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 }