J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 1 | /* |
| 2 | * Copyright 1998-2006 Sun Microsystems, Inc. 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. Sun designates this |
| 8 | * particular file as subject to the "Classpath" exception as provided |
| 9 | * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 22 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 23 | * have any 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 pa = new PrivilegedAction() { |
| 132 | public Object 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 implements Comparable { |
| 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 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 = (LatencyRequest)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(); |
| 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(Object o) { |
| 244 | LatencyRequest r = (LatencyRequest)o; |
| 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 | } |