Java Virtual Machine Heap

I came across an issue regarding memory allocation, and decided it would be in my best interests to how the Java Virtual Machine (JVM) manages it's memory space.

JVM is comprised of three memory segments: heap memory, non-heap memory, and the code for itself.

Heap Memory

A JVM heap is allocated at the start-up of every program.  By default the initial heap size is 2MB and the max heap size is set to 64MB.  But, in the event that your program needs more memory you can initialize it to have more using:

%java -Xmsn

Specifies the initial size of the memory allocation pool.   This  value  must  be  a  multiple  of 1024 greater than 1 MB.  Append the letter  k  or  K  to indicate  kilobytes,  the letter m or M to indicate megabytes, the letter g or G to indicate gigabytes, or  the  letter  t or T to indicate terabytes. The default value is 2MB. Examples:

• -Xms6291456
• -Xms6144k
• -Xms6m

Likewise, we can set a maximum heap size using:

%java -Xmsn

Heap Dump Snapshots

There are a couple of commands you can use to obtain a view of your program's heap. The first one being Java's tool, HPROF, which enables CPU, heap, or monitor profiling.  The second command is the Java Heap Analysis Tool (JHAT).

%java -Xrunhprof:format=b, file=snapshot.hprof Classname
%jhat snapshot.hprof

Without formatting the snapshot, HPROF will return the heap dump in a text file.  But because sifting through possibly 40k+ lines of text, we can format the dump so that it easer to browse, as well as make use of Object Query Language (OQL).

After running these two commands, you can view the heap file by opening up your browser and going to http://localhost:7000.


JConsole

Overview tab

This is a tool that was included in JDK 5.0 and later distributions.  It provides the user with a GUI for monitoring specific data such as memory and thread usage.

Some useful tabs that are provided: Overview, Memory, Threads, Classes, and the VM Summary.  You can adjust the time frames for even more specific analysis.