001: /*
002: * @(#)GC.java 1.15 06/10/10
003: *
004: * Copyright 1990-2006 Sun Microsystems, Inc. All Rights Reserved.
005: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
006: *
007: * This program is free software; you can redistribute it and/or
008: * modify it under the terms of the GNU General Public License version
009: * 2 only, as published by the Free Software Foundation.
010: *
011: * This program is distributed in the hope that it will be useful, but
012: * WITHOUT ANY WARRANTY; without even the implied warranty of
013: * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
014: * General Public License version 2 for more details (a copy is
015: * included at /legal/license.txt).
016: *
017: * You should have received a copy of the GNU General Public License
018: * version 2 along with this work; if not, write to the Free Software
019: * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
020: * 02110-1301 USA
021: *
022: * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
023: * Clara, CA 95054 or visit www.sun.com if you need additional
024: * information or have any questions.
025: *
026: */
027:
028: package sun.misc;
029:
030: import java.security.AccessController;
031: import java.security.PrivilegedAction;
032: import java.util.SortedSet;
033: import java.util.TreeSet;
034: import sun.misc.ThreadRegistry;
035:
036: /**
037: * Support for garbage-collection latency requests.
038: *
039: * @version 1.8, 00/05/16
040: * @author Mark Reinhold
041: * @since JDK1.2
042: */
043:
044: public class GC {
045:
046: private GC() {
047: } /* To prevent instantiation */
048:
049: /* Latency-target value indicating that there's no active target
050: */
051: private static final long NO_TARGET = Long.MAX_VALUE;
052:
053: /* The current latency target, or NO_TARGET if there is no target
054: */
055: private static long latencyTarget = NO_TARGET;
056:
057: /* The daemon thread that implements the latency-target mechanism,
058: * or null if there is presently no daemon thread
059: */
060: private static Thread daemon = null;
061:
062: /* The lock object for the latencyTarget and daemon fields. The daemon
063: * thread, if it exists, waits on this lock for notification that the
064: * latency target has changed.
065: */
066: private static class LatencyLock extends Object {
067: };
068:
069: private static Object lock = new LatencyLock();
070:
071: /**
072: * Returns the maximum <em>object-inspection age</em>, which is the number
073: * of real-time milliseconds that have elapsed since the
074: * least-recently-inspected heap object was last inspected by the garbage
075: * collector.
076: *
077: * <p> For simple stop-the-world collectors this value is just the time
078: * since the most recent collection. For generational collectors it is the
079: * time since the oldest generation was most recently collected. Other
080: * collectors are free to return a pessimistic estimate of the elapsed
081: * time, or simply the time since the last full collection was performed.
082: *
083: * <p> Note that in the presence of reference objects, a given object that
084: * is no longer strongly reachable may have to be inspected multiple times
085: * before it can be reclaimed.
086: */
087: public static native long maxObjectInspectionAge();
088:
089: private static class Daemon extends Thread {
090:
091: public void run() {
092: while (!ThreadRegistry.exitRequested()) {
093: long l;
094: synchronized (lock) {
095:
096: l = latencyTarget;
097: if (l == NO_TARGET) {
098: /* No latency target, so exit */
099: GC.daemon = null;
100: return;
101: }
102:
103: long d = maxObjectInspectionAge();
104: if (d >= l) {
105: /* Do a full collection. There is a remote possibility
106: * that a full collection will occurr between the time
107: * we sample the inspection age and the time the GC
108: * actually starts, but this is sufficiently unlikely
109: * that it doesn't seem worth the more expensive JVM
110: * interface that would be required.
111: */
112: System.gc();
113: d = 0;
114: }
115:
116: /* Wait for the latency period to expire,
117: * or for notification that the period has changed
118: */
119: try {
120: lock.wait(l - d);
121: } catch (InterruptedException x) {
122: continue;
123: }
124: }
125: }
126: }
127:
128: private Daemon(ThreadGroup tg) {
129: super (tg, "GC Daemon");
130: }
131:
132: /* Create a new daemon thread in the root thread group */
133: public static void create() {
134: PrivilegedAction pa = new PrivilegedAction() {
135: public Object run() {
136: ThreadGroup tg = Thread.currentThread()
137: .getThreadGroup();
138: for (ThreadGroup tgn = tg; tgn != null; tg = tgn, tgn = tg
139: .getParent())
140: ;
141: Daemon d = new Daemon(tg);
142: d.setDaemon(true);
143: d.setPriority(Thread.MIN_PRIORITY + 1);
144: d.start();
145: GC.daemon = d;
146: return null;
147: }
148: };
149: AccessController.doPrivileged(pa);
150: }
151:
152: }
153:
154: /* Sets the latency target to the given value.
155: * Must be invoked while holding the lock.
156: */
157: private static void setLatencyTarget(long ms) {
158: latencyTarget = ms;
159: if (daemon == null) {
160: /* Create a new daemon thread */
161: Daemon.create();
162: } else {
163: /* Notify the existing daemon thread
164: * that the lateency target has changed
165: */
166: lock.notify();
167: }
168: }
169:
170: /**
171: * Represents an active garbage-collection latency request. Instances of
172: * this class are created by the <code>{@link #requestLatency}</code>
173: * method. Given a request, the only interesting operation is that of
174: * cancellation.
175: */
176: public static class LatencyRequest implements Comparable {
177:
178: /* Instance counter, used to generate unique identifers */
179: private static long counter = 0;
180:
181: /* Sorted set of active latency requests */
182: private static SortedSet requests = null;
183:
184: /* Examine the request set and reset the latency target if necessary.
185: * Must be invoked while holding the lock.
186: */
187: private static void adjustLatencyIfNeeded() {
188: if ((requests == null) || requests.isEmpty()) {
189: if (latencyTarget != NO_TARGET) {
190: setLatencyTarget(NO_TARGET);
191: }
192: } else {
193: LatencyRequest r = (LatencyRequest) requests.first();
194: if (r.latency != latencyTarget) {
195: setLatencyTarget(r.latency);
196: }
197: }
198: }
199:
200: /* The requested latency, or NO_TARGET
201: * if this request has been cancelled
202: */
203: private long latency;
204:
205: /* Unique identifier for this request */
206: private long id;
207:
208: private LatencyRequest(long ms) {
209: if (ms <= 0) {
210: throw new IllegalArgumentException(
211: "Non-positive latency: " + ms);
212: }
213: this .latency = ms;
214: synchronized (lock) {
215: this .id = ++counter;
216: if (requests == null) {
217: requests = new TreeSet();
218: }
219: requests.add(this );
220: adjustLatencyIfNeeded();
221: }
222: }
223:
224: /**
225: * Cancels this latency request.
226: *
227: * @throws IllegalStateException
228: * If this request has already been cancelled
229: */
230: public void cancel() {
231: synchronized (lock) {
232: if (this .latency == NO_TARGET) {
233: throw new IllegalStateException("Request already"
234: + " cancelled");
235: }
236: if (!requests.remove(this )) {
237: throw new InternalError("Latency request " + this
238: + " not found");
239: }
240: if (requests.isEmpty())
241: requests = null;
242: this .latency = NO_TARGET;
243: adjustLatencyIfNeeded();
244: }
245: }
246:
247: public int compareTo(Object o) {
248: LatencyRequest r = (LatencyRequest) o;
249: long d = this .latency - r.latency;
250: if (d == 0)
251: d = this .id - r.id;
252: return (d < 0) ? -1 : ((d > 0) ? +1 : 0);
253: }
254:
255: public String toString() {
256: return (LatencyRequest.class.getName() + "[" + latency
257: + "," + id + "]");
258: }
259:
260: }
261:
262: /**
263: * Makes a new request for a garbage-collection latency of the given
264: * number of real-time milliseconds. A low-priority daemon thread makes a
265: * best effort to ensure that the maximum object-inspection age never
266: * exceeds the smallest of the currently active requests.
267: *
268: * @param latency
269: * The requested latency
270: *
271: * @throws IllegalArgumentException
272: * If the given <code>latency</code> is non-positive
273: */
274: public static LatencyRequest requestLatency(long latency) {
275: return new LatencyRequest(latency);
276: }
277:
278: /**
279: * Returns the current smallest garbage-collection latency request, or zero
280: * if there are no active requests.
281: */
282: public static long currentLatencyTarget() {
283: long t = latencyTarget;
284: return (t == NO_TARGET) ? 0 : t;
285: }
286:
287: }
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