How is the default max Java heap size determined

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If I omit the -Xmxn option from the Java command line then a default value will be used  According to Java documentation       the default value is chosen at runtime based on system configuration    What system configuration settings influence the default value

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Finally   As of Java 8u191 you now have the options   -XX InitialRAMPercentage -XX MaxRAMPercentage -XX MinRAMPercentage   that can be used to size the heap as a percentage of the usable physical RAM   which is same as the RAM installed less what the kernel uses    See Release Notes for Java8 u191 for more information  Note that the options are mentioned under a Docker heading but in fact they apply whether you are in Docker environment or in a traditional environment   The default value for MaxRAMPercentage is 25   This is extremely conservative    My own rule  If your host is more or less dedicated to running the given java application  then you can without problems increase dramatically  If you are on Linux   only running standard daemons and have installed RAM from somewhere around 1 Gb and up then I wouldn t hesitate to use 75  for the JVM s heap  Again  remember that this is 75  of the RAM available  not the RAM installed  What is left is the other user land processes that may be running on the host and the other types of memory that the JVM needs  eg for stack   All together  this will typically fit nicely in the 25  that is left  Obviously  with even more installed RAM the 75  is a safer and safer bet   I wish the JDK folks had implemented an option where you could specify a ladder   Setting the MaxRAMPercentage option look like this   java -XX MaxRAMPercentage 75 0         Note that these percentage values are of  double  type and therefore you must specify them with a decimal dot  You get a somewhat odd error if you use  75  instead of  75 0

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For the IBM JVM  the command is the following   java -verbose sizes -version   For more information about the IBM SDK for Java 8  http   www-01 ibm com support knowledgecenter SSYKE2 8 0 0 com ibm java lnx 80 doc diag appendixes defaults html lang en

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On Windows  you can use the following command to find out the defaults on the system where your applications runs       java -XX  PrintFlagsFinal -version   findstr HeapSize   Look for the options MaxHeapSize  for -Xmx  and InitialHeapSize for -Xms   On a Unix Linux system  you can do     java -XX  PrintFlagsFinal -version   grep HeapSize   I believe the resulting output is in bytes

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A number of parameters affect generation size  The following diagram illustrates the difference between committed space and virtual space in the heap  At initialization of the virtual machine  the entire space for the heap is reserved  The size of the space reserved can be specified with the -Xmx option  If the value of the -Xms parameter is smaller than the value of the -Xmx parameter  not all of the space that is reserved is immediately committed to the virtual machine  The uncommitted space is labeled  virtual  in this figure  The different parts of the heap  permanent generation  tenured generation and young generation  can grow to the limit of the virtual space as needed     By default  the virtual machine grows or shrinks the heap at each collection to try to keep the proportion of free space to live objects at each collection within a specific range  This target range is set as a percentage by the parameters -XX MinHeapFreeRatio  lt minimum gt  and -XX MaxHeapFreeRatio  lt maximum gt   and the total size is bounded below by -Xms lt min gt  and above by -Xmx lt max gt    Parameter           Default Value  MinHeapFreeRatio    40  MaxHeapFreeRatio    70  -Xms    3670k  -Xmx    64m  Default values of heap size parameters on 64-bit systems have been scaled up by approximately 30   This increase is meant to compensate for the larger size of objects on a 64-bit system   With these parameters  if the percent of free space in a generation falls below 40   the generation will be expanded to maintain 40  free space  up to the maximum allowed size of the generation  Similarly  if the free space exceeds 70   the generation will be contracted so that only 70  of the space is free  subject to the minimum size of the generation   Large server applications often experience two problems with these defaults  One is slow startup  because the initial heap is small and must be resized over many major collections  A more pressing problem is that the default maximum heap size is unreasonably small for most server applications  The rules of thumb for server applications are    Unless you have problems with pauses  try granting as much memory as possible to the virtual machine  The default size  64MB  is often too small  Setting -Xms and -Xmx to the same value increases predictability by removing the most important sizing decision from the virtual machine  However  the virtual machine is then unable to compensate if you make a poor choice  In general  increase the memory as you increase the number of processors  since allocation can be parallelized   There is the full article

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Ernesto is right   According to the link he posted  1       Updated Client JVM heap configuration       In the Client JVM            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              Server JVM heap configuration ergonomics are now the same as the Client  except that the default maximum heap size for 32-bit JVMs is 1 gigabyte  corresponding to a physical memory size of 4 gigabytes  and for 64-bit JVMs is 32 gigabytes  corresponding to a physical memory size of 128 gigabytes        1  http   www oracle com technetwork java javase 6u18-142093 html

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This is changed in Java 6 update 18   Assuming that we have more than 1 GB of physical memory  quite common these days   it s always 1 4th of your physical memory for the server vm

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Java 8 takes more than 1 64th of your physical memory for your Xmssize  Minimum HeapSize  and less than 1 4th of your physical memory for your -Xmxsize  Maximum HeapSize    You can check the default Java heap size by   In Windows   java -XX  PrintFlagsFinal -version   findstr  i  HeapSize PermSize ThreadStackSize    In Linux   java -XX  PrintFlagsFinal -version   grep -iE  HeapSize PermSize ThreadStackSize       What system configuration settings influence the default value    The machine s physical memory  amp  Java version

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For Java SE 5  According to Garbage Collector Ergonomics  Oracle       initial heap size       Larger of 1 64th of the machine s physical memory on the machine or some   reasonable minimum  Before J2SE 5 0    the default initial heap size was a   reasonable minimum  which varies by   platform  You can override this   default using the -Xms command-line   option       maximum heap size       Smaller of 1 4th of the physical memory or 1GB  Before J2SE 5 0  the   default maximum heap size was 64MB    You can override this default using   the -Xmx command-line option    UPDATE   As pointed out by Tom Anderson in his comment  the above is for server-class machines  From Ergonomics in the 5 0 JavaTM Virtual Machine      In the J2SE platform version 5 0 a   class of machine referred to as a   server-class machine has been defined   as a machine with         2 or more physical processors   2 or more Gbytes of physical memory         with the exception of 32 bit platforms   running a version of the Windows   operating system  On all other   platforms the default values are the   same as the default values for version   1 4 2        In the J2SE platform version 1 4 2 by   default the following selections were   made         initial heap size of 4 Mbyte   maximum heap size of 64 Mbyte

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default value is chosen at runtime based on system configuration  Have a look at the documentation page  Default Heap Size Unless the initial and maximum heap sizes are specified on the command line  they are calculated based on the amount of memory on the machine    Client JVM Default Initial and Maximum Heap Sizes  The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes  MB  and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte  GB    Server JVM Default Initial and Maximum Heap Sizes  On 32-bit JVMs  the default maximum heap size can be up to 1 GB if there is 4 GB or more of physical memory  On 64-bit JVMs  the default maximum heap size can be up to 32 GB if there is 128 GB or more of physical memory    What system configuration settings influence the default value   You can specify the initial and maximum heap sizes using the flags -Xms  initial heap size  and -Xmx  maximum heap size   If you know how much heap your application needs to work well  you can set -Xms and -Xmx to the same value

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The Xms and Xmx are flag of Java virtual machine  JVM    Xms  initial and minimum JVM heap size  Format  -Xms lt size gt  g G m M k K  Default Size   -server mode  25  of free physical memory   gt  8MB and  lt   64MB -client mode  25  of free physical memory   gt  8MB and  lt   16MB   Typical Size   -Xms128M -Xms256M -Xms512M   Function Effect   - gt  JVM start with allocate Xms size memory     Xmx  maximum JVM heap size  Format  -Xmx lt size gt  g G m M k K  Default Size    lt   R27 2  Windows  75  of total physical memory up to 1GB Linux Solaris  50  of available physical memory up to 1GB    gt   R27 3  Windows X64  75  of total physical memory up to 2GB Linux Solaris X64  50  of available physical memory up to 2GB Windows x86  75  of total physical memory up to 1GB Linux Solaris X86  50  of available physical memory up to 1GB     Typical Size   -Xmx1g -Xmx2084M -Xmx4g -Xmx6g -Xmx8g   Function Effect   - gt  JVM allow use maxium of Xmx size memory  when exceed Xmx  will java lang OutOfMemoryError  How to fix OutOfMemoryError    exceed Xmx value  eg  from -Xmx4g to -Xmx8g              More detail see  official doc  -X Command-line Options

[java] How is the default max Java heap size determined?

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