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    1   /*
    2    * Copyright (c) 1998, 2008, Oracle and/or its affiliates. All rights reserved.
    3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
    4    *
    5    * This code is free software; you can redistribute it and/or modify it
    6    * under the terms of the GNU General Public License version 2 only, as
    7    * published by the Free Software Foundation.  Oracle designates this
    8    * particular file as subject to the "Classpath" exception as provided
    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
   11    * This code is distributed in the hope that it will be useful, but WITHOUT
   12    * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
   16    *
   17    * You should have received a copy of the GNU General Public License version
   18    * 2 along with this work; if not, write to the Free Software Foundation,
   19    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   20    *
   21    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   22    * or visit www.oracle.com if you need additional information or have any
   23    * questions.
   24    */
   25   
   26   package sun.misc;
   27   
   28   import java.security.AccessController;
   29   import java.security.PrivilegedAction;
   30   import java.util.SortedSet;
   31   import java.util.TreeSet;
   32   
   33   
   34   /**
   35    * Support for garbage-collection latency requests.
   36    *
   37    * @author   Mark Reinhold
   38    * @since    1.2
   39    */
   40   
   41   public class GC {
   42   
   43       private GC() { }            /* To prevent instantiation */
   44   
   45   
   46       /* Latency-target value indicating that there's no active target
   47        */
   48       private static final long NO_TARGET = Long.MAX_VALUE;
   49   
   50       /* The current latency target, or NO_TARGET if there is no target
   51        */
   52       private static long latencyTarget = NO_TARGET;
   53   
   54       /* The daemon thread that implements the latency-target mechanism,
   55        * or null if there is presently no daemon thread
   56        */
   57       private static Thread daemon = null;
   58   
   59       /* The lock object for the latencyTarget and daemon fields.  The daemon
   60        * thread, if it exists, waits on this lock for notification that the
   61        * latency target has changed.
   62        */
   63       private static class LatencyLock extends Object { };
   64       private static Object lock = new LatencyLock();
   65   
   66   
   67       /**
   68        * Returns the maximum <em>object-inspection age</em>, which is the number
   69        * of real-time milliseconds that have elapsed since the
   70        * least-recently-inspected heap object was last inspected by the garbage
   71        * collector.
   72        *
   73        * <p> For simple stop-the-world collectors this value is just the time
   74        * since the most recent collection.  For generational collectors it is the
   75        * time since the oldest generation was most recently collected.  Other
   76        * collectors are free to return a pessimistic estimate of the elapsed
   77        * time, or simply the time since the last full collection was performed.
   78        *
   79        * <p> Note that in the presence of reference objects, a given object that
   80        * is no longer strongly reachable may have to be inspected multiple times
   81        * before it can be reclaimed.
   82        */
   83       public static native long maxObjectInspectionAge();
   84   
   85   
   86       private static class Daemon extends Thread {
   87   
   88           public void run() {
   89               for (;;) {
   90                   long l;
   91                   synchronized (lock) {
   92   
   93                       l = latencyTarget;
   94                       if (l == NO_TARGET) {
   95                           /* No latency target, so exit */
   96                           GC.daemon = null;
   97                           return;
   98                       }
   99   
  100                       long d = maxObjectInspectionAge();
  101                       if (d >= l) {
  102                           /* Do a full collection.  There is a remote possibility
  103                            * that a full collection will occurr between the time
  104                            * we sample the inspection age and the time the GC
  105                            * actually starts, but this is sufficiently unlikely
  106                            * that it doesn't seem worth the more expensive JVM
  107                            * interface that would be required.
  108                            */
  109                           System.gc();
  110                           d = 0;
  111                       }
  112   
  113                       /* Wait for the latency period to expire,
  114                        * or for notification that the period has changed
  115                        */
  116                       try {
  117                           lock.wait(l - d);
  118                       } catch (InterruptedException x) {
  119                           continue;
  120                       }
  121                   }
  122               }
  123           }
  124   
  125           private Daemon(ThreadGroup tg) {
  126               super(tg, "GC Daemon");
  127           }
  128   
  129           /* Create a new daemon thread in the root thread group */
  130           public static void create() {
  131               PrivilegedAction<Void> pa = new PrivilegedAction<Void>() {
  132                   public Void run() {
  133                       ThreadGroup tg = Thread.currentThread().getThreadGroup();
  134                       for (ThreadGroup tgn = tg;
  135                            tgn != null;
  136                            tg = tgn, tgn = tg.getParent());
  137                       Daemon d = new Daemon(tg);
  138                       d.setDaemon(true);
  139                       d.setPriority(Thread.MIN_PRIORITY + 1);
  140                       d.start();
  141                       GC.daemon = d;
  142                       return null;
  143                   }};
  144               AccessController.doPrivileged(pa);
  145           }
  146   
  147       }
  148   
  149   
  150       /* Sets the latency target to the given value.
  151        * Must be invoked while holding the lock.
  152        */
  153       private static void setLatencyTarget(long ms) {
  154           latencyTarget = ms;
  155           if (daemon == null) {
  156               /* Create a new daemon thread */
  157               Daemon.create();
  158           } else {
  159               /* Notify the existing daemon thread
  160                * that the lateency target has changed
  161                */
  162               lock.notify();
  163           }
  164       }
  165   
  166   
  167       /**
  168        * Represents an active garbage-collection latency request.  Instances of
  169        * this class are created by the <code>{@link #requestLatency}</code>
  170        * method.  Given a request, the only interesting operation is that of
  171        * cancellation.
  172        */
  173       public static class LatencyRequest
  174           implements Comparable<LatencyRequest> {
  175   
  176           /* Instance counter, used to generate unique identifers */
  177           private static long counter = 0;
  178   
  179           /* Sorted set of active latency requests */
  180           private static SortedSet<LatencyRequest> requests = null;
  181   
  182           /* Examine the request set and reset the latency target if necessary.
  183            * Must be invoked while holding the lock.
  184            */
  185           private static void adjustLatencyIfNeeded() {
  186               if ((requests == null) || requests.isEmpty()) {
  187                   if (latencyTarget != NO_TARGET) {
  188                       setLatencyTarget(NO_TARGET);
  189                   }
  190               } else {
  191                   LatencyRequest r = requests.first();
  192                   if (r.latency != latencyTarget) {
  193                       setLatencyTarget(r.latency);
  194                   }
  195               }
  196           }
  197   
  198           /* The requested latency, or NO_TARGET
  199            * if this request has been cancelled
  200            */
  201           private long latency;
  202   
  203           /* Unique identifier for this request */
  204           private long id;
  205   
  206           private LatencyRequest(long ms) {
  207               if (ms <= 0) {
  208                   throw new IllegalArgumentException("Non-positive latency: "
  209                                                      + ms);
  210               }
  211               this.latency = ms;
  212               synchronized (lock) {
  213                   this.id = ++counter;
  214                   if (requests == null) {
  215                       requests = new TreeSet<LatencyRequest>();
  216                   }
  217                   requests.add(this);
  218                   adjustLatencyIfNeeded();
  219               }
  220           }
  221   
  222           /**
  223            * Cancels this latency request.
  224            *
  225            * @throws  IllegalStateException
  226            *          If this request has already been cancelled
  227            */
  228           public void cancel() {
  229               synchronized (lock) {
  230                   if (this.latency == NO_TARGET) {
  231                       throw new IllegalStateException("Request already"
  232                                                       + " cancelled");
  233                   }
  234                   if (!requests.remove(this)) {
  235                       throw new InternalError("Latency request "
  236                                               + this + " not found");
  237                   }
  238                   if (requests.isEmpty()) requests = null;
  239                   this.latency = NO_TARGET;
  240                   adjustLatencyIfNeeded();
  241               }
  242           }
  243   
  244           public int compareTo(LatencyRequest r) {
  245               long d = this.latency - r.latency;
  246               if (d == 0) d = this.id - r.id;
  247               return (d < 0) ? -1 : ((d > 0) ? +1 : 0);
  248           }
  249   
  250           public String toString() {
  251               return (LatencyRequest.class.getName()
  252                       + "[" + latency + "," + id + "]");
  253           }
  254   
  255       }
  256   
  257   
  258       /**
  259        * Makes a new request for a garbage-collection latency of the given
  260        * number of real-time milliseconds.  A low-priority daemon thread makes a
  261        * best effort to ensure that the maximum object-inspection age never
  262        * exceeds the smallest of the currently active requests.
  263        *
  264        * @param   latency
  265        *          The requested latency
  266        *
  267        * @throws  IllegalArgumentException
  268        *          If the given <code>latency</code> is non-positive
  269        */
  270       public static LatencyRequest requestLatency(long latency) {
  271           return new LatencyRequest(latency);
  272       }
  273   
  274   
  275       /**
  276        * Returns the current smallest garbage-collection latency request, or zero
  277        * if there are no active requests.
  278        */
  279       public static long currentLatencyTarget() {
  280           long t = latencyTarget;
  281           return (t == NO_TARGET) ? 0 : t;
  282       }
  283   
  284   }

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