1 /* |
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2 * Copyright (c) 1998, 2008, Oracle and/or its affiliates. All rights reserved. |
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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4 * |
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5 * This code is free software; you can redistribute it and/or modify it |
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6 * under the terms of the GNU General Public License version 2 only, as |
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7 * published by the Free Software Foundation. Oracle designates this |
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8 * particular file as subject to the "Classpath" exception as provided |
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9 * by Oracle in the LICENSE file that accompanied this code. |
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10 * |
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11 * This code is distributed in the hope that it will be useful, but WITHOUT |
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12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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14 * version 2 for more details (a copy is included in the LICENSE file that |
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15 * accompanied this code). |
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16 * |
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17 * You should have received a copy of the GNU General Public License version |
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18 * 2 along with this work; if not, write to the Free Software Foundation, |
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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20 * |
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21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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22 * or visit www.oracle.com if you need additional information or have any |
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23 * questions. |
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24 */ |
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25 |
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26 package sun.misc; |
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27 |
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28 import java.security.AccessController; |
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29 import java.security.PrivilegedAction; |
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30 import java.util.SortedSet; |
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31 import java.util.TreeSet; |
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32 |
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33 |
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34 /** |
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35 * Support for garbage-collection latency requests. |
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36 * |
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37 * @author Mark Reinhold |
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38 * @since 1.2 |
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39 */ |
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40 |
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41 public class GC { |
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42 |
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43 private GC() { } /* To prevent instantiation */ |
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44 |
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45 |
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46 /* Latency-target value indicating that there's no active target |
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47 */ |
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48 private static final long NO_TARGET = Long.MAX_VALUE; |
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49 |
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50 /* The current latency target, or NO_TARGET if there is no target |
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51 */ |
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52 private static long latencyTarget = NO_TARGET; |
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53 |
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54 /* The daemon thread that implements the latency-target mechanism, |
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55 * or null if there is presently no daemon thread |
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56 */ |
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57 private static Thread daemon = null; |
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58 |
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59 /* The lock object for the latencyTarget and daemon fields. The daemon |
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60 * thread, if it exists, waits on this lock for notification that the |
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61 * latency target has changed. |
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62 */ |
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63 private static class LatencyLock extends Object { }; |
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64 private static Object lock = new LatencyLock(); |
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65 |
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66 |
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67 /** |
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68 * Returns the maximum <em>object-inspection age</em>, which is the number |
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69 * of real-time milliseconds that have elapsed since the |
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70 * least-recently-inspected heap object was last inspected by the garbage |
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71 * collector. |
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72 * |
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73 * <p> For simple stop-the-world collectors this value is just the time |
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74 * since the most recent collection. For generational collectors it is the |
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75 * time since the oldest generation was most recently collected. Other |
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76 * collectors are free to return a pessimistic estimate of the elapsed |
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77 * time, or simply the time since the last full collection was performed. |
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78 * |
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79 * <p> Note that in the presence of reference objects, a given object that |
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80 * is no longer strongly reachable may have to be inspected multiple times |
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81 * before it can be reclaimed. |
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82 */ |
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83 public static native long maxObjectInspectionAge(); |
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84 |
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85 private static class Daemon extends Thread { |
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86 |
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87 public void run() { |
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88 for (;;) { |
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89 long l; |
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90 synchronized (lock) { |
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91 |
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92 l = latencyTarget; |
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93 if (l == NO_TARGET) { |
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94 /* No latency target, so exit */ |
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95 GC.daemon = null; |
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96 return; |
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97 } |
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98 |
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99 long d = maxObjectInspectionAge(); |
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100 if (d >= l) { |
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101 /* Do a full collection. There is a remote possibility |
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102 * that a full collection will occurr between the time |
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103 * we sample the inspection age and the time the GC |
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104 * actually starts, but this is sufficiently unlikely |
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105 * that it doesn't seem worth the more expensive JVM |
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106 * interface that would be required. |
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107 */ |
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108 System.gc(); |
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109 d = 0; |
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110 } |
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111 |
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112 /* Wait for the latency period to expire, |
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113 * or for notification that the period has changed |
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114 */ |
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115 try { |
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116 lock.wait(l - d); |
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117 } catch (InterruptedException x) { |
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118 continue; |
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119 } |
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120 } |
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121 } |
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122 } |
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123 |
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124 private Daemon(ThreadGroup tg) { |
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125 super(tg, null, "GC Daemon", 0L, false); |
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126 } |
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127 |
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128 /* Create a new daemon thread in the root thread group */ |
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129 public static void create() { |
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130 PrivilegedAction<Void> pa = new PrivilegedAction<Void>() { |
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131 public Void run() { |
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132 ThreadGroup tg = Thread.currentThread().getThreadGroup(); |
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133 for (ThreadGroup tgn = tg; |
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134 tgn != null; |
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135 tg = tgn, tgn = tg.getParent()); |
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136 Daemon d = new Daemon(tg); |
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137 d.setDaemon(true); |
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138 d.setPriority(Thread.MIN_PRIORITY + 1); |
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139 d.start(); |
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140 GC.daemon = d; |
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141 return null; |
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142 }}; |
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143 AccessController.doPrivileged(pa); |
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144 } |
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145 |
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146 } |
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147 |
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148 |
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149 /* Sets the latency target to the given value. |
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150 * Must be invoked while holding the lock. |
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151 */ |
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152 private static void setLatencyTarget(long ms) { |
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153 latencyTarget = ms; |
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154 if (daemon == null) { |
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155 /* Create a new daemon thread */ |
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156 Daemon.create(); |
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157 } else { |
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158 /* Notify the existing daemon thread |
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159 * that the lateency target has changed |
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160 */ |
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161 lock.notify(); |
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162 } |
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163 } |
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164 |
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165 |
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166 /** |
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167 * Represents an active garbage-collection latency request. Instances of |
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168 * this class are created by the <code>{@link #requestLatency}</code> |
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169 * method. Given a request, the only interesting operation is that of |
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170 * cancellation. |
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171 */ |
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172 public static class LatencyRequest |
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173 implements Comparable<LatencyRequest> { |
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174 |
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175 /* Instance counter, used to generate unique identifers */ |
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176 private static long counter = 0; |
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177 |
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178 /* Sorted set of active latency requests */ |
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179 private static SortedSet<LatencyRequest> requests = null; |
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180 |
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181 /* Examine the request set and reset the latency target if necessary. |
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182 * Must be invoked while holding the lock. |
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183 */ |
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184 private static void adjustLatencyIfNeeded() { |
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185 if ((requests == null) || requests.isEmpty()) { |
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186 if (latencyTarget != NO_TARGET) { |
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187 setLatencyTarget(NO_TARGET); |
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188 } |
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189 } else { |
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190 LatencyRequest r = requests.first(); |
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191 if (r.latency != latencyTarget) { |
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192 setLatencyTarget(r.latency); |
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193 } |
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194 } |
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195 } |
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196 |
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197 /* The requested latency, or NO_TARGET |
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198 * if this request has been cancelled |
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199 */ |
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200 private long latency; |
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201 |
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202 /* Unique identifier for this request */ |
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203 private long id; |
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204 |
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205 private LatencyRequest(long ms) { |
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206 if (ms <= 0) { |
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207 throw new IllegalArgumentException("Non-positive latency: " |
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208 + ms); |
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209 } |
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210 this.latency = ms; |
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211 synchronized (lock) { |
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212 this.id = ++counter; |
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213 if (requests == null) { |
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214 requests = new TreeSet<LatencyRequest>(); |
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215 } |
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216 requests.add(this); |
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217 adjustLatencyIfNeeded(); |
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218 } |
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219 } |
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220 |
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221 /** |
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222 * Cancels this latency request. |
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223 * |
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224 * @throws IllegalStateException |
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225 * If this request has already been cancelled |
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226 */ |
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227 public void cancel() { |
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228 synchronized (lock) { |
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229 if (this.latency == NO_TARGET) { |
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230 throw new IllegalStateException("Request already" |
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231 + " cancelled"); |
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232 } |
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233 if (!requests.remove(this)) { |
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234 throw new InternalError("Latency request " |
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235 + this + " not found"); |
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236 } |
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237 if (requests.isEmpty()) requests = null; |
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238 this.latency = NO_TARGET; |
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239 adjustLatencyIfNeeded(); |
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240 } |
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241 } |
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242 |
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243 public int compareTo(LatencyRequest r) { |
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244 long d = this.latency - r.latency; |
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245 if (d == 0) d = this.id - r.id; |
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246 return (d < 0) ? -1 : ((d > 0) ? +1 : 0); |
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247 } |
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248 |
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249 public String toString() { |
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250 return (LatencyRequest.class.getName() |
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251 + "[" + latency + "," + id + "]"); |
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252 } |
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253 |
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254 } |
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255 |
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256 |
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257 /** |
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258 * Makes a new request for a garbage-collection latency of the given |
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259 * number of real-time milliseconds. A low-priority daemon thread makes a |
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260 * best effort to ensure that the maximum object-inspection age never |
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261 * exceeds the smallest of the currently active requests. |
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262 * |
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263 * @param latency |
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264 * The requested latency |
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265 * |
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266 * @throws IllegalArgumentException |
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267 * If the given <code>latency</code> is non-positive |
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268 */ |
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269 public static LatencyRequest requestLatency(long latency) { |
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270 return new LatencyRequest(latency); |
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271 } |
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272 |
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273 |
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274 /** |
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275 * Returns the current smallest garbage-collection latency request, or zero |
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276 * if there are no active requests. |
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277 */ |
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278 public static long currentLatencyTarget() { |
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279 long t = latencyTarget; |
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280 return (t == NO_TARGET) ? 0 : t; |
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281 } |
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282 |
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283 } |
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