我已经有这个问题很长一段时间了，试图阅读大量资源并了解正在发生的事情 - 但我仍然无法很好地理解为什么事情会这样。
简而言之，我正在尝试测试 a在竞争环境中的CAS表现与非环境中的表现。synchronized我已经提出了这个JMH测试：
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Warmup(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
@State(Scope.Benchmark)
public class SandBox {
Object lock = new Object();
public static void main(String[] args) throws RunnerException {
Options opt = new OptionsBuilder().include(SandBox.class.getSimpleName())
.jvmArgs("-ea", "-Xms10g", "-Xmx10g")
.shouldFailOnError(true)
.build();
new Runner(opt).run();
}
@State(Scope.Thread)
public static class Holder {
private long number;
private AtomicLong atomicLong;
@Setup
public void setUp() {
number = ThreadLocalRandom.current().nextLong();
atomicLong = new AtomicLong(number);
}
}
@Fork(1)
@Benchmark
public long sync(Holder holder) {
long n = holder.number;
synchronized (lock) {
n = n * 123;
}
return n;
}
@Fork(1)
@Benchmark
public AtomicLong cas(Holder holder) {
AtomicLong al = holder.atomicLong;
al.updateAndGet(x -> x * 123);
return al;
}
private Object anotherLock = new Object();
private long anotherNumber = ThreadLocalRandom.current().nextLong();
private AtomicLong anotherAl = new AtomicLong(anotherNumber);
@Fork(1)
@Benchmark
public long syncShared() {
synchronized (anotherLock) {
anotherNumber = anotherNumber * 123;
}
return anotherNumber;
}
@Fork(1)
@Benchmark
public AtomicLong casShared() {
anotherAl.updateAndGet(x -> x * 123);
return anotherAl;
}
@Fork(value = 1, jvmArgsAppend = "-XX:-UseBiasedLocking")
@Benchmark
public long syncSharedNonBiased() {
synchronized (anotherLock) {
anotherNumber = anotherNumber * 123;
}
return anotherNumber;
}
}
结果：
Benchmark Mode Cnt Score Error Units
spinLockVsSynchronized.SandBox.cas avgt 5 212.922 ± 18.011 ns/op
spinLockVsSynchronized.SandBox.casShared avgt 5 4106.764 ± 1233.108 ns/op
spinLockVsSynchronized.SandBox.sync avgt 5 2869.664 ± 231.482 ns/op
spinLockVsSynchronized.SandBox.syncShared avgt 5 2414.177 ± 85.022 ns/op
spinLockVsSynchronized.SandBox.syncSharedNonBiased avgt 5 2696.102 ± 279.734 ns/op
在非共享情况下CAS，速度要快得多，这是我所期望的。但在共同的情况下，事情是相反的——这是我无法理解的。我不认为这与偏向锁定有关，因为这会在线程持有锁定 5 秒（AFAIK）之后发生，而这不会发生，测试就是证明。
老实说，我希望只是我的测试有问题，并且有jmh专业知识的人会出现并指出我这里的错误设置。