JMM and volatile analysis

1.JMM: Java Memory Model, java thread memory model

JMM: It is an abstract concept, communication between threads and memory as described, similar to the model java thread and CPU cache memory model, it is standardized, for shielding hardware and operating systems of the differences in memory access .

2.JMM large atomic binding and 8

3.volatile applications and the underlying principles of inquiry

volatile: lightweight synchronized, in the development of multiprocessor ensure that the "visibility" of shared variables. Visibility means: When a thread modifies a shared variables, other threads to use the shared variable values can be modified in time to read. Properly modified, it is lower than the cost of execution of synchronized, because it does not cause a context switch and thread scheduling.

public class VolatileTest {
    private static volatile boolean  flag = false;
    public static void main(String[] args) {
        update();
    }

    public static void update(){
        flag = true;
        System.out.println(flag);
    }
}
Volatile JIT编译器编译java代码为汇编指令查看
1.在jdk\jre\bin\ 目录下添加 hsdis-amd64.lib
2.在jdk1.8\jre\bin\server\目录下添加hsdis-amd64.dll文件
3.在IDEA中设置 JVM参数
-server -Xcomp -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly 
-XX:CompileCommand=compileonly,VolatileTest.update
4.运行Java程序即可打印出

CompilerOracle: compileonly *VolatileTest.update
Loaded disassembler from E:\EclipseDev\jdk\jdk1.8\jre\bin\server\hsdis-amd64.dll
Decoding compiled method 0x000000000f11aad0:
Code:
Argument 0 is unknown.RIP: 0xf11ac40 Code size: 0x000002a8
[Disassembling for mach='amd64']
[Entry Point]
[Verified Entry Point]
[Constants]
# {method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest'
# [sp+0x40] (sp of caller)
0x000000000f11ac40: mov dword ptr [rsp+0ffffffffffffa000h],eax
0x000000000f11ac47: push rbp
0x000000000f11ac48: sub rsp,30h
0x000000000f11ac4c: mov r8,8792d70h ; {metadata(method data for {method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest')}
0x000000000f11ac56: mov edx,dword ptr [r8+0dch]
0x000000000f11ac5d: add edx,8h
0x000000000f11ac60: mov dword ptr [r8+0dch],edx
0x000000000f11ac67: mov r8,8792b70h ; {metadata({method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest')}
0x000000000f11ac71: and edx,0h
0x000000000f11ac74: cmp edx,0h
0x000000000f11ac77: je 0f11ad68h ;*iconst_1
; - com.yew.test.VolatileTest::update@0 (line 17)

0x000000000f11ac7d: mov r8,0d7b08a30h ; {oop(a 'java/lang/Class' = 'com/yew/test/VolatileTest')}
0x000000000f11ac87: mov edx,1h
0x000000000f11ac8c: mov byte ptr [r8+68h],dl

volatile修饰
0x000000000f11ac90: lock add dword ptr [rsp],0h ;*putstatic flag
; - com.yew.test.VolatileTest::update@1 (line 17)

无Volatile修饰

0x000000000f113707: mov byte ptr [r8+68h],1h ;*putstatic flag
; - com.yew.test.VolatileTest::update@1 (line 17)

通过比较可知：改变共享变量flag的值为true,该变量由Volatile修饰，进行汇编打印时，会有lock前缀修饰，根据IA-32架构软件开发者手册可知，lock前缀指令在多核CPU处理器下会引发两件事情：

【1】将当前处理器缓存行的数据立即写回系统内存

【2】wirte操作会使其他处理器中缓存该内存地址的数据无效

LOCK#声言期间，处理器独占任何共享内存。IA-32处理器和Intel 64处理器使用MESI（修改、独占、共享、无效）控制协议去维护内部缓存和其他处理器缓存的一致性。通过嗅探技术保证处理器内部缓存、系统缓存和其他处理器缓存的数据再总线上保持一致。当其他处理器打算回写内存地址，该地址是共享内存区域，那么嗅探的处理器会将它的缓存行设置为无效，下次访问相同内存时，强制执行缓存行填充。

0x000000000f11ac95: nop
0x000000000f11ac98: jmp 0f11add4h ; {no_reloc}
0x000000000f11ac9d: add byte ptr [rax],al
0x000000000f11ac9f: add byte ptr [rax],al
0x000000000f11aca1: add byte ptr [rsi+0fh],ah
0x000000000f11aca4: Fatal error: Disassembling failed with error code: 15Decoding compiled method 0x000000000f11ef50:
Code:
Argument 0 is unknown.RIP: 0xf11f080 Code size: 0x00000058
[Entry Point]
[Verified Entry Point]
[Constants]
# {method} {0x0000000008792b78} 'update' '()V' in 'com/yew/test/VolatileTest'
# [sp+0x20] (sp of caller)
0x000000000f11f080: mov dword ptr [rsp+0ffffffffffffa000h],eax
0x000000000f11f087: push rbp
0x000000000f11f088: sub rsp,10h
0x000000000f11f08c: mov r10,0d7b08a30h ; {oop(a 'java/lang/Class' = 'com/yew/test/VolatileTest')}
0x000000000f11f096: mov byte ptr [r10+68h],1h
0x000000000f11f09b: lock add dword ptr [rsp],0h ;*putstatic flag
; - com.yew.test.VolatileTest::update@1 (line 17)

0x000000000f11f0a0: mov edx,1ch
0x000000000f11f0a5: nop
0x000000000f11f0a7: call 0f0557a0h ; OopMap{off=44}
;*getstatic out
; - com.yew.test.VolatileTest::update@4 (line 18)
; {runtime_call}
0x000000000f11f0ac: int3 ;*getstatic out
; - com.yew.test.VolatileTest::update@4 (line 18)

0x000000000f11f0ad: hlt
0x000000000f11f0ae: hlt
0x000000000f11f0af: hlt
0x000000000f11f0b0: hlt
0x000000000f11f0b1: hlt
0x000000000f11f0b2: hlt
0x000000000f11f0b3: hlt
0x000000000f11f0b4: hlt
0x000000000f11f0b5: hlt
0x000000000f11f0b6: hlt
0x000000000f11f0b7: hlt
0x000000000f11f0b8: hlt
0x000000000f11f0b9: hlt
0x000000000f11f0ba: hlt
0x000000000f11f0bb: hlt
0x000000000f11f0bc: hlt
0x000000000f11f0bd: hlt
0x000000000f11f0be: hlt
0x000000000f11f0bf: hlt
[Exception Handler]
[Stub Code]
0x000000000f11f0c0: jmp 0f0883a0h ; {no_reloc}
[Deopt Handler Code]
0x000000000f11f0c5: call 0f11f0cah
0x000000000f11f0ca: sub qword ptr [rsp],5h
0x000000000f11f0cf: jmp 0f057600h ; {runtime_call}
0x000000000f11f0d4: hlt
0x000000000f11f0d5: hlt
0x000000000f11f0d6: hlt
0x000000000f11f0d7: hlt
true

4.volatile的使用优化

java并发大师Doug Li在jdk7并发包中新增了一个队列集合LinkeTransferQueue,它在使用Volatile关键字修饰变量时，采用追加字节的方式将变量填充到64字节

volatile修饰变量在进行修改时，会进行LOCK前置指令加锁，锁住缓存行的数据独占

适用于：缓存行字节为64字节处理器如 I7 酷睿 Pentium M等

不适用：非64字节宽的缓存行 P6系列或者奔腾共享变量不会被频繁的写

4.并发编程的三大特性：可见性、原子性、有序性

volatile可以保证可见性、有序性，但是不保证原子性。

JMM and Volatile underlying principle analysis