xms and xmx Java

What is xmx and xms?

xmx and xms are JVM, Java Virtual Machine, command-line options/flags that are used by programmers to manipulate the heap size used by JVM. This allows them to optimize the speed and memory of different java applications.

About xms

• xms is used to specify the initial lower bound of java heap memory size.
• Default xms size is 1/64th of the physical memory size.
• Format for specifying xms:

java -xms{Numerical Size}{Unit}

where Unit is $k/K$ for kilobytes, $m/M$ for megabytes, and g/G for gigabytets.

• The numerical size must be a whole number.
• By default, the size is in bytes.

About xmx

• xmx is used to specify the upper bound of java heap memory size.
• Default xms size is 1/4th of the physical memory size.
• Format for specifying xms:

java -xms{Numerical Size}{Unit}

where Unit is k/K for kilobytes, m/M for megabytes, and g/G for gigabytes.

• The numerical size must be a whole number.
• By default, the size is in bytes.

Pitfalls

• If the programmer initializes a fairly large heap size, then the Garbage Collector will consume more time while clearing the memory, which will slow down the program.
• If the programmer initializes a fairly small heap size, then it is possible to run out of memory. Insufficient memory will throw the following exception: java.lang.OutOfMemoryError: Java heap space.