理解与使用Treiber Stack
背景
最近在很多JDK源码中都看到了Treiber stack这个单词。
- 比如CompletableFuture中的：
volatile Completion stack; // Top of Treiber stack of dependent actions
- 比如FutureTask中的：
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
- 比如Phaser中的：
/**
* Wait nodes for Treiber stack representing wait queue
*/
static final class QNode implements ForkJoinPool.ManagedBlocker {
final Phaser phaser;
final int phase;
final boolean interruptible;
final boolean timed;
boolean wasInterrupted;
long nanos;
final long deadline;
volatile Thread thread; // nulled to cancel wait
QNode next;
- 还比如ForkJoinPool中的描述：
* Bits and masks for field ctl, packed with 4 16 bit subfields:
* AC: Number of active running workers minus target parallelism
* TC: Number of total workers minus target parallelism
* SS: version count and status of top waiting thread
* ID: poolIndex of top of Treiber stack of waiters
感觉这种名词出现的频率有点高，需要深入了解一下。
名称由来
Treiber Stack在 R. Kent Treiber在1986年的论文Systems Programming: Coping with Parallelism中首次出现。它是一种无锁并发栈，其无锁的特性是基于CAS原子操作实现的。
CompletableFuture源码实现
CompletableFuture的Treiber stack实现感觉有点复杂，因为有其他逻辑掺杂，代码不容易阅读，其实抽象来看，Treiber stack首先是个单向链表，链表头部即栈顶元素，在入栈和出现过程中，需要对栈顶元素进行CAS控制，防止多线程情况下数据错乱。
// Either the result or boxed AltResult
volatile Object result;
// Top of Treiber stack of dependent actions（Treiber stack栈顶元素）
volatile Completion stack;
/** Returns true if successfully pushed c onto stack. */
final boolean tryPushStack(Completion c) {
Completion h = stack;
lazySetNext(c, h);
return UNSAFE.compareAndSwapObject(this, STACK, h, c);
}
/** Unconditionally pushes c onto stack, retrying if necessary. */
final void pushStack(Completion c) {
do {} while (!tryPushStack(c));
}
简单来看，入栈的步骤如下：
- 尝试入栈，利用CAS将新的节点作为栈顶元素，新节点next赋值为旧栈顶元素
- 尝试入栈成功，即结束；入栈失败，继续重试上面的操作
FutureTask实现
FutureTask用了Treiber Stack来维护等待任务完成的线程，在FutureTask的任务完成/取消/异常后在finishCompletion钩子方法中会唤醒栈中等待的线程。
Treiber Stack抽象实现
入栈
void push(Node new) {
do {
} while(!tryPush(new)) // 尝试入栈
}
boolean tryPush(node) {
Node oldHead = head;
node.next = oldHead; // 新节点next赋值为旧栈顶元素
return CAS(oldHead, node); // 利用CAS将新的节点作为栈顶元素
}
出栈
对于出栈，要做的工作就是将原来的栈顶节点移除，等待垃圾回收；新栈顶元素CAS为第一个子元素。伪代码：
E pop() {
Node<E> oldHead;
Node<E> newHead;
do {
oldHead = top.get();
// 判断栈是否为空，为空直接返回
if (oldHead == null)
return null;
newHead = oldHead.next;
} while (!CAS(oldHead, newHead));
// 旧的节点删掉next引用，等待gc
oldHead.item = null;
return oldHead.item;
}
示例
import sun.misc.Unsafe;
import java.lang.reflect.Field;
import java.util.Objects;
import java.util.Optional;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* 基于Unsafe实现TreiberStack
* @author Charles
*/
public class TreiberStack<E> {
private volatile Node<E> head;
public void push(E item) {
Objects.requireNonNull(item);
Node<E> newHead = new Node<>(item);
Node<E> oldHead;
int count = 0;
do {
oldHead = head;
count++;
} while (!tryPush(oldHead, newHead, count));
newHead.next = oldHead;
}
private boolean tryPush(Node<E> oldHead, Node<E> newHead, int count) {
boolean isSuccess = UNSAFE.compareAndSwapObject(this, HEAD, oldHead, newHead);
System.out.println(currentThreadName() + " try push [" + count + "]," +
" oldHead = " + getValue(oldHead) +
" newHead = " + getValue(newHead) +
" isSuccess = " + isSuccess);
return isSuccess;
}
public E pop() {
Node<E> oldHead;
Node<E> newHead;
do {
oldHead = head;
System.out.println(currentThreadName() + " do pop:" +
" oldHead = " + getValue(oldHead) +
" newHead = " + Optional.ofNullable(head).map(s -> s.next.item).orElse(null));
if (oldHead == null) {
return null;
}
newHead = oldHead.next;
} while (!tryPop(oldHead, newHead));
oldHead.next = null;
return oldHead.item;
}
private boolean tryPop(Node<E> oldHead, Node<E> newHead) {
boolean isSuccess = UNSAFE.compareAndSwapObject(this, HEAD, oldHead, newHead);
System.out.println(currentThreadName() + " try pop:" +
" oldHead = " + getValue(oldHead) +
" currentHead = " + getValue(head) +
" newHead = " + getValue(newHead) +
" isSuccess: " + isSuccess);
return isSuccess;
}
private E getValue(Node<E> n) {
return Optional.ofNullable(n).map(t -> t.item).orElse(null);
}
private static class Node<E> {
E item;
Node<E> next;
Node(E item) {
this.item = item;
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long HEAD;
private static final long NEXT;
static {
try {
Field getUnsafe = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
getUnsafe.setAccessible(true);
UNSAFE = (Unsafe) getUnsafe.get(null);
Class<?> k = TreiberStack.class;
HEAD = UNSAFE.objectFieldOffset(k.getDeclaredField("head"));
NEXT = UNSAFE.objectFieldOffset(TreiberStack.Node.class.getDeclaredField("next"));
} catch (Exception x) {
throw new Error(x);
}
}
private static class RandomValue {
private final Integer value;
public RandomValue() {
this.value = new Random().nextInt(Integer.MAX_VALUE);
}
public Integer getValue() {
return value;
}
@Override
public String toString() {
return value.toString();
}
}
private static String currentThreadName() {
return System.nanoTime() + " / " + Thread.currentThread().getName();
}
public static void main(String[] args) throws InterruptedException {
TreiberStack<RandomValue> ts = new TreiberStack<>();
ExecutorService es = Executors.newFixedThreadPool(10);
Thread.sleep(2000);
for (int i = 0; i < 5; i++) {
es.submit(() -> ts.push(new RandomValue()));
}
for (int i = 0; i < 50; i++) {
es.submit((Runnable) ts::pop);
}
}
}
