for tuning the JEE server platform.

The basic primer for JDK 1.4 is at http://java.sun.com/docs/hotspot/gc1.4.2/

The key points you need to know are:

Total JVM Heap = Young + Tenured(also called Old)
Young = Eden + From (SS1) + To (SS2)

In the diagram below [taken from the Sun website], "From" and "To" are the names of the two Survivor Spaces (SS) within the "Young".

Perm Space (and code cache): stores JVM’s own stuff. This is outside the Heap you assign using Xms and Xmx. A good explanation of this is available here

The JVM Heap is at default initial 2Mb and max 64Mb (for JDK 1.4 on Solaris).
Default Perm Size is 16MB (for JDK 1.4 on Solaris)
The defaults change for each JDK and are different on each OS - so look up the values on the respective websites.

The ratios are as shown below



Now the object life cycle and garbage collection occurs like this:

1. Objects when created are always first allocated to Eden.
2. When Eden fills up, a fast but not comprehensive GC (minor collection) is run over the young generation only.
3. All surviving objects are moved from Eden into one Survivor Space.
4. In consequent minor collections, new objects move from Eden into the other Survivor Space, plus everything from the first Survivor Space (survivors from the previous minor collection) is also moved into the second Survivor Space. Thus one survivor should be empty at that time.
5. When objects in Survivor Space are old enough (or survivor fills up), they are moved to Tenured. By default the long-lived objects may be copied up to 31 times between the Survivor Spaces before they are finally promoted to the Old generation.
6. When tenured fills up, a Full GC collection is run that is comprehensive: the entire heap is analyzed, all objects that can be destroyed are killed and memory is reclaimed.

Note: the above lifecycle changes slightly when advanced options such as ConcurrentMarkSweep etc are enabled.

Look Closer

What do these values mean ?

A full list of options available at http://java.sun.com/docs/hotspot/VMOptions.html and http://java.sun.com/javase/technologies/hotspot/vmoptions.jsp


The absolute basic ones are listed in the table below. Note: This is for JDK 1.4
Some of these have changed in JDK 1.6