完善"如何保证变量的可见性"问题答案

[shark-ctrl]

参见Issue 关于Java 并发常见面试题总结（中）“如何保证变量的可见性？”问题的答案详解 #1849

[jizhuozhi]

关于TLAB的描述并不严谨，实际上不论是否有TLAB，JVM都可以通过锁或CAS或其他同步原语来完成原子性内存分配。分配TLAB本身就是从堆中分配连续内存空间的过程，通过CAS移动碰撞指针（Bump Pointer）来完成的分配。只不过使用TLAB可以避免在分配对象过程中使用同步原语，尽最大程度减少并发冲突。

引用2004年G1论文Garbage-First Garbage Collection中的描述

2.1 Heap Layout/Heap Regions/Allocation
The Garbage-First heap is divided into equal-sized heap regions, each a contiguous range of virtual memory. Alloca- tion in a heap region consists of incrementing a boundary, top, between allocated and unallocated space. One region is the current allocation region from which storage is be- ing allocated. Since we are mainly concerned with multi- processors, mutator threads allocate only thread-local allo- cation buffers, or TLABs, directly in this heap region, using a compare-and-swap, or CAS, operation. They then allocate objects privately within those buffers, to minimize allocation contention. When the current allocation region is filled, a new allocation region is chosen. Empty regions are orga- nized into a linked list to make region allocation a constant time operation.
Larger objects may be allocated directly in the current allocation region, outside of TLABs. Objects whose size exceeds 3/4 of the heap region size, however, are termed humongous. Humongous objects are allocated in dedicated (contiguous sequences of) heap regions; these regions con- tain only the humongous object.1

[jizhuozhi]

同时volatile本身并没有关注体系结构上是如何实现的缓存一致性协议，它仅仅关心在读之前使用读写屏障，在写之后使用写读屏障而已。

具体的源码可以看这一部分
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/interpreter/zero/bytecodeInterpreter.cpp#L1768-L1805
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/interpreter/zero/bytecodeInterpreter.cpp#L1899-L1934

实际上是Java内存模型开发时间远远早于C++内存顺序定义时间，不然Java可以不应定义访问顺序，直接使用C++内存顺序的实现的（https://en.cppreference.com/w/cpp/atomic/memory_order）

3. Read-write coherence: if a value computation A of some atomic M (a read) happens-before an operation B on M (a write), then the value of A comes from a side-effect (a write) X that appears earlier than B in the modification order of M
4. Write-read coherence: if a side effect (a write) X on an atomic object M happens-before a value computation (a read) B of M, then the evaluation B shall take its value from X or from a side effect Y that follows X in the modification order of M