JVM の仮想スレッド メカニズムでのピン留めの調査

Java's virtual threads offer a lightweight alternative to traditional OS threads, enabling efficient concurrency management. But understanding their behavior is crucial for optimal performance. This blog post dives into pinning, a scenario that can impact virtual thread execution, and explores techniques to monitor and address it.

Virtual Threads: A Lightweight Concurrency Approach

Java's virtual threads are managed entities that run on top of the underlying operating system threads (carrier threads). They provide a more efficient way to handle concurrency compared to creating numerous OS threads, as they incur lower overhead. The JVM maps virtual threads to carrier threads dynamically, allowing for better resource utilization.

• Managed by the JVM: Unlike OS threads that are directly managed by the operating system, virtual threads are created and scheduled by the Java Virtual Machine (JVM). This allows for finer-grained control and optimization within the JVM environment.

• Reduced Overhead: Creating and managing virtual threads incurs significantly lower overhead compared to OS threads. This is because the JVM can manage a larger pool of virtual threads efficiently, utilizing a smaller number of underlying OS threads.

• Compatibility with Existing Code: Virtual threads are designed to be seamlessly integrated with existing Java code. They can be used alongside traditional OS threads and work within the familiar constructs like Executor and ExecutorService for managing concurrent.

The figure below shows the relationship between virtual threads and platform threads:

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Pinning: When a Virtual Thread Gets Stuck

Pinning occurs when a virtual thread becomes tied to its carrier thread. This essentially means the virtual thread cannot be preempted (switched to another carrier thread) while it's in a pinned state. Here are common scenarios that trigger pinning:

• Synchronized Blocks and Methods: Executing code within a synchronized block or method leads to pinning. This ensures exclusive access to shared resources, preventing data corruption issues.

Code Example:

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {

  public static void main(String[] args) throws InterruptedException {

    final Counter counter = new Counter();

    Runnable task = () -> {
      for (int i = 0; i be3c6ba0a699aca98d67df8bee3b84fd {
      for (int i = 0; i 4596771306681ecdae85c561b8883cd6 {
      for (int i = 0; i < 100; i++) {
        counter.increment();
      }
    };

    Thread thread1 = Thread.ofVirtual().start(task);
    Thread thread2 = Thread.ofVirtual().start(task);

    try {
      thread1.join();
      thread2.join();
    } catch (InterruptedException e) {
      throw new RuntimeException(e);
    }

    System.out.println("Final counter value: " + counter.getCount());
  }
}

class Counter {

  private int count = 0;
  private final ReentrantLock lock = new ReentrantLock();

  public void increment() {
    lock.lock();
    try {
      Thread.sleep(100); // This simulates a blocking call
      count++;
    } catch (InterruptedException e) {
      e.printStackTrace();
    } finally {
      lock.unlock();
    }
  }

  public int getCount() {
    return count;
  }
}

In the updated example, we use a ReentrantLock instead of a synchronized block. The thread can acquire the lock and release it immediately after it completes its operation, potentially reducing the duration of pinning compared to a synchronized block which might hold the lock for a longer period.

結論は

Java の仮想スレッドは、言語の進化と機能の証拠となります。これらは、従来の OS スレッドに代わる新鮮で軽量な代替手段を提供し、効率的な同時実行管理への架け橋となります。時間をかけてスレッドの固定などの重要な概念を深く掘り下げて理解することで、開発者はこれらの軽量スレッドの可能性を最大限に活用するためのノウハウを得ることができます。この知識は、開発者が今後の機能を活用するための準備を整えるだけでなく、現在のプロジェクトで複雑な同時実行制御の問題をより効果的に解決できるようにするためにも役立ちます。

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