我试图证明当有很多读者而只有一些作者时，synchronized 会更慢。不知何故，我证明了相反的情况。
RW 示例，执行时间为 313 ms:
package zad3readWriteLockPerformance;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class Main {
public static long start, end;
public static void main(String[] args) {
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
end = System.currentTimeMillis();
System.out.println("Time of execution " + (end - start) + " ms");
}));
start = System.currentTimeMillis();
final int NUMBER_OF_THREADS = 1000;
ThreadSafeArrayList<Integer> threadSafeArrayList = new ThreadSafeArrayList<>();
ArrayList<Thread> consumerThreadList = new ArrayList<Thread>();
for (int i = 0; i < NUMBER_OF_THREADS; i++) {
Thread t = new Thread(new Consumer(threadSafeArrayList));
consumerThreadList.add(t);
t.start();
}
ArrayList<Thread> producerThreadList = new ArrayList<Thread>();
for (int i = 0; i < NUMBER_OF_THREADS/10; i++) {
Thread t = new Thread(new Producer(threadSafeArrayList));
producerThreadList.add(t);
t.start();
}
// System.out.println("Printing the First Element : " + threadSafeArrayList.get(1));
}
}
class Consumer implements Runnable {
public final static int NUMBER_OF_OPERATIONS = 100;
ThreadSafeArrayList<Integer> threadSafeArrayList;
public Consumer(ThreadSafeArrayList<Integer> threadSafeArrayList) {
this.threadSafeArrayList = threadSafeArrayList;
}
@Override
public void run() {
for (int j = 0; j < NUMBER_OF_OPERATIONS; j++) {
Integer obtainedElement = threadSafeArrayList.getRandomElement();
}
}
}
class Producer implements Runnable {
public final static int NUMBER_OF_OPERATIONS = 100;
ThreadSafeArrayList<Integer> threadSafeArrayList;
public Producer(ThreadSafeArrayList<Integer> threadSafeArrayList) {
this.threadSafeArrayList = threadSafeArrayList;
}
@Override
public void run() {
for (int j = 0; j < NUMBER_OF_OPERATIONS; j++) {
threadSafeArrayList.add((int) (Math.random() * 1000));
}
}
}
class ThreadSafeArrayList<E> {
private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
private final Lock readLock = readWriteLock.readLock();
private final Lock writeLock = readWriteLock.writeLock();
private final List<E> list = new ArrayList<>();
public void add(E o) {
writeLock.lock();
try {
list.add(o);
//System.out.println("Adding element by thread" + Thread.currentThread().getName());
} finally {
writeLock.unlock();
}
}
public E getRandomElement() {
readLock.lock();
try {
//System.out.println("Printing elements by thread" + Thread.currentThread().getName());
if (size() == 0) {
return null;
}
return list.get((int) (Math.random() * size()));
} finally {
readLock.unlock();
}
}
public int size() {
return list.size();
}
}
同步示例，执行时间仅为241ms:
package zad3readWriteLockPerformanceZMIENONENENASYNCHRO;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class Main {
public static long start, end;
public static void main(String[] args) {
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
end = System.currentTimeMillis();
System.out.println("Time of execution " + (end - start) + " ms");
}));
start = System.currentTimeMillis();
final int NUMBER_OF_THREADS = 1000;
List<Integer> list = Collections.synchronizedList(new ArrayList<Integer>());
ArrayList<Thread> consumerThreadList = new ArrayList<Thread>();
for (int i = 0; i < NUMBER_OF_THREADS; i++) {
Thread t = new Thread(new Consumer(list));
consumerThreadList.add(t);
t.start();
}
ArrayList<Thread> producerThreadList = new ArrayList<Thread>();
for (int i = 0; i < NUMBER_OF_THREADS / 10; i++) {
Thread t = new Thread(new Producer(list));
producerThreadList.add(t);
t.start();
}
// System.out.println("Printing the First Element : " + threadSafeArrayList.get(1));
}
}
class Consumer implements Runnable {
public final static int NUMBER_OF_OPERATIONS = 100;
List<Integer> list;
public Consumer(List<Integer> list) {
this.list = list;
}
@Override
public void run() {
for (int j = 0; j < NUMBER_OF_OPERATIONS; j++) {
if (list.size() > 0)
list.get((int) (Math.random() * list.size()));
}
}
}
class Producer implements Runnable {
public final static int NUMBER_OF_OPERATIONS = 100;
List<Integer> threadSafeArrayList;
public Producer(List<Integer> threadSafeArrayList) {
this.threadSafeArrayList = threadSafeArrayList;
}
@Override
public void run() {
for (int j = 0; j < NUMBER_OF_OPERATIONS; j++) {
threadSafeArrayList.add((int) (Math.random() * 1000));
}
}
}
当我的读者比作者多十倍时，为什么同步收集更快。如何显示我在许多文章中读到的 RW 锁的进展？
最佳答案
获取 ReadWriteLock 的实际成本通常比获取简单互斥锁的成本要慢得多。 ReadWriteLock 的 javadoc 是这样的:
因此，您的线程正在执行非常简单的操作这一事实可能意味着性能取决于实际获取锁所花费的时间。
您的基准测试还有另一个问题，那就是 Math.random 是同步的。从它的 javadoc :
因此，即使您的并发读取器在获取 ReadWriteLock 时不会相互阻塞，但它们可能仍在争夺 Math.random 中获取的锁，从而破坏了使用 ReadWriteLock 的一些好处。您可以改用 ThreadLocalRandom 来改进这一点。
此外，正如 assylias 指出的那样，不考虑 JIT 编译和其他运行时怪癖的朴素 Java 基准测试是不可靠的。您应该将 Java Microbenchmarking Harness (JMH) 用于此类基准测试。