What is the JVM runtime data area and the JMM memory model

What is the JVM runtime data area and JMM memory model

This article "What is the JVM runtime data area and the JMM memory model?" Most people do not understand the knowledge points, so the editor summarizes the following content for you, the content is detailed, and the steps are clear, has a certain reference value. I hope you can gain something after reading this article. Let's take a look at this article "What is the JVM runtime data area and the JMM memory model".

1. JVM runtime data area

The JVM runtime data area can be divided into five blocks: metaspace, heap, virtual machine stack, native method stack, and program counter.

• Metaspace (method area): Store class template objects, it is an area shared by threads, on disk, generally not GC

• Heap space:The area shared by threads, the main position for object creation and GC

• Virtual machine stack: The thread is private, the basic unit is the stack frame, each stack frame corresponds to a method, the stack frame is composed as follows

• Local variable table: store method variable information

• Operand stack: the area where methods run

• Dynamic linking: point to the method template object, implement method rewriting with the virtual method table

• Return address: the return address of the method

• Native method stack:Thread private, execution area for native methods

• Program Counter: Thread private, responsible for recording where the thread executes during the thread context switch process

2. JMM memory model

Hardware memory model

Usually, when the CPU needs to read the main memory, it will read the part of the main memory into the CPU cache or the internal register, and then perform the operation in the register.When the CPU needs to write the result back to main memory, it flushes the value of the internal registers to the cache, and then at some point flushes the value back to main memory.

In a multiprocessor system, each processor has its own cache, and they share the same main memory, so there is a cache coherency problem.In order to solve the problem of consistency, each processor needs to follow some protocols when accessing the cache, and operate according to the protocol when reading and writing. Such protocols include MSI, MESI, etc.

JMM

The JMM memory model divides the content into two parts: thread private memory and main memory. The corresponding relationship with the hardware memory model we mentioned earlier is as follows:

The interaction of private memory with main memory is controlled by the following eight operations:

3. Visible line and volatile keyword

In a nutshell, the volatile keyword prevents instruction reordering in the form of memory barriers to maintain variable ordering and visibility.

A line of code goes through the following stages from execution to execution:

The volatile keyword serves two purposes:

• Guarantees that volatile-modified shared variables are always visible to all threads, that is, when a thread modifies the value of a volatile-modified shared variable, the new value is always immediately available to other threads.

• Disable instruction reordering optimization.

There are four types of memory barrier instructions available in the JVM:

• loadload:Between two read operations

• storestore:Between two writes

• loadstore:Between read and write operations

• storelosd:Between writes and reads

The above is about the content of the article "What is the JVM runtime data area and the JMM memory model?" I believe everyone has a certain understanding. I hope the content shared by the editor will be helpful to you.If you want to know more related knowledge content, please pay attention to the Yisu Cloud Industry Information Channel.