How is the default Java heap size determined?
If I omit the -Xmxn option from the Java command line a default value is used. According to Java documentation "the default value is chosen at runtime based on system configuration".
What system configuration settings influence the default value?
On Windows systems, you can use the following command to find out the defaults on the system where your applications runs.
java -XX:+PrintFlagsFinal -version | findstr HeapSize
Look for the options MaxHeapSize (for -Xmx ) and InitialHeapSize for -Xms .
On a Unix/Linux system, you can do
java -XX:+PrintFlagsFinal -version | grep HeapSize
I believe the resulting output is in bytes.
According to Garbage Collector Ergonomics [Oracle]:
initial heap size:
Larger of 1/64th of the machine's physical memory on the machine or some reasonable minimum. Before J2SE 5.0, the default initial heap size was a reasonable minimum, which varies by platform. You can override this default using the -Xms command-line option.
maximum heap size:
Smaller of 1/4th of the physical memory or 1GB. Before J2SE 5.0, the default maximum heap size was 64MB. You can override this default using the -Xmx command-line option.
UPDATE:
As pointed out by Tom Anderson in his comment, the above is for server-class machines. From Ergonomics in the 5.0 JavaTM Virtual Machine:
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
with the exception of 32 bit platforms running a version of the Windows operating system. On all other platforms the default values are the same as the default values for version 1.4.2.
In the J2SE platform version 1.4.2 by default the following selections were made
This is changed with Java 6 update 18.
Assuming that we have more than 1 GB of physical memory (quite common these days), it's always 1/4th of your physical memory.
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