java-topology/test/jdk/java/lang/Thread/virtual/stress/LotsOfContendedMonitorEnter.java

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/*
 * @test id=default
 * @summary Test virtual threads entering a lot of monitors with contention
 * @library /test/lib
 * @run main/timeout=480 LotsOfContendedMonitorEnter
 */

import java.time.Instant;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.CountDownLatch;
import jdk.test.lib.thread.VThreadRunner;

public class LotsOfContendedMonitorEnter {
    static int depth;

    public static void main(String[] args) throws Exception {
        if (args.length > 0) {
            depth = Integer.parseInt(args[0]);
        } else {
            depth = 1024;
        }

        var exRef = new AtomicReference<Throwable>();
        var thread = Thread.ofVirtual().start(() -> {
            try {
                testContendedEnter(depth);
            } catch (Exception e) {
                exRef.set(e);
            }
        });
        thread.join();
        assert exRef.get() == null;
    }

    /**
     * Enter the monitor for a new object, racing with another virtual thread that
     * attempts to enter around the same time, then repeat to the given depth.
     */
    private static void testContendedEnter(int depthRemaining) throws Exception {
        boolean doLog = depthRemaining % 10 == 0;
        if (depthRemaining > 0) {
            var lock = new Object();

            // start thread to enter monitor for brief period, then enters again when signalled
            var started = new CountDownLatch(1);
            var signal = new CountDownLatch(1);
            var thread = Thread.ofVirtual().start(() -> {
                started.countDown();

                // enter, may be contended
                synchronized (lock) {
                    Thread.onSpinWait();
                }

                // wait to be signalled
                try {
                    signal.await();
                } catch (InterruptedException e) { }

                // enter again, this will block until the main thread releases
                synchronized (lock) {
                    // do nothing
                }
            });
            try {
                // wait for thread to start
                started.await();

                // enter, may be contended
                synchronized (lock) {
                    // signal thread to enter monitor again, it should block
                    signal.countDown();
                    await(thread, Thread.State.BLOCKED);
                    if (doLog) {
                        System.out.println(Instant.now() + " => at depth: " + (depth - depthRemaining));
                    }
                    testContendedEnter(depthRemaining - 1);
                }
            } finally {
                thread.join();
            }
        }
        if (doLog) {
            System.out.println(Instant.now() + " => returning from depth: " + (depth - depthRemaining));
        }
    }

    /**
     * Waits for the given thread to reach a given state.
     */
    private static void await(Thread thread, Thread.State expectedState) {
        Thread.State state = thread.getState();
        while (state != expectedState) {
            assert state != Thread.State.TERMINATED : "Thread has terminated";
            Thread.yield();
            state = thread.getState();
        }
    }
}