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 }
Save This Page