JVM(Java) Heap size

Two JVM options are often used to tune JVM heap size: -Xmx for maximum heap size, and -Xms for initial heap size. Here are some common mistakes made by developers while using them:

1. Missing m, M, g or G at the end (they are case insensitive). For example,
   

   java -Xmx128 BigApp
   java.lang.OutOfMemoryError: Java heap space
   

   The correct command should be: java -Xmx128m BigApp. To be precise, -Xmx128 is a valid setting for very small apps, like HelloWorld. But in real life, I guess you really mean -Xmx128m
   
   
2. Extra space in JVM options, or incorrectly use =. For example,
   

   java -Xmx 128m BigApp
   Invalid maximum heap size: -Xmx
   Could not create the Java virtual machine.
   
   java -Xmx=512m HelloWorld
   Invalid maximum heap size: -Xmx=512m
   Could not create the Java virtual machine.
   

   The correct command should be java -Xmx128m BigApp, with no whitespace nor =. -X options are different than -Dkey=value system properties, where = is used.
   
   
3. Only setting -Xms JVM option and its value is greater than the default maximum heap size, which is 64m. The default minimum heap size seems to be 0. For example,
   

   java -Xms128m BigApp
   Error occurred during initialization of VM
   Incompatible initial and maximum heap sizes specified
   

   The correct command should be java -Xms128m -Xmx128m BigApp. Its a good idea to set the minimum and maximum heap size to the same value. In any case, dont let the minimum heap size exceed the maximum heap size.
   
   
4. Heap size is larger than your computers physical memory. For example,
   

   java -Xmx2g BigApp
   Error occurred during initialization of VM
   Could not reserve enough space for object heap
   Could not create the Java virtual machine.
   

   The fix is to make it lower than the physical memory: java -Xmx1g BigApp
   
   
5. Incorrectly use mb as the unit, where m or M should be used instead.
   

   java -Xms256mb -Xmx256mb BigApp
   Invalid initial heap size: -Xms256mb
   Could not create the Java virtual machine.

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