What is the default maximum heap size for Sun s JVM from Java SE 6

Question

What is the default maximum heap size for Sun s JVM from Java SE 6  i e  equivalent to setting -Xmx    Looks like for Java SE 5 with a server-class machine  it s     Smaller of 1 4th of the physical memory or 1GB    Bonus question  Looks like for IBM s JVM you can ask it  java -verbose sizes -version   Can you similarly ask Sun s JVM   Edit   I used Runtime getRuntime   maxMemory to confirm min physical memory 4  1G   referenced in Sun documentation

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java 1 6 0 21 or later  or so       java -XX  PrintFlagsFinal -version 2 gt  amp 1   grep MaxHeapSize uintx MaxHeapSize                            12660904960       product    It looks like the min 1G  has been removed   Or on Windows using findstr  C   gt java -XX  PrintFlagsFinal -version 2 gt  amp 1   findstr MaxHeapSize

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To answer this question it s critical whether the Java VM is in CLIENT or SERVER mode   You can specify  -client  or  -server  options   Otherwise java uses internal rules  basically win32 is always client and Linux is always server  but see the table here   http   docs oracle com javase 6 docs technotes guides vm server-class html  Sun Oracle jre6u18 doc says re client  the VM gets 1 2 of physical memory if machine has  lt   192MB  1 4 of memory if machine has  lt   1Gb  max 256Mb   In my test on a 32bit WindowsXP system with 2Gb phys mem  Java allocated 256Mb  which agrees with the doc   Sun Oracle jre6u18 doc says re server  same as client  then adds confusing language  for 32bit JVM the default max is 1Gb  and for 64 bit JVM the default is 32Gb   In my test on a 64bit linux machine with 8Gb physical  Java allocates 2Gb  which is 1 4 of physical  on a 64bit linux machine with 128Gb physical Java allocates 32Gb  again 1 4 of physical   Thanks to this SO post for guiding me   Definition of server-class machine changed recently

User · Answer

one way is if you have a jdk installed   in bin folder there is a utility called jconsole even visualvm can be used   Launch it and connect to the relevant java process and you can see what are the heap size settings  set and many other details  When running headless or cli only   jConsole can be used over lan  if you specify a port to connect on when starting the service in question

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As of JDK6U18 following are configurations for the Heap Size       In the Client JVM  the default Java heap configuration has been   modified to improve the performance of today s rich client   applications  Initial and maximum heap sizes are larger and settings   related to generational garbage collection are better tuned       The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes and otherwise one fourth of   the physical memory up to a physical memory size of 1 gigabyte        For example  if your machine has 128 megabytes of physical memory  then the maximum heap size is 64 megabytes  and greater than or equal   to 1 gigabyte of physical memory results in a maximum heap size of 256   megabytes        The maximum heap size is not actually used by the JVM unless your program creates enough objects to require it  A much smaller amount    termed the initial heap size  is allocated during JVM initialization    This amount is at least 8 megabytes and otherwise 1 64 of physical   memory up to a physical memory size of 1 gigabyte    Source   http   www oracle com technetwork java javase 6u18-142093 html

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With JDK   You can also use jinfo to connect to the JVM for the  lt PROCESS ID gt  in question and get the value for MaxHeapSize   jinfo -flag MaxHeapSize  lt PROCESS ID gt

User · Answer

One can ask with some Java code   long maxBytes   Runtime getRuntime   maxMemory    System out println  Max memory      maxBytes   1024   1024    M      See javadoc

[java] What is the default maximum heap size for Sun's JVM from Java SE 6?

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