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/*
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* Copyright (c) 1998, 2008, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package sun.misc;
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import java.security.AccessController;
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import java.security.PrivilegedAction;
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import java.util.SortedSet;
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import java.util.TreeSet;
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/**
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* Support for garbage-collection latency requests.
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*
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* @author Mark Reinhold
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* @since 1.2
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*/
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public class GC {
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private GC() { } /* To prevent instantiation */
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/* Latency-target value indicating that there's no active target
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*/
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private static final long NO_TARGET = Long.MAX_VALUE;
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/* The current latency target, or NO_TARGET if there is no target
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*/
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private static long latencyTarget = NO_TARGET;
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/* The daemon thread that implements the latency-target mechanism,
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* or null if there is presently no daemon thread
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*/
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private static Thread daemon = null;
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/* The lock object for the latencyTarget and daemon fields. The daemon
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* thread, if it exists, waits on this lock for notification that the
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* latency target has changed.
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*/
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private static class LatencyLock extends Object { };
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private static Object lock = new LatencyLock();
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/**
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* Returns the maximum <em>object-inspection age</em>, which is the number
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* of real-time milliseconds that have elapsed since the
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* least-recently-inspected heap object was last inspected by the garbage
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* collector.
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*
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* <p> For simple stop-the-world collectors this value is just the time
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* since the most recent collection. For generational collectors it is the
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* time since the oldest generation was most recently collected. Other
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* collectors are free to return a pessimistic estimate of the elapsed
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* time, or simply the time since the last full collection was performed.
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*
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* <p> Note that in the presence of reference objects, a given object that
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* is no longer strongly reachable may have to be inspected multiple times
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* before it can be reclaimed.
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*/
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public static native long maxObjectInspectionAge();
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private static class Daemon extends Thread {
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public void run() {
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for (;;) {
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long l;
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synchronized (lock) {
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l = latencyTarget;
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if (l == NO_TARGET) {
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/* No latency target, so exit */
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GC.daemon = null;
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return;
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}
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long d = maxObjectInspectionAge();
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if (d >= l) {
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/* Do a full collection. There is a remote possibility
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* that a full collection will occurr between the time
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* we sample the inspection age and the time the GC
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* actually starts, but this is sufficiently unlikely
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* that it doesn't seem worth the more expensive JVM
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* interface that would be required.
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*/
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System.gc();
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d = 0;
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}
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/* Wait for the latency period to expire,
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* or for notification that the period has changed
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*/
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try {
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lock.wait(l - d);
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} catch (InterruptedException x) {
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continue;
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}
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}
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}
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}
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private Daemon(ThreadGroup tg) {
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super(tg, "GC Daemon");
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}
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/* Create a new daemon thread in the root thread group */
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public static void create() {
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PrivilegedAction<Void> pa = new PrivilegedAction<Void>() {
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public Void run() {
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ThreadGroup tg = Thread.currentThread().getThreadGroup();
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for (ThreadGroup tgn = tg;
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tgn != null;
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tg = tgn, tgn = tg.getParent());
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Daemon d = new Daemon(tg);
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d.setDaemon(true);
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d.setPriority(Thread.MIN_PRIORITY + 1);
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d.start();
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GC.daemon = d;
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return null;
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}};
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AccessController.doPrivileged(pa);
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}
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}
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/* Sets the latency target to the given value.
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* Must be invoked while holding the lock.
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*/
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private static void setLatencyTarget(long ms) {
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latencyTarget = ms;
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if (daemon == null) {
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/* Create a new daemon thread */
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Daemon.create();
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} else {
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/* Notify the existing daemon thread
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* that the lateency target has changed
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*/
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lock.notify();
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}
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}
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/**
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* Represents an active garbage-collection latency request. Instances of
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* this class are created by the <code>{@link #requestLatency}</code>
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* method. Given a request, the only interesting operation is that of
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* cancellation.
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*/
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public static class LatencyRequest
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implements Comparable<LatencyRequest> {
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/* Instance counter, used to generate unique identifers */
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private static long counter = 0;
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/* Sorted set of active latency requests */
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private static SortedSet<LatencyRequest> requests = null;
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/* Examine the request set and reset the latency target if necessary.
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* Must be invoked while holding the lock.
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*/
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private static void adjustLatencyIfNeeded() {
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if ((requests == null) || requests.isEmpty()) {
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if (latencyTarget != NO_TARGET) {
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setLatencyTarget(NO_TARGET);
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}
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} else {
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LatencyRequest r = requests.first();
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if (r.latency != latencyTarget) {
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setLatencyTarget(r.latency);
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}
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}
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}
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/* The requested latency, or NO_TARGET
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* if this request has been cancelled
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*/
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private long latency;
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/* Unique identifier for this request */
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private long id;
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private LatencyRequest(long ms) {
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if (ms <= 0) {
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throw new IllegalArgumentException("Non-positive latency: "
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+ ms);
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}
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this.latency = ms;
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synchronized (lock) {
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this.id = ++counter;
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if (requests == null) {
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requests = new TreeSet<LatencyRequest>();
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}
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requests.add(this);
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adjustLatencyIfNeeded();
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}
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}
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/**
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* Cancels this latency request.
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*
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* @throws IllegalStateException
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* If this request has already been cancelled
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*/
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public void cancel() {
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synchronized (lock) {
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if (this.latency == NO_TARGET) {
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throw new IllegalStateException("Request already"
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+ " cancelled");
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}
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if (!requests.remove(this)) {
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throw new InternalError("Latency request "
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+ this + " not found");
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}
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if (requests.isEmpty()) requests = null;
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this.latency = NO_TARGET;
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adjustLatencyIfNeeded();
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}
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}
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public int compareTo(LatencyRequest r) {
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long d = this.latency - r.latency;
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if (d == 0) d = this.id - r.id;
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return (d < 0) ? -1 : ((d > 0) ? +1 : 0);
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}
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public String toString() {
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return (LatencyRequest.class.getName()
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+ "[" + latency + "," + id + "]");
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}
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}
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/**
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* Makes a new request for a garbage-collection latency of the given
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* number of real-time milliseconds. A low-priority daemon thread makes a
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* best effort to ensure that the maximum object-inspection age never
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* exceeds the smallest of the currently active requests.
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*
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* @param latency
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* The requested latency
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*
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* @throws IllegalArgumentException
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* If the given <code>latency</code> is non-positive
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*/
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public static LatencyRequest requestLatency(long latency) {
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return new LatencyRequest(latency);
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}
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/**
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* Returns the current smallest garbage-collection latency request, or zero
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* if there are no active requests.
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*/
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public static long currentLatencyTarget() {
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long t = latencyTarget;
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return (t == NO_TARGET) ? 0 : t;
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}
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}
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