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

Question

Please explain the use of Xms and Xmx parameters in JVMs  What are the default values for them

User · Accepted Answer

The flag Xmx specifies the maximum memory allocation pool for a Java Virtual Machine  JVM   while Xms specifies the initial memory allocation pool  This means that your JVM will be started with Xms amount of memory and will be able to use a maximum of Xmx amount of memory  For example  starting a JVM like below will start it with 256  MB of memory and will allow the process to use up to 2048  MB of memory  java -Xms256m -Xmx2048m  The memory flag can also be specified in different sizes  such as kilobytes  megabytes  and so on  -Xmx1024k -Xmx512m -Xmx8g  The Xms flag has no default value  and Xmx typically has a default value of 256  MB  A common use for these flags is when you encounter a java lang OutOfMemoryError  When using these settings  keep in mind that these settings are for the JVM s heap  and that the JVM can and will use more memory than just the size allocated to the heap  From Oracle s documentation   Note that the JVM uses more memory than just the heap  For example Java methods  thread stacks and native handles are allocated in memory separate from the heap  as well as JVM internal data structures

User · Answer

Run the command java -X and you will get a list of all -X options   C  Users Admin gt java -X -Xmixed           mixed mode execution  default  -Xint             interpreted mode execution only -Xbootclasspath  lt directories and zip jar files separated by   gt                        set search path for bootstrap classes and resources -Xbootclasspath a  lt directories and zip jar files separated by   gt                        append to end of bootstrap class path -Xbootclasspath p  lt directories and zip jar files separated by   gt                        prepend in front of bootstrap class path -Xdiag            show additional diagnostic messages -Xnoclassgc       disable class garbage collection -Xincgc           enable incremental garbage collection -Xloggc  lt file gt     log GC status to a file with time stamps -Xbatch           disable background compilation -Xms lt size gt         set initial Java heap size                          -Xmx lt size gt         set maximum Java heap size                          -Xss lt size gt         set java thread stack size -Xprof            output cpu profiling data -Xfuture          enable strictest checks  anticipating future default -Xrs              reduce use of OS signals by Java VM  see documentation  -Xcheck jni       perform additional checks for JNI functions -Xshare off       do not attempt to use shared class data -Xshare auto      use shared class data if possible  default  -Xshare on        require using shared class data  otherwise fail  -XshowSettings    show all settings and continue -XshowSettings all         show all settings and continue -XshowSettings vm          show all vm related settings and continue -XshowSettings properties  show all property settings and continue -XshowSettings locale      show all locale related settings and continue   The -X options are non-standard and subject to change without notice   I hope this will help you understand Xms  Xmx as well as many other things that matters the most

User · Answer

The question itself has already been addressed above   Just adding part of the default values   As per http   docs oracle com cd E13150 01 jrockit jvm jrockit jrdocs refman optionX html  The default value of Xmx will depend on platform and amount of memory available in the system

User · Answer

-Xms initial heap size for the startup  however  during the working process the heap size can be less than -Xms  due to users  inactivity or GC iterations  This is not a minimal required heap size   -Xmx maximal heap size

User · Answer

You can specify it in your IDE  For example  for Eclipse in Run Configurations   VM arguments  You can enter -Xmx800m -Xms500m as

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

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