为何JDK21中获取已锁定ReentrantLock的性能低于JDK11？

测试背景与发现 #

在优化锁相关代码时，通过JMH基准测试对比ReentrantLock单次锁定与重入锁定的性能开销，意外发现JDK11在重入场景下的表现优于JDK21。同时补充的synchronized块及无锁场景测试结果符合预期：synchronized经JVM优化后性能几乎与无锁场景持平，且不同JDK版本间无性能衰减。

基准测试代码 #

@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 10, time = 1, timeUnit = TimeUnit.SECONDS)
@Fork(1)
@State(Scope.Benchmark)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class LockNoLockBenchmark {
  int counter;

  ReentrantLock lock = new ReentrantLock();


  @Benchmark
  public void noLock() {
    ++counter;
  }

  @Benchmark
  public void syncLock() {
    synchronized (new Object()) {
      ++counter;
    }
  }

  @Benchmark
  public void lockUnlock() {
    lock.lock();
    try {
      ++counter;
    } finally {
      lock.unlock();
    }
  }

  @Benchmark
  public void lockLockUnlockUnlock() {
    lock.lock();
    try {
      lock.lock();
      try {
        ++counter;
      } finally {
        lock.unlock();
      }
    } finally {
      lock.unlock();
    }
  }
}

测试环境 #

• 处理器：Intel Rocket Lake 12代Intel(R) Core(TM) i9-12950HX（12核）
• 内存：64GB

测试结果 #

JDK 21测试结果 #

openjdk 21.0.2 2024-01-16
OpenJDK Runtime Environment (build 21.0.2+13-58)
OpenJDK 64-Bit Server VM (build 21.0.2+13-58, mixed mode, sharing)

Benchmark                                 Mode  Cnt   Score   Error  Units
LockNoLockBenchmark.lockLockUnlockUnlock  avgt   10  27.457 ± 0.876  ns/op
LockNoLockBenchmark.lockUnlock            avgt   10  11.409 ± 0.256  ns/op
LockNoLockBenchmark.noLock                avgt   10   0.280 ± 0.010  ns/op
LockNoLockBenchmark.syncLock              avgt   10   0.280 ± 0.008  ns/op

JDK 11测试结果 #

openjdk 11.0.21 2023-10-17
OpenJDK Runtime Environment (build 11.0.21+9-post-Ubuntu-0ubuntu122.04)
OpenJDK 64-Bit Server VM (build 11.0.21+9-post-Ubuntu-0ubuntu122.04, mixed mode, sharing)

Benchmark                                 Mode  Cnt   Score   Error  Units
LockNoLockBenchmark.lockLockUnlockUnlock  avgt   10  22.414 ± 1.366  ns/op
LockNoLockBenchmark.lockUnlock            avgt   10  11.690 ± 0.407  ns/op
LockNoLockBenchmark.noLock                avgt   10   0.283 ± 0.021  ns/op
LockNoLockBenchmark.syncLock              avgt   10   0.289 ± 0.012  ns/op

核心疑问与优化需求 #

1. 原本预期JDK21在该场景下不会出现性能衰减，想了解ReentrantLock重入场景JDK21性能低于JDK11的原因，同时确认本次基准测试是否准确。
2. 希望获取针对**需要获取已锁定锁（重入）**场景的代码优化方法。

内容的提问来源于stack exchange，提问作者alexid