How is the default max Java heap size determined?

Question

If I omit the -Xmxn option from the Java command line then a default value will be used. According to Java documentation

\"the default v

Answer 1

The Xms and Xmx are flag of Java virtual machine (JVM):

• Xms: initial and minimum JVM heap size
  • Format: -Xms[g|G|m|M|k|K]
  • Default Size:
    • -server mode: 25% of free physical memory, >=8MB and <= 64MB
    • -client mode: 25% of free physical memory, >=8MB and <= 16MB
  • Typical Size:
    • -Xms128M
    • -Xms256M
    • -Xms512M
  • Function/Effect:
    • -> JVM start with allocate Xms size memory
• Xmx: maximum JVM heap size
  • Format: -Xmx[g|G|m|M|k|K]
  • Default Size:
    • <= R27.2
      • Windows: 75% of total physical memory up to 1GB
      • Linux/Solaris: 50% of available physical memory up to 1GB
    • >= 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:
    • -> 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