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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 | +} | |