JAVA 并发：CLH 锁 与 AbstractQueuedSynchronizer
首先向Doug Lea致敬。
CLH
以下是CLH锁的一个简单实现：
class SimpleCLHLock {
/**
* initialized with a dummy node
*/
private Node dummy = new Node();
private AtomicReference<Node> tail = new AtomicReference<Node>(dummy);
/**
* implicit single linked list node
*/
private static class Node {
public volatile boolean locked = false;
}
private ThreadLocal<Node> threadLockNode = new ThreadLocal<Node>();
public void lock() {
Node last_tail;
Node new_tail = new Node();
new_tail.locked = true;
while (true) {
last_tail = tail.get();
if (tail.compareAndSet(last_tail, new_tail)) {
break;
}
}
// we just keep previous tail node as current node's direct predecessor
// and waiting it jump into unlocked state, so we form a single linked list implicitly
while (last_tail.locked) {
Thread.yield();
}
// we need the node reference when we want to unlock
// some threads may waiting on its state to be unlocked in above while-loop statement.
threadLockNode.set(new_tail);
}
public void unlock() {
// retrieve the lock state held by current thread
Node node = threadLockNode.get();
// if null then the current thread has not own the lock, also can't perform unlock operation
if (node == null) {
throw new IllegalMonitorStateException();
}
node.locked = false;
threadLockNode.set(null);
}
}
测试了不同的同步方式的速度
package lock;
import java.util.List;
import java.util.ArrayList;
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.ReentrantLock;
/**
* Created by hegaofeng on 6/30/15.
*/
public class CLH {
private static volatile long count;
private static AtomicLong atomicLong = new AtomicLong();
private static ExecutorService pool = Executors.newCachedThreadPool();
public static void main(String[] args) {
final int deltaPerThreads = 10000;
final int numsOfThreads = 10;
Callable<Boolean> raw_counter = new Callable<Boolean>() {
@Override
public Boolean call() {
for (int i=0; i<deltaPerThreads; i++) count++;
return true;
}
};
Callable<Boolean> syn_counter = new Callable<Boolean>() {
@Override
public Boolean call() throws Exception {
for (int i=0; i<deltaPerThreads; i++) {
synchronized (CLH.class) {
count++;
}
}
return true;
}
};
Callable<Boolean> clh_counter = new Callable<Boolean>() {
private SimpleCLHLock lock = new SimpleCLHLock();
@Override
public Boolean call() throws Exception {
for (int i=0; i<deltaPerThreads; i++) {
lock.lock();
count++;
lock.unlock();
}
return true;
}
};
final Callable<Boolean> lck_counter = new Callable<Boolean>() {
private ReentrantLock lock = new ReentrantLock();
@Override
public Boolean call() throws Exception {
for (int i=0; i<deltaPerThreads; i++) {
lock.lock();
count++;
lock.unlock();
}
return true;
}
};
final Callable<Boolean> lck_fair_counter = new Callable<Boolean>() {
private ReentrantLock lock = new ReentrantLock(true);
@Override
public Boolean call() throws Exception {
for (int i=0; i<deltaPerThreads; i++) {
lock.lock();
count++;
lock.unlock();
}
return true;
}
};
Callable<Boolean> sem_counter = new Callable<Boolean>() {
private Semaphore sem = new Semaphore(1);
@Override
public Boolean call() throws Exception {
for(int i=0; i<deltaPerThreads; i++) {
sem.acquire();
count++;
sem.release();
}
return true;
}
};
Callable<Boolean> sem_fair_counter = new Callable<Boolean>() {
private Semaphore sem = new Semaphore(1, true);
@Override
public Boolean call() throws Exception {
for(int i=0; i<deltaPerThreads; i++) {
sem.acquire();
count++;
sem.release();
}
return true;
}
};
Callable<Boolean> cas_counter = new Callable<Boolean>() {
private AtomicBoolean locked = new AtomicBoolean();
@Override
public Boolean call() throws Exception {
for (int i=0; i<deltaPerThreads; i++) {
while (true) {
if (locked.compareAndSet(false, true)) {
count++;
locked.getAndSet(false);
break;
}
}
}
return true;
}
};
Callable<Boolean> pure_cas = new Callable<Boolean>() {
@Override
public Boolean call() throws Exception {
for (int i=0; i<deltaPerThreads; i++) {
atomicLong.incrementAndGet();
}
return true;
}
};
test("raw", raw_counter, deltaPerThreads, numsOfThreads);
test("syn", syn_counter, deltaPerThreads, numsOfThreads);
test("CLH", clh_counter, deltaPerThreads, numsOfThreads);
test("lock", lck_counter, deltaPerThreads, numsOfThreads);
test("lock_fair", lck_fair_counter, deltaPerThreads, numsOfThreads);
test("sem", sem_counter, deltaPerThreads, numsOfThreads);
test("sem_fair", sem_fair_counter, deltaPerThreads, numsOfThreads);
test("CAS", cas_counter, deltaPerThreads, numsOfThreads);
test("pure_CAS", pure_cas, deltaPerThreads, numsOfThreads);
}
public static void test(String name, Callable<Boolean> callable, final int deltaPerThreads, final int numsOfThreads) {
count = 0;
atomicLong.set(0);
pool = Executors.newCachedThreadPool();
List<Callable<Boolean>> calls = new ArrayList<Callable<Boolean>>();
for (int i=0; i<numsOfThreads; i++) {
calls.add(callable);
}
long start = System.nanoTime();
try {
pool.invokeAll(calls);
} catch (InterruptedException e) {
e.printStackTrace();
}
pool.shutdown();
long end = System.nanoTime();
System.out.println("case : " + name);
System.out.println("counted: " + (name.equals("pure_CAS") ? atomicLong.get() : count));
System.out.println("target : " + numsOfThreads * deltaPerThreads);
System.out.println("time : " + (end - start) / 1e6 + "ms");
System.out.println();
}
}
结果：
case : raw
counted: 79270
target : 100000
time : 11.278ms
case : syn
counted: 100000
target : 100000
time : 13.553ms
case : CLH
counted: 100000
target : 100000
time : 1037.665ms
case : lock
counted: 100000
target : 100000
time : 29.134ms
case : lock_fair
counted: 100000
target : 100000
time : 638.944ms
case : sem
counted: 100000
target : 100000
time : 45.154ms
case : sem_fair
counted: 100000
target : 100000
time : 552.855ms
case : CAS
counted: 100000
target : 100000
time : 158.422ms
case : pure_CAS
counted: 100000
target : 100000
time : 31.964ms
- raw 表示不进行任何同步，现在没有保护的情况下计数累加产生了错误
- syn 表示使用内置锁同步方式即使用
synchronized块，在几种方式里表现最好 - CLH 表示使用自己实现的一个CLH锁（具有排队功能的自旋锁），这里的CLH的实现方式决定了它是一个公平锁
- lock 表示用ReentrantLock进行数据保护，速度仅次于内置锁，lock_fair是它的公平版本，不过速度上有大幅下降，变慢了将近20倍
- sem 表示使用Semaphore也就是信号量进行数据保护，速度也不错，sem_fair是它的公平版本，和ReentrantLock上出现的情况一样，公平版本比非公平版本出现了大幅的速度下降，慢了10倍
- CAS 用CAS操作实现简单的自旋锁，不具有排队功能
- pure_CAS 表示直接用使用AtomicLong类型的count变量进行计数，就不需要锁保护了，速度也是非常快得
调整测试的参数将线程数改为2，每线程增量改为50000即总增量不变的情况下得到的结果如下：
case : raw
counted: 91460
target : 100000
time : 4.767ms
case : syn
counted: 100000
target : 100000
time : 12.385ms
case : CLH
counted: 100000
target : 100000
time : 86.12ms
case : lock
counted: 100000
target : 100000
time : 33.91ms
case : lock_fair
counted: 100000
target : 100000
time : 108.119ms
case : sem
counted: 100000
target : 100000
time : 62.503ms
case : sem_fair
counted: 100000
target : 100000
time : 83.035ms
case : CAS
counted: 100000
target : 100000
time : 13.528ms
case : pure_CAS
counted: 100000
target : 100000
time : 11.377ms
基本锁
ReentrantLock的非公平版本在两次测试中并没有很大的变化，比较稳定。
公平锁
在不必要的情况下不去使用锁或者信号量的公平版本，它们相比非公平版本要慢很多，尤其当竞争非常激烈时。从第二组数据来看公平锁与非公平锁的差距缩小了很多，因为在竞争程度比较低（线程数少）的时候花在维护队列上的时间将大大减少。
CAS
正如Java并发编程中提到的CAS在高竞争的环境下性能不如直接使用锁，因为当竞争非常激烈时CAS花在重试上的时间将会大量增加（因为竞争激烈变量变化越快，CAS操作失败可能性越大）。而在竞争不是非常激烈的情况下CAS的开销更小，比如在第二次此时中的CAS自旋锁时间就比直接用ReentrantLock来得少。
内置锁
虽然说在Java并发编程一书中提到JDK1.6的Lock实现比synchronized要略快，这在JDK1.7中恐怕已经不成立了。
AbstractQueuedSynchronizer
http://javarticles.com/2012/10/abstractqueuedsynchronizer-aqs.html#prettyPhoto
