RPM: Java can set maximum memory

Test method: At the command line with Java-xmxxxxxm-version, such as: java-xmx1024m-version command to test, and then gradually increase the value of XXXX, if performed normally indicates that the specified memory size is available, otherwise the error message will be printed.

• Heap and non-heap (non-heap) memory
  According to the official statement, "Java virtual machines have a heap, the heap is a runtime data region, and all class instances and arrays of memory are allocated from here." The heap is created when the Java virtual machine is started. "" The memory outside the heap in the JVM is called non-heap (non-heap memory) ". You can see that the JVM primarily manages two types of memory: heap and non-heap. In a nutshell, a heap is a Java code-readable memory that is left to the developer, not a heap, which is left to itself by the JVM, so the method area, the JVM internally processes or optimizes the required memory (such as the JIT-compiled code cache), each class structure (such as running a constant pool, field, and method data) And the code for methods and construction methods are in non-heap memory.
• Heap memory allocation
  The initial memory allocated by the JVM is specified by-XMS, the default is physical memory 1/64;JVM the maximum allocated memory is specified by-XMX, which defaults to 1/4 of the physical memory. When the default free heap memory is less than 40%, the JVM increases the heap until the maximum limit of-xmx, and when the free heap memory is greater than 70%, the JVM reduces the heap until the minimum limit of-XMS. So the server generally sets-xms,-xmx equal to avoid resizing the heap after each GC.
• Non-heap memory allocation
  The JVM uses-xx:permsize to set the non-heap memory initial value, which defaults to 1/64 of the physical memory, and the maximum non-heap memory by Xx:maxpermsize, which by default is 1/4 of physical memory.
• JVM Memory Limit (max)
  First, the JVM memory is limited to the actual maximum physical memory (nonsense!). hehe), assuming that the physical memory is infinitely large, the maximum value of the JVM memory is very much related to the operating system. In short, the 32-bit processor, although the controllable memory space has 4GB, but the specific operating system will give a limit, This limit is generally 2GB-3GB (typically under Windows systems for the 1.5g-2g,linux system 2g-3g), and processors over 64bit are not limited.

  So there are several reasons why setting a VM parameter causes the program to fail to start:

  1) The value of-XMS in the parameter is greater than-XMX, or the value of-xx:permsize is greater than-xx:maxpermsize;

  2) The sum of-XMX values and-xx:maxpermsize exceeds the maximum limit of JVM memory, such as the maximum memory limit of the current operating system, or actual physical memory, and so on. When it comes to physical memory, it is important to note that if your memory is 1024MB, the actual system is not likely to be 1024MB, because a portion of it is consumed by the hardware.

RPM: Java can set maximum memory