从缓存行出发理解volatile变量、伪共享False sharing、disruptor

待我称王封你为后i 2022-08-14 03:44 208阅读 0赞

**备注，现在已经进入多核时代了，考虑问题要考虑多核啊**

**volatile关键字**

当变量被某个线程A修改值之后，其它线程比如B若读取此变量的话，立刻可以看到原来线程A修改后的值

注：普通变量与volatile变量的区别是volatile的特殊规则保证了新值能立即同步到主内存，以及每次使用前可以立即从内存刷新，即一个线程修改了某个变量的值，其它线程读取的话肯定能看到新的值；

**普通变量：**

**写命中：当处理器将操作数写回到一个内存缓存的区域时，它首先会检查这个缓存的内存地址是否在缓存行中，如果不存在一个有效的缓存行，则处理器将这个操作数写回到缓存，而不是写回到内存，这个操作被称为写命中。**

<table style="border:1px solid silver; border-collapse:collapse; color:rgb(0,0,0); font-family:Verdana,Geneva,Arial,Helvetica,sans-serif; font-size:13px; line-height:20.799999237060547px"> 
 <tbody> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">术语</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">英文单词</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">描述</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">共享变量</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> &nbsp;</td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">在多个线程之间能够被共享的变量被称为共享变量。共享变量包括所有的实例变量，静态变量和数组元素。他们都被存放在堆内存中，Volatile只作用于共享变量。</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">内存屏障</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">Memory Barriers</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">是一组处理器指令，用于实现对内存操作的顺序限制。</p> <p style="margin:10px auto">备注：&nbsp;In the&nbsp;<a href="http://en.wikipedia.org/wiki/Java_Memory_Model" style="color:rgb(7,93,179)" rel="nofollow">Java Memory Model</a>&nbsp;a&nbsp;<em>volatile&nbsp;</em>field has a store barrier inserted after a write to it and a load barrier inserted before a read of it.</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">缓冲行</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">Cache line</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">缓存中可以分配的最小存储单位。处理器填写缓存线时会加载整个缓存线，需要使用多个主内存读周期。</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">原子操作</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">Atomic operations</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">不可中断的一个或一系列操作。</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">缓存行填充</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">cache line fill</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">当处理器识别到从内存中读取操作数是可缓存的，处理器读取整个缓存行到适当的缓存（L1，L2，L3的或所有）</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">缓存命中</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">cache hit</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">如果进行高速缓存行填充操作的内存位置仍然是下次处理器访问的地址时，处理器从缓存中读取操作数，而不是从内存。</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">写命中</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">write hit</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">当处理器将操作数写回到一个内存缓存的区域时，它首先会检查这个缓存的内存地址是否在缓存行中，如果不存在一个有效的缓存行，则处理器将这个操作数写回到缓存，而不是写回到内存，这个操作被称为写命中。</p> </td> 
  </tr> 
  <tr> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">写缺失</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">write misses the cache</p> </td> 
   <td style="border:1px solid silver; border-collapse:collapse; padding:3px"> <p style="margin:10px auto">一个有效的缓存行被写入到不存在的内存区域。</p> </td> 
  </tr> 
 </tbody> 
</table>

**单核CPU缓存结构**

**![Center][]**

**单核CPU缓存**

**![Center 1][]**

**多核CPU缓存**

所谓缓存航就是缓存中可以分配的最小存储单位。

处理器填写缓存线时会加载整个缓存线，需要使用多个主内存读周期。

下面降到伪缓存时会介绍，多核CPU、内存的缓存系统;

Information transfer between the cache and the memory** is in terms of complete cache lines, rather than individual bytes.** **Thus if the program needs a particular byte, the entire cache line containing that byte is obtained from the memory.** For example, suppose that the cache of Figure 2 was being used and the program fetches the word (two bytes) at location 0004736. If none of the cache lines contain the 16 bytes stored in addresses 0004730 through 000473F, then these 16 bytes are transferred from the memory to one of the cache lines. Because of the spatial locality of the program, we expect that other values in the cache line thus loaded will be referenced in the near future.

## Volatile的实现原理 ##

那么Volatile是如何来保证可见性的呢？在x86处理器下通过工具获取JIT编译器生成的汇编指令来看看对Volatile进行写操作CPU会做什么事情。

有volatile变量修饰的共享变量进行写操作的时候会多第二行汇编代码，通过查IA-32架构软件开发者手册可知，lock前缀的指令在多核处理器下会引发了两件事情

*  **将当前处理器缓存行的数据会写回到系统内存。**
 *  **这个写回内存的操作会引起在其他CPU里缓存了该内存地址的数据无效。**

**关键点：其实相当于线程 像缓存行写数据的时候，会锁住缓存行，是其他线程不能读，写完后失效缓存行，其他线程便可以从内存读到共享变量的最新值了；**

**深度解析：**

处理器为了提高处理速度，不直接和内存进行通讯，而是先将系统内存的数据读到内部缓存（L1,L2或其他）后再进行操作，但操作完之后不知道何时会写到内存；如果对声明了Volatile变量进行写操作，JVM就会向处理器发送一条Lock前缀的指令，将这个变量所在缓存行的数据写回到系统内存。但是就算写回到内存，如果其他处理器缓存的值还是旧的，再执行计算操作就会有问题，所以**在多处理器下，为了保证各个处理器的缓存是一致的，就会实现缓存一致性协议**，每个处理器通过嗅探在总线上传播的数据来检查自己缓存的值是不是过期了，当处理器发现自己缓存行对应的内存地址被修改，就会将当前处理器的缓存行设置成无效状态，当处理器要对这个数据进行修改操作的时候，会强制重新从系统内存里把数据读到处理器缓存里。

**Lock前缀指令会引起处理器缓存回写到内存**。Lock前缀指令导致在执行指令期间，声言处理器的 LOCK\# 信号。在多处理器环境中，LOCK\# 信号确保在声言该信号期间，处理器可以独占使用任何共享内存。（因为它会锁住总线，导致其他CPU不能访问总线，不能访问总线就意味着不能访问系统内存），但是在最近的处理器里，LOCK＃信号一般不锁总线，而是锁缓存，毕竟锁总线开销比较大。在8.1.4章节有详细说明锁定操作对处理器缓存的影响，对于Intel486和Pentium处理器，在锁操作时，总是在总线上声言LOCK\#信号。但在P6和最近的处理器中，如果访问的内存区域已经缓存在处理器内部，则不会声言LOCK\#信号。相反地，它会锁定这块内存区域的缓存并回写到内存，并使用缓存一致性机制来确保修改的原子性，此操作被称为“缓存锁定”，**缓存一致性机制会阻止同时修改被两个以上处理器缓存的内存区域数据**。

**一个处理器的缓存回写到内存会导致其他处理器的缓存无效**。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。在多核处理器系统中进行操作的时候，IA-32 和Intel 64处理器能嗅探其他处理器访问系统内存和它们的内部缓存。它们使用嗅探技术保证它的内部缓存，系统内存和其他处理器的缓存的数据在总线上保持一致。例如在Pentium和P6 family处理器中，如果通过嗅探一个处理器来检测其他处理器打算写内存地址，而这个地址当前处理共享状态，**那么正在嗅探的处理器将无效它的缓存行，在下次访问相同内存地址时，强制执行缓存行填充。**

## Volatile的使用优化 ##

著名的Java并发编程大师Doug lea在JDK7的并发包里新增一个队列集合类LinkedTransferQueue，他在使用Volatile变量时，用一种追加字节的方式来优化队列出队和入队的性能。

追加字节能优化性能？这种方式看起来很神奇，但如果深入理解处理器架构就能理解其中的奥秘。让我们先来看看LinkedTransferQueue这个类，它使用一个内部类类型来定义队列的头队列（Head）和尾节点（tail），而这个内部类PaddedAtomicReference相对于父类AtomicReference只做了一件事情，就将共享变量追加到64字节。我们可以来计算下，一个对象的引用占4个字节，它追加了15个变量共占60个字节，再加上父类的Value变量，一共64个字节。

**为什么追加64字节能够提高并发编程的效率呢**？ 因为对于英特尔酷睿i7，酷睿， Atom和NetBurst， Core Solo和Pentium M处理器的L1，L2或L3缓存的高速缓存行是64个字节宽，不支持部分填充缓存行，这意味着如果队列的头节点和尾节点都不足64字节的话，处理器会将它们都读到同一个高速缓存行中，在多处理器下每个处理器都会缓存同样的头尾节点，当一个处理器试图修改头接点时会将整个缓存行锁定，那么在缓存一致性机制的作用下，会导致其他处理器不能访问自己高速缓存中的尾节点，而队列的入队和出队操作是需要不停修改头接点和尾节点，所以在多处理器的情况下将会严重影响到队列的入队和出队效率。Doug lea使用追加到64字节的方式来填满高速缓冲区的缓存行，避免头接点和尾节点加载到同一个缓存行，使得头尾节点在修改时不会互相锁定。

那么是不是在使用Volatile变量时都应该追加到64字节呢？不是的。在两种场景下不应该使用这种方式。第一：**缓存行非64字节宽的处理器**，如P6系列和奔腾处理器，它们的L1和L2高速缓存行是32个字节宽。第二：**共享变量不会被频繁的写**。因为使用追加字节的方式需要处理器读取更多的字节到高速缓冲区，这本身就会带来一定的性能消耗，共享变量如果不被频繁写的话，锁的几率也非常小，就没必要通过追加字节的方式来避免相互锁定。

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     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">private</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">transient</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">final</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">PaddedAtomicReference &lt; QNode &gt; tail;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">static</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">final</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">class</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">PaddedAtomicReference &lt; T &gt; </code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">extends</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">AtomicReference &lt; T &gt; { </code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,130,0)!important; background:none!important">// enough padding for 64bytes with 4byte refs </code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">PaddedAtomicReference(T r) { </code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">super</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">(r);</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">}</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">}</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">public</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">class</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">AtomicReference &lt; V &gt; </code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">implements</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">java.io.Serializable { </code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">private</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(0,0,255)!important; background:none!important">volatile</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">V value;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; background:none!important">}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

Java并发编程实践写道：“一个理解volatile变量好的方法：想想它们的行为与SynchrosizedInteger类相似，只不过用get和set方法取代了对volatile变量的读写操作。然而访问volatile变量的操作不会加锁，也就不会引起线程的阻塞，这使volatile相对于synchronized而言，只是轻量级的同步机制”

## [false-sharing][] ##

Memory is stored within the cache system in units know as cache lines.  Cache lines are a power of 2 of contiguous bytes which are typically 32-256 in size.  The most common cache line size is 64 bytes.   False sharing is a term which applies when threads unwittingly impact the performance of each other while modifying independent variables sharing the same cache line.  Write contention on cache lines is the single most limiting factor on achieving scalability for parallel threads of execution in an SMP system.  I’ve heard false sharing described as the silent performance killer because it is far from obvious when looking at code.

# class \{int x ,int y\}  x和y被放在同一个高速**缓存**区，如果一个线程修改x；那么另外一个线程修改y，必须等待x修改完成后才能实施。 #

![Center 2][]

L1　Cache(一级缓存)是CPU第一层高速缓存，分为数据缓存和指令缓存。内置的L1高速缓存的容量和结构对CPU的性能影响较大，不过高速缓冲存储器均由静态RAM组成，结构较复杂，在CPU管芯面积不能太大的情况下，L1级高速缓存的容量不可能做得太大。一般服务器CPU的L1缓存的容量通常在32—4096KB。

L2 Cache 都在CPU中

L3　Cache(三级缓存)，分为两种，早期的是外置，现在的都是内置的。而它的实际作用即是，L3缓存的应用可以进一步降低内存延迟，同时提升大数据量计算时处理器的性能。降低内存延迟和提升大数据量计算能力对游戏都很有帮助。而在服务器领域增加L3缓存在性能方面仍然有显著的提升。比方具有较大L3缓存的配置利用物理内存会更有效，故它比较慢的磁盘I/O子系统可以处理更多的数据请求。具有较大L3缓存的处理器提供更有效的文件系统缓存行为及较短消息和处理器队列长度。

从图中可以看出为多核共享的

To achieve linear scalability with number of threads, we must ensure no two threads write to the same variable or cache line.  Two threads writing to the same variable can be tracked down at a code level.   To be able to know if independent variables share the same cache line we need to know the memory layout, or we can get a tool to tell us.  Intel VTune is such a profiling tool.  In this article I’ll explain how memory is laid out for Java objects and how we can pad out our cache lines to avoid false sharing.

**可见每个cpu核心或者线程都会可能往同一个缓存行写数据；并且对共享变量，同时cpu核心会有各自的缓存行**

虽然两个线程修改各种独立变量，但是因为这些独立变量被放在同一个高速缓存区，性能就影响了。[测试结果。][Link 1]当多核CPU线程同时修改在同一个高速缓存行各自独立的变量时，会不自不觉地影响性能，这就发生了伪共享False sharing，伪共享是性能的无声杀手。

# 当多核CPU线程同时修改在同一个高速缓存行各自独立的变量时，会不自不觉地影响性能，这就发生了伪共享False sharing，伪共享是性能的无声杀手。 #

这里强调多核，是因为单核CPU模拟出的多线程不会严格意义上同时访问缓存行，所以性能影响不大

解决方便是将高速缓存剩余的字节填充填满(pad)，确保不发生多个字段被挤入一个高速缓存区，下面测试结果图就是和填充后性能比较。

实现字节填充的框架有 Disruptor，在RingBuffer中实现填充。关于[Disruptor可见infoQ这个视频][Disruptor_infoQ]，用1毫秒的延时得到100K+ TPS吞吐量，

JDK的ArrayQueue并行环境不见得是最快的，该视频后面讨论很多，让人大跌眼镜啊，开放源码多有好处啊，别人能发现你不能发现的漏洞。

# Disruptor #

Disruptor没有像JDK的LinkedBlockQueue等那样使用锁，针对CPU高速缓存进行了优化。

**原来我们以为多个线程同时写一个类的字段会发生争夺，这是多线程基本原理，所以使用了锁机制，保证这个共用字段(资源)能够某个时刻只能一个线程写，但是这样做的坏处是：有可能发生死锁。**

比如1号线程先后访问共享资源A和B；而2号线程先后访问共享资源B和A，因为在资源A和资源B都有锁，那么1号在访问资源A时，资源A上锁了，准备访问资源B，但是无法访问，因为与此同时；而2号线程在访问资源B，资源B锁着呢，正准备访问资源A，发现资源A被1号线程锁着呢，结果彼此无限等待彼此下去，死锁类似逻辑上自指悖论。

**所以，锁是坏的，破坏性能，锁是并发计算的大敌。**

我们回到队列上，一把一个队列有至少两个线程：生产者和消费者，这就具备了资源争夺的前提，这两个线程一般彼此守在队列的进出两端，表面上好像没有访问共享资源，实际上队列存在两个共享资源：队列大小或指针.

除了共享资源写操作上存在资源争夺问题外，Disruptor的LMAX团队发现**Java在多核CPU情况下有伪共享问题：**CPU会把数据从内存加载到高速缓存中 ,这样可以获得更好的性能，高速缓存默认大小是64 Byte为一个区域，**CPU机制限制只能一个CPU的一个线程访问(写)这个高速缓存区。**

CPU在将主内存中数据加载到高速缓冲时，如果发现被加载的数据不足64字节，那么就会加载多个数据，以填满自己的64字节，悲催就发生了，恰恰hotspot JVM中对象指针等大小都不会超过64字节，这样一个高速缓冲中可能加载了两个对象指针，一个CPU一个高速缓冲，双核就是两个CPU各自一个高速缓冲，那么两个高速缓冲中各有两个对象指针，都是指向相同的两个对象。

因为一个CPU只能访问(写)自己高速缓存区中数据，相当于给这个数据加锁，那么另外一个CPU**同时访问自己**高速缓冲行中同样数据时将会被锁定不能访问。

这就发生与锁机制类似的性能陷进，**Disruptor的解决办法是填满高速缓冲的64字节，不是对象指针等数据不够64字节吗？那么加一些字节填满64字节，这样CPU将数据加载到高速缓冲时，就只能加载一个了，刚刚好啊。**

所以，**尽管两个线程是在写两个不同的字段值，也会因为双核CPU底层机制发生伪装的共享，并没有真正共享，其实还是排他性的独享。**

现在我们大概知道RingBuffer是个什么东东了：1.ring buffer是一个大的数组.2.RingBuffer里所有指针都是Java longs (64字节) 不断永远向前计数，如后面图，不断在圆环中循环。3.RingBuffer只有当前序列号，没有终点序列号，其中数据不会被取出后消除，这样以便实现从过去某个序列号到当前序列号的重放，这样当消费者说没有接受到生产者发送的消息，生产者还可以再次发送，这点是一种原子性的“事务”机制。

This new pattern is an ideal foundation for any asynchronous event processing architecture where high-throughput and low-latency is required.

Concurrent execution of code is about two things, mutual exclusion and visibility of change.

Read and write operations require that all changes are made visible to other threads. However only contended write operations require the mutual exclusion of the changes.

Locks provide mutual exclusion and ensure that the visibility of change occurs in an ordered manner. Locks are incredibly expensive because they require arbitration when contended.

We will illustrate the cost of locks with a simple demonstration. The focus of this experiment is to call a function which increments a 64-bit counter in a loop 500 million times. This can be executed by a single thread on a 2.4Ghz Intel Westmere EP in just 300ms if written in Java. The language is unimportant to this experiment and results will be similar across all languages with the same basic primitives.Once a lock is introduced to provide mutual exclusion, even when the lock is as yet un-contended, the cost goes up significantly. The cost increases again, by orders of magnitude, when two or more threads begin to contend. The results of this simple experiment are shown in the table below

![Center 3][]

However CAS operations are not free of cost. The processor must lock its instruction pipeline to ensure atomicity and employ a memory barrier to make the changes visible to other threads. CAS operations are available in Java by using the java.util.concurrent.Atomic\* classes.

The processors need only guarantee that program logic produces the same results regardless of execution order.

barriter:make the memory state within a processor visible to other processors.

disruptor 是为了解决消费者大于生产者的问题

参考

伪共享 ：[http://mechanical-sympathy.blogspot.com/2011/07/false-sharing.html][http_mechanical-sympathy.blogspot.com_2011_07_false-sharing.html]

[http://www.jdon.com/jivejdon/thread/42451\#23138636][Link 1]

volatile:[http://www.infoq.com/cn/articles/ftf-java-volatile][http_www.infoq.com_cn_articles_ftf-java-volatile]

缓存行：[http://ecee.colorado.edu/~ecen2120/Manual/caches/cache.html][http_ecee.colorado.edu_ecen2120_Manual_caches_cache.html]

disruptor:[http://mechanitis.blogspot.com/2011/07/dissecting-disruptor-why-its-so-fast\_22.html][http_mechanitis.blogspot.com_2011_07_dissecting-disruptor-why-its-so-fast_22.html]

源码：[http://code.google.com/p/disruptor/][http_code.google.com_p_disruptor]

memory barrier 屏障：[http://mechanical-sympathy.blogspot.com/2011/07/memory-barriersfences.html][http_mechanical-sympathy.blogspot.com_2011_07_memory-barriersfences.html]

转帖：http://www.cnblogs.com/pingyuyue/archive/2012/02/20/2360596.html

///

# [What is @Contended and False Sharing ?][What is _Contended and False Sharing] #

[Java 8][] introduces a new annotation called [@Contended][Contended], this is to reduce false sharing. This article covers @Contended and explains why False Sharing is a silent performance killer.

#### Cache Line Overview ####

CPU's don't read memory in single bytes, rather they read 'chunks of memory' usually in blocks of 64 bytes, these chunks are referred to as cache lines.  
  If you had two threads ( let's call them Thread 1 and Thread 2  ) both modifying a volatile variable, which we shall call ‘x' :

> volatile long x;

If Thread 1 was to change the value of ‘x’, and then Thread 2  was to read it :

> **Thread 1: ** x=3;

> **Thread 2: ** System.out.print(x);

For the value of x to be passed between the two threads ( Thread 1 to Thread 2  ) a whole 64 bytes will be exchanged, as cores only exchange data in cache lines. It is possible that Thread 1 and Thread 2, may actually be processed on the same core, but for this over simplified example lets assume that each thread is processed on its own core.

Given that long values are stored in 8 bytes, and in our example our cache line is 64 bytes, then the cache line could store 8 longs, we already have one long value of 'x' stored in the cache line, lets assume that the rest of the cache line was full of 7 other longs, for example v1 to v7

> x, v1, v2, v3, v4, v5 ,v6 ,v7

#### False Sharing ####

This cache line could be used by a number of different threads. If another thread was to modify v2, this would then force  Thread 1  and  Thread 2   to reload the cache line. You maybe wondering why should  Thread 1  and  Thread 2  reload this cache line, as the update to v2 should not effect them. Well, even though these updates are logically independent of each other, coherence is maintained on a cache-line basis, and not on individual elements. This apparent unnecessary sharing of data is referred to as false sharing.

#### Padding  ####

A core can execute hundreds of instructions in the time taken to fetch a single cache line.  If a core has to wait for a cache line to be reloaded, the core will run out of things to do, this is called a stall. Stalls can be avoided by reducing false sharing, one technique to reduce false sharing is to pad out data structure so that threads working on independent variables fall in separate cache lines.  An example of a padded class, attempting to place 'x' and 'v1' on separate cache lines :

public class FalseSharingWithPadding { 
     
        public volatile long x; 
        public volatile long p2;   // padding 
        public volatile long p3;   // padding 
        public volatile long p4;   // padding 
        public volatile long p5;   // padding 
        public volatile long p6;   // padding 
        public volatile long p7;   // padding 
        public volatile long p8;   // padding 
        public volatile long v1; 
    }

Before you go ahead an pad all your data structures its worth bearing in mind that the JVM can eliminate or re-order unused fields, thus re-introducing false sharing. Also there is no guarantee where objects will be placed on the heap.   
  
 To reduce the chance of your unused padding fields from being eliminated, it usually helps if you set them volatile. I suggest you only apply padding to highly contended concurrent classes and then, only if profiling o n your target architecture,  actually shows a difference. Usually best to do this after at least 10,000 iterations, to eliminate the effects of JVM realtime optimisations.

#### Java 8 and @Contended  ####

Rather than introduce padding fields a cleaner approach would be to annotate the fields that were likely to fall victim to false sharing, this could act as a hint to the JVM, which could split your fields into separate cache lines. This is the aim of [JEP 142][].  
  
 This JEP introduces the @Contended annotation. This annotation serves as a hint that such objects and fields should reside in locations isolated from those of other objects or fields

public class Point { 
            
                 int x;
            
                 @Contended
            
                 int y; 
    }

The code above will place both x and y on separate cache lines. The @Contended «shifts» the 'y' field away from the object header.

#### References ####

http://openjdk.java.net/projects/jdk8/features

http://beautynbits.blogspot.co.uk/2012/11/the-end-for-false-sharing-in-java.html  
http://openjdk.java.net/jeps/142   
http://mechanical-sympathy.blogspot.co.uk/2011/08/false-sharing-java-7.html  
http://stackoverflow.com/questions/19892322/when-will-jvm-use-intrinsics  
https://blogs.oracle.com/dave/entry/java\_contented\_annotation\_to\_help

转帖：http://robsjava.blogspot.com/2014/03/what-is-false-sharing.html

[Center]: /images/20220810/b54f361aa02c4cfe9ff0cc3470d9e275.png
[Center 1]: /images/20220810/8f804ee9a83d4fa6b604b97a958175df.png
[false-sharing]: http://www.cnblogs.com/pingyuyue/archive/2012/03/15/2399429.html
[Center 2]: /images/20220810/021fbd8584a144a69668b3a348f6b848.png
[Link 1]: http://www.jdon.com/jivejdon/thread/42451#23138636
[Disruptor_infoQ]: http://www.infoq.com/presentations/LMAX
[Center 3]: /images/20220810/55634ca301db4fa383fdac45294535e4.png
[http_mechanical-sympathy.blogspot.com_2011_07_false-sharing.html]: http://mechanical-sympathy.blogspot.com/2011/07/false-sharing.html
[http_www.infoq.com_cn_articles_ftf-java-volatile]: http://www.infoq.com/cn/articles/ftf-java-volatile
[http_ecee.colorado.edu_ecen2120_Manual_caches_cache.html]: http://ecee.colorado.edu/~ecen2120/Manual/caches/cache.html
[http_mechanitis.blogspot.com_2011_07_dissecting-disruptor-why-its-so-fast_22.html]: http://mechanitis.blogspot.com/2011/07/dissecting-disruptor-why-its-so-fast_22.html
[http_code.google.com_p_disruptor]: http://code.google.com/p/disruptor/
[http_mechanical-sympathy.blogspot.com_2011_07_memory-barriersfences.html]: http://mechanical-sympathy.blogspot.com/2011/07/memory-barriersfences.html
[What is _Contended and False Sharing]: http://robsjava.blogspot.com/2014/03/what-is-false-sharing.html
[Java 8]: http://openjdk.java.net/projects/jdk8/features
[Contended]: http://gee.cs.oswego.edu/dl/jsr166/src/dl/sun/misc/sun/misc/Contended.html
[JEP 142]: http://openjdk.java.net/jeps/142