Explain JMM in detail

One 、 Computer hardware storage system

CPU Run is not a direct operation of memory, but first read the data in memory to the cache , And memory read and write operations will cause inconsistencies .JVM An attempt was made in the specification to define a Java Memory model (java Memory Model, abbreviation JMM) To screen out the memory access differences between various hardware and operating systems , In order to make Java The program can achieve consistent memory access effect under various platforms .

Two 、 brief introduction

1.JMM brief introduction

JMM(Java Memory model Java Memory Model, abbreviation JMM) It is an abstract concept and does not really exist. It only describes a set of conventions or specifications , Through this set of specifications, the... In the program is defined ( Especially multithreading ) The read-write access mode of each variable and determines when and how a thread's writing to a shared variable becomes visible to another thread , The key technical points are around the atomicity of multithreading 、 Visibility and order unfold .
principle ：JMM The key technology points are all around the atomicity of multithreading 、 Visibility and order unfold . It works as follows ：

1. adopt JMM To realize the abstract relationship between thread and main memory .
2. Shield the memory access differences of various hardware platforms and operating systems to achieve Java The program can achieve consistent memory access effect under various platforms .

2. The concept of cache rows

A multi-core cpu Each core of has its own L2 cache , All cores share a L3 cache . When the data in the main memory is written to the cache , It will also write in the nearby data by the way . This write range is called a cache row （ size 64 byte &#