001 /*
002 * Copyright 2009-2014 UnboundID Corp.
003 * All Rights Reserved.
004 */
005 /*
006 * Copyright (C) 2009-2014 UnboundID Corp.
007 *
008 * This program is free software; you can redistribute it and/or modify
009 * it under the terms of the GNU General Public License (GPLv2 only)
010 * or the terms of the GNU Lesser General Public License (LGPLv2.1 only)
011 * as published by the Free Software Foundation.
012 *
013 * This program is distributed in the hope that it will be useful,
014 * but WITHOUT ANY WARRANTY; without even the implied warranty of
015 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
016 * GNU General Public License for more details.
017 *
018 * You should have received a copy of the GNU General Public License
019 * along with this program; if not, see <http://www.gnu.org/licenses>.
020 */
021 package com.unboundid.util;
022
023
024
025 import java.io.Serializable;
026 import java.util.ArrayList;
027 import java.util.Collections;
028 import java.util.List;
029 import java.util.logging.Level;
030
031 import static com.unboundid.util.Debug.*;
032
033
034
035 /**
036 * Instances of this class are used to ensure that certain actions are performed
037 * at a fixed rate per interval (e.g. 10000 search operations per second).
038 * <p>
039 * Once a class is constructed with the duration of an interval and the target
040 * per interval, the {@link #await} method only releases callers at the
041 * specified number of times per interval. This class is most useful when
042 * the target number per interval exceeds the limits of other approaches
043 * such as {@code java.util.Timer} or
044 * {@code java.util.concurrent.ScheduledThreadPoolExecutor}. For instance,
045 * this does a good job of ensuring that something happens about 10000 times
046 * per second, but it's overkill to ensure something happens five times per
047 * hour. This does come at a cost. In the worst case, a single thread is
048 * tied up in a loop doing a small amount of computation followed by a
049 * Thread.yield(). Calling Thread.sleep() is not possible because many
050 * platforms sleep for a minimum of 10ms, and all platforms require sleeping
051 * for at least 1ms.
052 * <p>
053 * Testing has shown that this class is accurate for a "no-op"
054 * action up to two million per second, which vastly exceeds its
055 * typical use in tools such as {@code searchrate} and {@code modrate}. This
056 * class is designed to be called by multiple threads, however, it does not
057 * make any fairness guarantee between threads; a single-thread might be
058 * released from the {@link #await} method many times before another thread
059 * that is blocked in that method.
060 * <p>
061 * This class attempts to smooth out the target per interval throughout each
062 * interval. At a given ratio, R between 0 and 1, through the interval, the
063 * expected number of actions to have been performed in the interval at that
064 * time is R times the target per interval. That is, 10% of the way through
065 * the interval, approximately 10% of the actions have been performed, and
066 * 80% of the way through the interval, 80% of the actions have been performed.
067 */
068 @ThreadSafety(level=ThreadSafetyLevel.COMPLETELY_THREADSAFE)
069 public final class FixedRateBarrier
070 implements Serializable
071 {
072 /**
073 * The serial version UID for this serializable class.
074 */
075 private static final long serialVersionUID = -3490156685189909611L;
076
077 /**
078 * The minimum number of milliseconds that Thread.sleep() can handle
079 * accurately. This varies from platform to platform, so we measure it
080 * once in the static initializer below. When using a low rate (such as
081 * 100 per second), we can often sleep between iterations instead of having
082 * to spin calling Thread.yield().
083 */
084 private static final long minSleepMillis;
085
086 static
087 {
088 // Calibrate the minimum number of milliseconds that we can reliably
089 // sleep on this system. We take several measurements and take the median,
090 // which keeps us from choosing an outlier.
091 //
092 // It varies from system to system. Testing on three systems, yielded
093 // three different measurements Solaris x86 (10 ms), RedHat Linux (2 ms),
094 // Windows 7 (1 ms).
095
096 final List<Long> minSleepMillisMeasurements = new ArrayList<Long>();
097
098 for (int i = 0; i < 11; i++)
099 {
100 final long timeBefore = System.currentTimeMillis();
101 try
102 {
103 Thread.sleep(1);
104 }
105 catch (InterruptedException e)
106 {
107 debugException(e);
108 }
109 final long sleepMillis = System.currentTimeMillis() - timeBefore;
110 minSleepMillisMeasurements.add(sleepMillis);
111 }
112
113 Collections.sort(minSleepMillisMeasurements);
114 final long medianSleepMillis = minSleepMillisMeasurements.get(
115 minSleepMillisMeasurements.size()/2);
116
117 minSleepMillis = Math.max(medianSleepMillis, 1);
118
119 final String message = "Calibrated FixedRateBarrier to use " +
120 "minSleepMillis=" + minSleepMillis + ". " +
121 "Minimum sleep measurements = " + minSleepMillisMeasurements;
122 debug(Level.INFO, DebugType.OTHER, message);
123 }
124
125
126 // The duration of the target interval in nano-seconds.
127 private final long intervalDurationNanos;
128
129 // This tracks the number of milliseconds between each iteration if they
130 // were evenly spaced.
131 //
132 // If intervalDurationMs=1000 and perInterval=100, then this is 100.
133 // If intervalDurationMs=1000 and perInterval=10000, then this is .1.
134 private final double millisBetweenIterations;
135
136 // The target number of times to release a thread per interval.
137 private final int perInterval;
138
139 // This tracks when this class is shutdown. Calls to await() after
140 // shutdownRequested() is called, will return immediately with a value of
141 // true.
142 private volatile boolean shutdownRequested = false;
143
144 //
145 // The following class variables are guarded by synchronized(this).
146 //
147
148 // A count of the number of times that await has returned within the current
149 // interval.
150 private long countInThisInterval = 0;
151
152 // The start of this interval in terms of System.nanoTime().
153 private long intervalStartNanos = 0;
154
155 // The end of this interval in terms of System.nanoTime().
156 private long intervalEndNanos = 0;
157
158
159
160 /**
161 * Constructs a new FixedRateBarrier, which is active until
162 * {@link #shutdownRequested} is called.
163 *
164 * @param intervalDurationMs The duration of the interval in milliseconds.
165 * @param perInterval The target number of times that {@link #await} should
166 * return per interval.
167 */
168 public FixedRateBarrier(final long intervalDurationMs, final int perInterval)
169 {
170 Validator.ensureTrue(intervalDurationMs > 0,
171 "FixedRateBarrier.intervalDurationMs must be at least 1.");
172 Validator.ensureTrue(perInterval > 0,
173 "FixedRateBarrier.perInterval must be at least 1.");
174
175 this.perInterval = perInterval;
176
177 intervalDurationNanos = 1000L * 1000L * intervalDurationMs;
178
179 millisBetweenIterations = (double)intervalDurationMs/(double)perInterval;
180 }
181
182
183
184 /**
185 * This method waits until it is time for the next 'action' to be performed
186 * based on the specified interval duration and target per interval. This
187 * method can be called by multiple threads simultaneously. This method
188 * returns immediately if shutdown has been requested.
189 *
190 * @return {@code true} if shutdown has been requested and {@code} false
191 * otherwise.
192 */
193 public synchronized boolean await()
194 {
195 // Loop forever until we are requested to shutdown or it is time to perform
196 // the next 'action' in which case we break from the loop.
197 while (!shutdownRequested)
198 {
199 final long now = System.nanoTime();
200
201 if ((intervalStartNanos == 0) || // Handles the first time we're called.
202 (now < intervalStartNanos)) // Handles a change in the clock.
203 {
204 intervalStartNanos = now;
205 intervalEndNanos = intervalStartNanos + intervalDurationNanos;
206 }
207 else if (now >= intervalEndNanos) // End of an interval.
208 {
209 countInThisInterval = 0;
210
211 if (now < (intervalEndNanos + intervalDurationNanos))
212 {
213 // If we have already passed the end of the next interval, then we
214 // don't try to catch up. Instead we just reset the start of the
215 // next interval to now. This could happen if the system clock
216 // was set to the future, we're running in a debugger, or we have
217 // very short intervals and are unable to keep up.
218 intervalStartNanos = now;
219 }
220 else
221 {
222 // Usually we're some small fraction into the next interval, so
223 // we set the start of the current interval to the end of the
224 // previous one.
225 intervalStartNanos = intervalEndNanos;
226 }
227 intervalEndNanos = intervalStartNanos + intervalDurationNanos;
228 }
229
230 final long intervalRemaining = intervalEndNanos - now;
231 if (intervalRemaining <= 0)
232 {
233 // This shouldn't happen, but we're careful not to divide by 0.
234 continue;
235 }
236
237 final double intervalFractionRemaining =
238 (double) intervalRemaining / intervalDurationNanos;
239
240 final double expectedRemaining = intervalFractionRemaining * perInterval;
241 final long actualRemaining = perInterval - countInThisInterval;
242
243 if (actualRemaining >= expectedRemaining)
244 {
245 // We are on schedule or behind schedule so let the next 'action'
246 // happen.
247 countInThisInterval++;
248 break;
249 }
250 else
251 {
252 // If we can sleep until it's time to leave this barrier, then do
253 // so to keep from spinning on a CPU doing Thread.yield().
254
255 final double gapIterations = expectedRemaining - actualRemaining;
256 final double remainingMillis =
257 millisBetweenIterations * gapIterations;
258
259 if (remainingMillis >= minSleepMillis)
260 {
261 try
262 {
263 Thread.sleep((long)remainingMillis);
264 }
265 catch (InterruptedException e)
266 {
267 debugException(e);
268 }
269 }
270 else
271 {
272 // We're ahead of schedule so yield to other threads, and then try
273 // again. Note: this is the most costly part of the algorithm because
274 // we have to busy wait due to the lack of sleeping for very small
275 // amounts of time.
276 Thread.yield();
277 }
278 }
279 }
280
281 return shutdownRequested;
282 }
283
284
285
286 /**
287 * Shuts down this barrier. Future calls to await() will return immediately.
288 */
289 public void shutdownRequested()
290 {
291 shutdownRequested = true;
292 }
293
294
295
296 /**
297 * Returns {@code true} if shutdown has been requested.
298 *
299 * @return {@code true} if shutdown has been requested and {@code false}
300 * otherwise.
301 */
302 public boolean isShutdownRequested()
303 {
304 return shutdownRequested;
305 }
306 }