test/jdk/java/lang/Thread/virtual/YieldQueuing.java

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/**
 * @test
 * @summary Test Thread.yield submits the virtual thread task to the expected queue
 * @requires vm.continuations
 * @run junit/othervm -Djdk.virtualThreadScheduler.maxPoolSize=1 YieldQueuing
 */

import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.LockSupport;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class YieldQueuing {

    /**
     * Test Thread.yield submits the task for the current virtual thread to a scheduler
     * submission queue when there are no tasks in the local queue.
     */
    @Test
    void testYieldWithEmptyLocalQueue() throws Exception {
        var list = new CopyOnWriteArrayList<String>();

        var threadsStarted = new AtomicBoolean();

        var threadA = Thread.ofVirtual().unstarted(() -> {
            // pin thread until task for B is in submission queue
            while (!threadsStarted.get()) {
                Thread.onSpinWait();
            }

            list.add("A");
            Thread.yield();      // push task for A to submission queue, B should run
            list.add("A");
        });

        var threadB = Thread.ofVirtual().unstarted(() -> {
            list.add("B");
        });

        // push tasks for A and B to submission queue
        threadA.start();
        threadB.start();

        // release A
        threadsStarted.set(true);

        // wait for result
        threadA.join();
        threadB.join();
        assertEquals(list, List.of("A", "B", "A"));
    }

    /**
     * Test Thread.yield submits the task for the current virtual thread to the local
     * queue when there are tasks in the local queue.
     */
    @Test
    void testYieldWithNonEmptyLocalQueue() throws Exception {
        var list = new CopyOnWriteArrayList<String>();

        var threadsStarted = new AtomicBoolean();

        var threadA = Thread.ofVirtual().unstarted(() -> {
            // pin thread until tasks for B and C are in submission queue
            while (!threadsStarted.get()) {
                Thread.onSpinWait();
            }

            list.add("A");
            LockSupport.park();   // B should run
            list.add("A");
        });

        var threadB = Thread.ofVirtual().unstarted(() -> {
            list.add("B");
            LockSupport.unpark(threadA);  // push task for A to local queue
            Thread.yield();               // push task for B to local queue, A should run
            list.add("B");
        });

        var threadC = Thread.ofVirtual().unstarted(() -> {
            list.add("C");
        });

        // push tasks for A, B and C to submission queue
        threadA.start();
        threadB.start();
        threadC.start();

        // release A
        threadsStarted.set(true);

        // wait for result
        threadA.join();
        threadB.join();
        threadC.join();
        assertEquals(list, List.of("A", "B", "A", "B", "C"));
    }
}