读书笔记: 多处理器编程的艺术

https://book.douban.com/people/fleure/annotation/21334175/

Spin Locks and Contention

<原文开始> the TASLock performs very poorly, and the TTASLock performance, while substantially better, still falls far short of the ideal. </原文结束> TASLock: Test And Set Lock TTASLock: Test and Test And Set Lock，相比 TAS ，增加了一个 double check： <代码开始 lang="java"> public class TTASLock Implements Lock { AtomicBoolean state = new AtomicBoolean(false); public void lock() { while (true) { while (state.get()) {} ; if (! state.getAndSet(true)) { return ; } } } public void unlock() { state.set(false); } } </代码结束> <原文开始>Now consider the behavior of the TTASLock algorithm while the lock is held by a thread A. The first time thread B reads the lock it takes a cache miss, forcing B to block while the value is loaded into B's cache. As long as A holds the lock, B repeatly reread the value, but hits in the cache every time. </原文结束> TTASLock 比 TASLock 优良在于更多地读取本地缓存，而不是每次都锁总线。 <原文开始>This notion of local spinning, where threads repeatly reread cached values instead of repeatedly using the bus, is an important principle critical to the design of efficeient spin locks.</原文结束> <原文开始>Here is a key observation: if some other thread aquires the lock between hte first and second step, then, most likely, there is high conteniton for that lock.</原文结束> <原文开始>the larger the number of unsuccessful tries, the higher the likely contention, and the longer the thread should back off .</原文结束> <原文开始>backing off is common to several locking algorithms.</原文结束> <原文开始>the BackoffLock is easy to implement, and typically performs significantly better than TASLock on many architectures. </原文结束> 如果 double check 第一步失败了，那么它很有可能是在高的 contention，那么在这里退避是一条简单易行的优化路线；contention 越高，退避的时间可以越长。