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Wednesday, February 27, 2013

Java VM 5.0 Default Heap Size

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If you have Java 5.0 (1.5), you may wish to know the default heap size for a JVM.
If you don’t specify -Xmx or -Xms to control the heap size, then the defaults are used.

They are described in detail in the strangely title “Ergonomics in the 5.0 Java Virtual Machine” page here: http://www.oracle.com/technetwork/java/ergo5-140223.html

The page states:
"In the J2SE platform version 5.0 a class of machine referred to as a server-class machine has been defined as a machine with
2 or more physical processors
2 or more Gbytes of physical memory

On server-class machines by default the following are selected.
Throughput garbage collector
Heap sizes
initial heap size of 1/64 of physical memory up to 1Gbyte
maximum heap size of ¼ of physical memory up to 1Gbyte
Server runtime compiler
"

So if you have a “server class” machine, you could expect your Java 5.0 JVM to utilise a maximum of 1GB of heap with no tuning (-Xmx & -Xms).

Although it’s not very clear, it seems that a non-“server class” machine would allocate the same as a Java 1.4.2 virtual machine:

"In the J2SE platform version 1.4.2 by default the following selections were made
Serial garbage collector
Heap sizes
initial heap size of 4 Mbyte
maximum heap size of 64 Mbyte
Client runtime compiler
"

Therefore, a maximum of 64MB of heap would be utilised.

I don’t know if a single dual core CPU is recognised as a “server class” machine, but you can find out what your Java version thinks your machine is by running:

> java -help
Usage: java [-options] class [args...]
(to execute a class)
or java [-options] -jar jarfile [args...]
(to execute a jar file)

where options include:
-d32         use a 32-bit data model if available
-d64         use a 64-bit data model if available
-client       to select the "client" VM
-server      to select the "server" VM
-hotspot    is a synonym for the "client" VM [deprecated]
                 The default VM is server,
                 because you are running on a server-class machine
.

-cp <class search path of directo....


As you can see, the output tells you that it thinks you are running on a “server class” machine.

Either way, it’s probably best to use the “-server” command line option to be sure, plus the “-Xmx” option to restrict memory usage if you don’t need a whole 1GB heap.

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