gt:         for (Thread t : traces.keySet()) {
context: /*
   * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  import java.io.PrintStream;
  import java.util.ArrayList;
  import java.util.List;
  import java.util.Map;
  import java.util.Random;
  import sun.hotspot.WhiteBox;
  
  /*
   * @test TestMultiThreadStressRSet.java
   * @key stress
   * @requires vm.gc.G1
   * @requires os.maxMemory > 2G
   * @requires vm.opt.MaxGCPauseMillis == "null"
   *
   * @summary Stress G1 Remembered Set using multiple threads
   * @modules java.base/jdk.internal.misc
   * @library /test/lib
   * @build sun.hotspot.WhiteBox
   * @run main ClassFileInstaller sun.hotspot.WhiteBox
   *                              sun.hotspot.WhiteBox$WhiteBoxPermission
   * @run main/othervm -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
   *   -XX:+UseG1GC -XX:G1SummarizeRSetStatsPeriod=1 -Xlog:gc
   *   -Xmx500m -XX:G1HeapRegionSize=1m -XX:MaxGCPauseMillis=1000 TestMultiThreadStressRSet 10 4
   *
   * @run main/othervm -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
   *   -XX:+UseG1GC -XX:G1SummarizeRSetStatsPeriod=100 -Xlog:gc
   *   -Xmx1G -XX:G1HeapRegionSize=8m -XX:MaxGCPauseMillis=1000 TestMultiThreadStressRSet 60 16
   *
   * @run main/othervm/timeout=700 -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI
   *   -XX:+UseG1GC -XX:G1SummarizeRSetStatsPeriod=100 -Xlog:gc
   *   -Xmx500m -XX:G1HeapRegionSize=1m -XX:MaxGCPauseMillis=1000 TestMultiThreadStressRSet 600 32
   */
  public class TestMultiThreadStressRSet {
  
      private static final Random RND = new Random(2015 * 2016);
      private static final WhiteBox WB = WhiteBox.getWhiteBox();
      private static final int REF_SIZE = WB.getHeapOopSize();
      private static final int REGION_SIZE = WB.g1RegionSize();
  
      // How many regions to use for the storage
      private static final int STORAGE_REGIONS = 20;
  
      // Size a single obj in the storage
      private static final int OBJ_SIZE = 1024;
  
      // How many regions of young/old gen to use in the BUFFER
      private static final int BUFFER_YOUNG_REGIONS = 60;
      private static final int BUFFER_OLD_REGIONS = 40;
  
      // Total number of objects in the storage.
      private final int N;
  
      // The storage of byte[]
      private final List<Object> STORAGE;
  
      // Where references to the Storage will be stored
      private final List<Object[]> BUFFER;
  
      // The length of a buffer element.
      // RSet deals with "cards" (areas of 512 bytes), not with single refs
      // So, to affect the RSet the BUFFER refs should be allocated in different
      // memory cards.
      private final int BUF_ARR_LEN = 100 * (512 / REF_SIZE);
  
      // Total number of objects in the young/old buffers
      private final int YOUNG;
      private final int OLD;
  
      // To cause Remembered Sets change their coarse level the test uses a window
      // within STORAGE. All the BUFFER elements refer to only STORAGE objects
      // from the current window. The window is defined by a range.
      // The first element has got the index: 'windowStart',
      // the last one: 'windowStart + windowSize - 1'
      // The window is shifting periodically.
      private int windowStart;
      private final int windowSize;
  
      // Counter of created worker threads
      private int counter = 0;
  
      private volatile String errorMessage = null;
      private volatile boolean isEnough = false;
  
      public static void main(String args[]) {
          if (args.length != 2) {
              throw new IllegalArgumentException("TEST BUG: wrong arg count " + args.length);
          }
          long time = Long.parseLong(args[0]);
          int threads = Integer.parseInt(args[1]);
          new TestMultiThreadStressRSet().test(time * 1000, threads);
      }
  
      /**
       * Initiates test parameters, fills out the STORAGE and BUFFER.
       */
      public TestMultiThreadStressRSet() {
  
          N = (REGION_SIZE - 1) * STORAGE_REGIONS / OBJ_SIZE + 1;
          STORAGE = new ArrayList<>(N);
          int bytes = OBJ_SIZE - 20;
          for (int i = 0; i < N - 1; i++) {
              STORAGE.add(new byte[bytes]);
          }
          STORAGE.add(new byte[REGION_SIZE / 2 + 100]); // humongous
          windowStart = 0;
          windowSize = REGION_SIZE / OBJ_SIZE;
  
          BUFFER = new ArrayList<>();
          int sizeOfBufferObject = 20 + REF_SIZE * BUF_ARR_LEN;
          OLD = REGION_SIZE * BUFFER_OLD_REGIONS / sizeOfBufferObject;
          YOUNG = REGION_SIZE * BUFFER_YOUNG_REGIONS / sizeOfBufferObject;
          for (int i = 0; i < OLD + YOUNG; i++) {
              BUFFER.add(new Object[BUF_ARR_LEN]);
          }
      }
  
      /**
       * Does the testing. Steps:
       * <ul>
       * <li> starts the Shifter thread
       * <li> during the given time starts new Worker threads, keeping the number
       * of live thread under limit.
       * <li> stops the Shifter thread
       * </ul>
       *
       * @param timeInMillis how long to stress
       * @param maxThreads the maximum number of Worker thread working together.
       */
      public void test(long timeInMillis, int maxThreads) {
          if (timeInMillis <= 0 || maxThreads <= 0) {
              throw new IllegalArgumentException("TEST BUG: be positive!");
          }
          System.out.println("%% Time to work: " + timeInMillis / 1000 + "s");
          System.out.println("%% Number of threads: " + maxThreads);
          long finish = System.currentTimeMillis() + timeInMillis;
          Shifter shift = new Shifter(this, 1000, (int) (windowSize * 0.9));
          shift.start();
          for (int i = 0; i < maxThreads; i++) {
              new Worker(this, 100).start();
          }
          try {
              while (System.currentTimeMillis() < finish && errorMessage == null) {
                  Thread.sleep(100);
              }
          } catch (Throwable t) {
              printAllStackTraces(System.err);
              t.printStackTrace(System.err);
              this.errorMessage = t.getMessage();
          } finally {
              isEnough = true;
          }
          System.out.println("%% Total work cycles: " + counter);
          if (errorMessage != null) {
              throw new RuntimeException(errorMessage);
          }
      }
  
      /**
       * Returns an element from from the BUFFER (an object array) to keep
       * references to the storage.
       *
       * @return an Object[] from buffer.
       */
      private Object[] getFromBuffer() {
          int index = counter % (OLD + YOUNG);
          synchronized (BUFFER) {
              if (index < OLD) {
                  if (counter % 100 == (counter / 100) % 100) {
                      // need to generate garbage in the old gen to provoke mixed GC
                      return replaceInBuffer(index);
                  } else {
                      return BUFFER.get(index);
                  }
              } else {
                  return replaceInBuffer(index);
              }
          }
      }
  
      private Object[] replaceInBuffer(int index) {
          Object[] objs = new Object[BUF_ARR_LEN];
          BUFFER.set(index, objs);
          return objs;
      }
  
      /**
       * Returns a random object from the current window within the storage.
       * A storage element with index from windowStart to windowStart+windowSize.
       *
       * @return a random element from the current window within the storage.
       */
      private Object getRandomObject() {
          int index = (windowStart + RND.nextInt(windowSize)) % N;
          return STORAGE.get(index);
      }
  
      private static void printAllStackTraces(PrintStream ps) {
          Map<Thread, StackTraceElement[]> traces = Thread.getAllStackTraces();
  