JVM memory structure-program counter, virtual machine stack

Program counter

1.1 Definition

• The function is to remember the execution address of the next jvm instruction

• Features

  • Thread private, that is, each thread has its own program counter
  • No memory overflow

• In essence, the essence of the program counter is a register, which plays a role in the process of converting the binary code of the jvm into the machine code.

Virtual machine stack

• definition

  • Java Virtual Machine Stacks (Java virtual machine stack)
  • Each thread can only have one virtual machine stack (thread private), corresponding to the method currently executed

• effect

  • Whenever a thread executes a method, it is pushed into the virtual machine stack; whenever a method is executed, it is ejected from it.

• Thinking

  • Are local variables in methods thread safe?
    • If the local variables in the method are not accessed from the role of the escape method, it is thread-safe
    • If the local variable references the object and escapes the scope of the method, thread safety needs to be considered.
    • The core is whether there is a situation of multi-threaded access.

• Overflow of stack memory

  • Too many stack frames cause memory overflow (often)

    • For example, when the end condition of recursion is not well specified, there will be too many problems that cause overflow.

  • Excessive stack frame size causes stack memory overflow

• Thread runtime diagnosis

  • Use top to locate the process that appears (for example, CPU usage is too high)

  • ps H -eo pid,tid,% cpu | grep process id

  • jstack process id

    • The problematic thread can be found according to the thread id, and the source line number of the problem code can be further located