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     private static class Daemon extends Thread {

    86 

    87         public void run() {

    88             for (;;) {

    89                 long l;

    90                 synchronized (lock) {

    91 

    92                     l = latencyTarget;

    93                     if (l == NO_TARGET) {

    94                         /* No latency target, so exit */

    95                         GC.daemon = null;

    96                         return;

    97                     }

    98 

    99                     long d = maxObjectInspectionAge();

   100                     if (d >= l) {

   101                         /* Do a full collection.  There is a remote possibility

   102                          * that a full collection will occurr between the time

   103                          * we sample the inspection age and the time the GC

   104                          * actually starts, but this is sufficiently unlikely

   105                          * that it doesn't seem worth the more expensive JVM

   106                          * interface that would be required.

   107                          */

   108                         System.gc();

   109                         d = 0;

   110                     }

   111 

   112                     /* Wait for the latency period to expire,

   113                      * or for notification that the period has changed

   114                      */

   115                     try {

   116                         lock.wait(l - d);

   117                     } catch (InterruptedException x) {

   118                         continue;

   119                     }

   120                 }

   121             }

   122         }

   123 

   124         private Daemon(ThreadGroup tg) {

   125             super(tg, null, "GC Daemon", 0L, false);

   126         }

   127 

   128         /* Create a new daemon thread in the root thread group */

   129         public static void create() {

   130             PrivilegedAction<Void> pa = new PrivilegedAction<Void>() {

   131                 public Void run() {

   132                     ThreadGroup tg = Thread.currentThread().getThreadGroup();

   133                     for (ThreadGroup tgn = tg;

   134                          tgn != null;

   135                          tg = tgn, tgn = tg.getParent());

   136                     Daemon d = new Daemon(tg);

   137                     d.setDaemon(true);

   138                     d.setPriority(Thread.MIN_PRIORITY + 1);

   139                     d.start();

   140                     GC.daemon = d;

   141                     return null;

   142                 }};

   143             AccessController.doPrivileged(pa);

   144         }

   145 

   146     }

   147 

   148 

   149     /* Sets the latency target to the given value.

   150      * Must be invoked while holding the lock.

   151      */

   152     private static void setLatencyTarget(long ms) {

   153         latencyTarget = ms;

   154         if (daemon == null) {

   155             /* Create a new daemon thread */

   156             Daemon.create();

   157         } else {

   158             /* Notify the existing daemon thread

   159              * that the lateency target has changed

   160              */

   161             lock.notify();

   162         }

   163     }

   164 

   165 

   166     /**

   167      * Represents an active garbage-collection latency request.  Instances of

   168      * this class are created by the <code>{@link #requestLatency}</code>

   169      * method.  Given a request, the only interesting operation is that of

   170      * cancellation.

   171      */

   172     public static class LatencyRequest

   173         implements Comparable<LatencyRequest> {

   174 

   175         /* Instance counter, used to generate unique identifers */

   176         private static long counter = 0;

   177 

   178         /* Sorted set of active latency requests */

   179         private static SortedSet<LatencyRequest> requests = null;

   180 

   181         /* Examine the request set and reset the latency target if necessary.

   182          * Must be invoked while holding the lock.

   183          */

   184         private static void adjustLatencyIfNeeded() {

   185             if ((requests == null) || requests.isEmpty()) {

   186                 if (latencyTarget != NO_TARGET) {

   187                     setLatencyTarget(NO_TARGET);

   188                 }

   189             } else {

   190                 LatencyRequest r = requests.first();

   191                 if (r.latency != latencyTarget) {

   192                     setLatencyTarget(r.latency);

   193                 }

   194             }

   195         }

   196 

   197         /* The requested latency, or NO_TARGET

   198          * if this request has been cancelled

   199          */

   200         private long latency;

   201 

   202         /* Unique identifier for this request */

   203         private long id;

   204 

   205         private LatencyRequest(long ms) {

   206             if (ms <= 0) {

   207                 throw new IllegalArgumentException("Non-positive latency: "

   208                                                    + ms);

   209             }

   210             this.latency = ms;

   211             synchronized (lock) {

   212                 this.id = ++counter;

   213                 if (requests == null) {

   214                     requests = new TreeSet<LatencyRequest>();

   215                 }

   216                 requests.add(this);

   217                 adjustLatencyIfNeeded();

   218             }

   219         }

   220 

   221         /**

   222          * Cancels this latency request.

   223          *

   224          * @throws  IllegalStateException

   225          *          If this request has already been cancelled

   226          */

   227         public void cancel() {

   228             synchronized (lock) {

   229                 if (this.latency == NO_TARGET) {

   230                     throw new IllegalStateException("Request already"

   231                                                     + " cancelled");

   232                 }

   233                 if (!requests.remove(this)) {

   234                     throw new InternalError("Latency request "

   235                                             + this + " not found");

   236                 }

   237                 if (requests.isEmpty()) requests = null;

   238                 this.latency = NO_TARGET;

   239                 adjustLatencyIfNeeded();

   240             }

   241         }

   242 

   243         public int compareTo(LatencyRequest r) {

   244             long d = this.latency - r.latency;

   245             if (d == 0) d = this.id - r.id;

   246             return (d < 0) ? -1 : ((d > 0) ? +1 : 0);

   247         }

   248 

   249         public String toString() {

   250             return (LatencyRequest.class.getName()

   251                     + "[" + latency + "," + id + "]");

   252         }

   253 

   254     }

   255 

   256 

   257     /**

   258      * Makes a new request for a garbage-collection latency of the given

   259      * number of real-time milliseconds.  A low-priority daemon thread makes a

   260      * best effort to ensure that the maximum object-inspection age never

   261      * exceeds the smallest of the currently active requests.

   262      *

   263      * @param   latency

   264      *          The requested latency

   265      *

   266      * @throws  IllegalArgumentException

   267      *          If the given <code>latency</code> is non-positive

   268      */

   269     public static LatencyRequest requestLatency(long latency) {

   270         return new LatencyRequest(latency);

   271     }

   272 

   273 

   274     /**

   275      * Returns the current smallest garbage-collection latency request, or zero

   276      * if there are no active requests.

   277      */

   278     public static long currentLatencyTarget() {

   279         long t = latencyTarget;

   280         return (t == NO_TARGET) ? 0 : t;

   281     }

   282 

   283 }