Java – what is -Xms and -Xmx parameter?

By mkyong | Updated: May 3, 2019

Viewed: 4,891 (+17 pv/w)

Tags: heap size java memory

In Java, -Xms set initial Java heap size, while -Xmx set the maximum Java heap size.

1. Some java -Xms -Xmx examples :

# Start with 128MB of memory, and allow the Java process to use up to 1024MB of memory. 
java -Xms128m -Xmx1024m

# Start with 256MB of memory, and allow the Java process to use up to 4G (4096MB) of memory.
java -Xms256m -Xmx4g

2. The default initial and maximum Java heap size is allocated based on this ergonomics algorithm , also read this article – Find out your Java heap memory size

3. If the Java process has exceeded the -Xmx maximum Java heap size, the popular java.lang.OutOfMemoryError will be thrown.

4. For other options, run this java -X

> java -X

    -Xbatch           disable background compilation
    -Xbootclasspath/a:<directories and zip/jar files separated by ;>
                      append to end of bootstrap class path
    -Xcheck:jni       perform additional checks for JNI functions
    -Xcomp            forces compilation of methods on first invocation
    -Xdebug           provided for backward compatibility
    -Xdiag            show additional diagnostic messages
    -Xfuture          enable strictest checks, anticipating future default
    -Xint             interpreted mode execution only
    -Xinternalversion
                      displays more detailed JVM version information than the
                      -version option
    -Xloggc:<file>    log GC status to a file with time stamps
    -Xmixed           mixed mode execution (default)
    -Xmn<size>        sets the initial and maximum size (in bytes) of the heap
                      for the young generation (nursery)
    -Xms<size>        set initial Java heap size
    -Xmx<size>        set maximum Java heap size
    -Xnoclassgc       disable class garbage collection
    -Xrs              reduce use of OS signals by Java/VM (see documentation)
    -Xshare:auto      use shared class data if possible (default)
    -Xshare:off       do not attempt to use shared class data
    -Xshare:on        require using shared class data, otherwise fail.
    -XshowSettings    show all settings and continue
    -XshowSettings:all
                      show all settings and continue
    -XshowSettings:locale
                      show all locale related settings and continue
    -XshowSettings:properties
                      show all property settings and continue
    -XshowSettings:vm
                      show all vm related settings and continue
    -XshowSettings:system
                      (Linux Only) show host system or container
                      configuration and continue
    -Xss<size>        set java thread stack size
    -Xverify          sets the mode of the bytecode verifier

References

• Tuning Java Virtual Machines (JVMs)
• Eclipse -Xms / -Xmx
• ergonomics algorithm
• Find out your Java heap memory size