The -Xgenconfig Option

The young generation in the Java HotSpot VM consists of an eden and two equally sized semi-spaces, whereas the young generation in EVM consists of two equally size semi-spaces (with no eden). A command such as -Xgenconfig:32m,64m,semispaces:128m,512m,markcompact states that there are two 64mb semispaces each starting at 32mb and expandable to 64mb, and an old generation which starts at 128mb and can expand to 512mb. This creates a heap which is 640mb in size (maximum). In the Java HotSpot VM the equivalent command would be: -Xms256m -Xmx640m -XX:NewSize=32m -XX:MaxNewSize=64m As you can see, the Java HotSpot VM sets the total size of the heap with -Xms/-Xmx and then carves out the young generation from that space, whereas when using -Xgenconfig you must specify each generation's size.