What are the -Xms and -Xmx parameters when starting JVM?

The -Xms and -Xmx options are used to set the initial and maximum heap sizes, respectively, for the Java Virtual Machine (JVM). The heap is the area of memory where the JVM stores objects created by your application.

The -Xms option sets the initial size of the heap, while the -Xmx option sets the maximum size of the heap. Both options take a size parameter, which can be specified in bytes, kilobytes, megabytes, or gigabytes, with a trailing B, K, M, or G, respectively. For example, to set the initial heap size to 1 gigabyte and the maximum heap size to 4 gigabytes, you would use the following options:

-Xms1G -Xmx4G

It is generally recommended to set the initial and maximum heap sizes to the same value to avoid the overhead of resizing the heap. Setting the heap size too small can cause OutOfMemoryError exceptions to be thrown while setting it too large can waste memory and potentially lead to longer garbage collection (GC) pauses.

It's important to note that the -Xms and -Xmx options only control the size of the heap and do not affect the amount of memory used by the JVM for other purposes, such as thread stacks or native libraries.