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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 | } | ||