"Ask every day" briefly talk about JMM / talk about your understanding of JMM

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JMM What is it?

Java Memory Model , Java Memory model , abbreviation JMM.
Java Communication between threads is made by JMM control .JMM Determines when a thread's write to a shared resource is visible to another thread . From an abstract point of view ,JMM Defines the relationship between threads and main memory , Each thread has a local memory ¹, Storing shared resources in main memory 「 copy 」.

JMM Memory model

It said ,JMM Control thread communication . Thread as shown in the figure A、B How is the communication

1. Threads A Refresh the shared variables in your local memory to the main memory
2. Threads B Make the copy in your local memory invalid , Read the shared variable value in main memory

The above two steps can be seen

• Communication between threads must pass through main memory .
• JMM By controlling the interaction between local memory and main memory of each thread , Provide us with memory visibility assurance .

Reorder

When a program is executing , To optimize program performance , Compilers and processors often reorder instructions . Reordering can cause memory visibility problems in multithreaded programs .

JMM Treatment and requirements for these two types of rearrangement ：

• For compiler reordering ：JMM Will prevent certain types of compiler reordering . So what is it ？
• Reorder for processor ：JMM Will ask for Java When the compiler generates a sequence of instructions , Insert a specific type of memory barrier （Memory Barries） To disable certain types of processor reordering .

JMM As a language level memory model , On different compilers and processors , By prohibiting certain types of compiler and processor reordering , Provide consistent memory visibility assurance .

Data dependency

If two operations access the same variable , And one of these two operations is a write operation , Then there is a data dependency between the two operations .
There are three operations that generate data dependencies ：

1. After reading
2. Read after writing
3. Write after
   The above three operations , If the order is rearranged , Then the result of the program will be changed .

as-if-serial semantics

as-if-serial( Serialization ) The meaning of is ： No matter how the compiler and processor reorder to provide parallelism , The execution result of a single threaded program cannot be changed ².
So in order to achieve this effect , Compilers and processors do not reorder operations that have data dependencies .

Pictured ,C Respectively with A、B There are data dependencies , that C Will not be rearranged in A、B Before any one goes .
Then the last possible order is ：

happens-before

If an operation happens-before Another operation , Then the execution result of the first operation is visible to the second operation , But the execution order of the first operation is not necessarily ahead of the second . If these two operations are data dependent , Then the first operation is ahead of the second , vice versa .
Multi thread operation happens-before The establishment of the relationship depends on the correct synchronization of the code .

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1. Local memory is JMM An abstract concept of , It doesn't really exist , It actually covers caching 、 Write buffer 、 Optimization of registers and other hardware and compilers . ︎

2. as-if-serial Protect single threaded programs , At the same time, it will also create an illusion ： The execution order of single threaded code is consistent with the order of written code . ︎