JVM parameters in Java

On the basis of how we specify JVM option it can be divided into two parts, JVM Options which starts with –X and those which starts with -XX:

1)       JVM Options that begin with -X are non-standard (thy are not guaranteed to be supported on all JVM implementations), and are subject to change without notice in subsequent releases of the JDK.

2)       JVM Options or parameters which are specified with -XX are not stable and are not recommended for casual use. These options are subject to change without notice also.

 -Xmx

The flag Xmx specifies the maximum memory allocation pool for a Java Virtual Machine (JVM), while Xms specifies the initial memory allocation pool. A common use for these flags is when you encounter a java.lang.OutOfMemoryError.

 -Xms        set initial Java heap size

 -Xmx        set maximum Java heap size

      -Xss>         set java thread stack size

-XX:MaxPermSize

are used to set size for Permanent Generation.

Permanent Generation: The Permanent Generation is where class files are kept and never deallocated as the name suggests. These are the result of compiled classes and jsp pages. If this space is full, it triggers a Full Garbage Collection. If the Full Garbage Collection cannot clean out old unreferenced classes and there is no room left to expand the Permanent Space, an Out‐of‐ Memory error (OOME) is thrown and the JVM will crash

agentlib:jdwp 

Sun's VM implementations require command line options to load the JDWP agent for debugging.

From 5.0 onwards the -agentlib:jdwp option is used to load and specify options to the JDWP agent.

For releases prior to 5.0, the -Xdebug and -Xrunjdwp options are used 

The -agentlib:jdwp and -Xrunjdwp option can be further qualified with sub-options. The sub-options are specified as follows:

    -agentlib:jdwp=<name1>[=<value1>],<name2>[=<value2>]...

or

   -Xdebug -Xrunjdwp:<name1>[=<value1>],<name2>[=<value2>]...

Transports

A JPDA Transport is a method of communication between a debugger and the virtual machine that is being debugged (hereafter the target VM).  The communication is connection oriented - one side acts as a server, listening for a connection. The other side acts as a client and connects to the server. JPDA allows either the debugger application or the target VM to act as the server. 

1. Socket Transport

The JPDA reference implementation provides a socket transport for the Solaris, Linux, and Microsoft Windows platforms. The socket transport uses a TCP/IP connection between the debugger application and the target VM. With the socket transport, the debugger application and target VM can reside either on the same machine or on different machines.

The socket transport is identified through a unique string, dt_socket.

2. Shared Memory Transport

In addition to the socket transport, the JPDA reference implementation provides a shared memory transport on the Microsoft Windows platform. The shared memory transport uses a shared memory region to exchange JDWP packets between the debugger application and the target VM. With the shared memory transport, the debugger application and target VM must reside on the same machine.

The shared memory transport is identified through a unique string, dt_shmem.

Server

The option server=y opens a socket and listens for incoming debugger requests(cq acts as server).

With server=n the debugged application will try to connect actively to a debugger and run therefore as a client.

Suspend

In JVM DEBUG parameters there is a parameter called "suspend" which takes the value as "y" or "n". so if you want to debug the process from the start set this parameter as "suspend=y" and your Java application will wait until Eclipse remotely connects to it. Otherwise, if you want to run your program and later want eclipse to be connected that set this as "suspend=n" so your java application will run normally and after eclipse remotely connected to it, it will stop on breakpoints. (default  value suspend=y if not specified)


Source and References

http://javarevisited.blogspot.in/2011/11/hotspot-jvm-options-java-examples.html