org.apache.jackrabbit.oak.stats.Clock Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.jackrabbit.oak.stats;
import java.io.Closeable;
import java.util.Date;
import java.util.Random;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
/**
* Mechanism for keeping track of time at millisecond accuracy.
*/
public abstract class Clock {
/**
* Maximum amount (in ms) of random noise to include in the time
* signal reported by the {@link #SIMPLE} clock. Configurable by the
* "simple.clock.noise" system property to make it easier to test
* the effect of an inaccurate system clock.
*/
private static final int SIMPLE_CLOCK_NOISE =
Integer.getInteger("simple.clock.noise", 0);
/**
* Millisecond granularity of the {@link #ACCURATE} clock.
* Configurable by the "accurate.clock.granularity" system property
* to make it easier to test the effect of a slow-moving clock on
* code that relies on millisecond timestamps.
*/
private static final long ACCURATE_CLOCK_GRANULARITY =
Long.getLong("accurate.clock.granularity", 1);
/**
* Millisecond update interval of the {@link Fast} clock. Configurable
* by the "fast.clock.interval" system property to to make it easier
* to test the effect of different update frequencies.
*/
static final long FAST_CLOCK_INTERVAL =
Long.getLong("fast.clock.interval", 10);
private long monotonic = 0;
private long increasing = 0;
/**
* Returns the current time in milliseconds since the epoch.
*
* @see System#currentTimeMillis()
* @return current time in milliseconds since the epoch
*/
public abstract long getTime();
/**
* Returns a monotonically increasing timestamp based on the current time.
* A call to this method will always return a value that is greater than
* or equal to a value returned by any previous call. This contract holds
* even across multiple threads and in cases when the system time is
* adjusted backwards. In the latter case the returned value will remain
* constant until the previously reported timestamp is again reached.
*
* @return monotonically increasing timestamp
*/
public synchronized long getTimeMonotonic() {
long now = getTime();
if (now > monotonic) {
monotonic = now;
} else {
now = monotonic;
}
return now;
}
/**
* Returns a strictly increasing timestamp based on the current time.
* This method is like {@link #getTimeMonotonic()}, with the exception
* that two calls of this method will never return the same timestamp.
* Instead this method will explicitly wait until the current time
* increases beyond any previously returned value. Note that the wait
* may last long if this method is called frequently from many concurrent
* thread or if the system time is adjusted backwards. The caller should
* be prepared to deal with an explicit interrupt in such cases.
*
* @return strictly increasing timestamp
* @throws InterruptedException if the wait was interrupted
*/
public synchronized long getTimeIncreasing() throws InterruptedException {
long now = getTime();
while (now <= increasing) {
wait(0, 100000); // 0.1ms
now = getTime();
}
increasing = now;
return now;
}
/**
* Convenience method that returns the {@link #getTime()} value
* as a {@link Date} instance.
*
* @return current time
*/
public Date getDate() {
return new Date(getTime());
}
/**
* Convenience method that returns the {@link #getTimeMonotonic()} value
* as a {@link Date} instance.
*
* @return monotonically increasing time
*/
public Date getDateMonotonic() {
return new Date(getTimeMonotonic());
}
/**
* Convenience method that returns the {@link #getTimeIncreasing()} value
* as a {@link Date} instance.
*
* @return strictly increasing time
*/
public Date getDateIncreasing() throws InterruptedException {
return new Date(getTimeIncreasing());
}
/**
* Waits until the given point in time is reached. The current thread
* is suspended until the {@link #getTimeIncreasing()} method returns
* a time that's equal or greater than the given point in time.
*
* @param timestamp time in milliseconds since epoch
* @throws InterruptedException if the wait was interrupted
*/
public void waitUntil(long timestamp) throws InterruptedException {
long now = getTimeIncreasing();
while (now < timestamp) {
Thread.sleep(timestamp - now);
now = getTimeIncreasing();
}
}
/**
* Simple clock implementation based on {@link System#currentTimeMillis()},
* which is known to be rather slow on some platforms.
*/
public static Clock SIMPLE = createSimpleClock();
private static Clock createSimpleClock() {
final int noise = SIMPLE_CLOCK_NOISE;
if (noise > 0) {
return new Clock() {
private final Random random = new Random();
@Override
public synchronized long getTime() {
return System.currentTimeMillis() + random.nextInt(noise);
}
@Override
public String toString() {
return "Clock.SIMPLE (with noise)";
}
};
} else {
return new Clock() {
@Override
public long getTime() {
return System.currentTimeMillis();
}
@Override
public String toString() {
return "Clock.SIMPLE";
}
};
}
}
/**
* Accurate clock implementation that uses interval timings from the
* {@link System#nanoTime()} method to calculate an as accurate as possible
* time based on occasional calls to {@link System#currentTimeMillis()}
* to prevent clock drift.
*/
public static Clock ACCURATE = new Clock() {
private static final long NS_IN_MS = 1000000;
private long ms = SIMPLE.getTime();
private long ns = System.nanoTime();
@Override
public synchronized long getTime() {
long nowns = System.nanoTime();
long nsIncrease = Math.max(nowns - ns, 0); // >= 0
long msIncrease = (nsIncrease + NS_IN_MS/2) / NS_IN_MS; // round up
if (ACCURATE_CLOCK_GRANULARITY > 1) {
msIncrease -= msIncrease % ACCURATE_CLOCK_GRANULARITY;
}
// If last clock sync was less than one second ago, the nanosecond
// timer drift will be insignificant and there's no need to re-sync.
if (msIncrease < 1000) {
return ms + msIncrease;
}
// Last clock sync was up to ten seconds ago, so we synchronize
// smoothly to avoid both drift and sudden jumps.
long nowms = SIMPLE.getTime();
if (msIncrease < 10000) {
// 1) increase the ms and ns timestamps as if the estimated
// ms increase was entirely correct
ms += msIncrease;
ns += msIncrease * NS_IN_MS;
// 2) compare the resulting time with the wall clock to see
// if we're out of sync and to adjust accordingly
long jump = nowms - ms;
if (jump == 0) {
// 2a) No deviation from wall clock.
return ms;
} else if (0 < jump && jump < 100) {
// 2b) The wall clock is up to 100ms ahead of us, probably
// because of its low granularity. Adjust the ns timestamp
// 0.1ms backward for future clock readings to jump that
// much ahead to eventually catch up with the wall clock.
ns -= NS_IN_MS / 10;
return ms;
} else if (0 > jump && jump > -100) {
// 2c) The wall clock is up to 100ms behind us, probably
// because of its low granularity. Adjust the ns timestamp
// 0.1ms forward for future clock readings to stay constant
// (because of the Math.max(..., 0) above) for that long
// to eventually catch up with the wall clock.
ns += NS_IN_MS / 10;
return ms;
}
}
// Last clock sync was over 10s ago or the nanosecond timer has
// drifted more than 100ms from the wall clock, so it's best to
// to a hard sync with no smoothing.
if (nowms >= ms + 1000) {
ms = nowms;
ns = nowns;
} else {
// Prevent the clock from moving backwards by setting the
// ms timestamp to exactly 1s ahead of the last sync time
// (to account for the time between clock syncs), and
// adjusting the ns timestamp ahead so that the reported time
// will stall until the clock would again move ahead.
ms = ms + 1000; // the 1s clock sync interval from above
ns = nowns + (ms - nowms) * NS_IN_MS;
}
return ms;
}
@Override
public String toString() {
return "Clock.ACCURATE";
}
};
/**
* Fast clock implementation whose {@link #getTime()} method returns
* instantaneously thanks to a background task that takes care of the
* actual time-keeping work.
*/
public static class Fast extends Clock implements Closeable {
private volatile long time = ACCURATE.getTime();
private final ScheduledFuture future;
public Fast(ScheduledExecutorService executor) {
future = executor.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
time = ACCURATE.getTime();
}
}, FAST_CLOCK_INTERVAL, FAST_CLOCK_INTERVAL, TimeUnit.MILLISECONDS);
}
@Override
public long getTime() {
return time;
}
@Override
public String toString() {
return "Clock.Fast";
}
public void close() {
future.cancel(false);
}
}
/**
* A virtual clock that has no connection to the actual system time.
* Instead the clock maintains an internal counter that's incremented
* atomically whenever the current time is requested. This guarantees
* that the reported time signal is always strictly increasing.
*/
public static class Virtual extends Clock {
private final AtomicLong time = new AtomicLong();
@Override
public long getTime() {
return time.getAndIncrement();
}
@Override
public void waitUntil(long timestamp) {
long now = time.get();
while (now < timestamp && !time.compareAndSet(now, timestamp)) {
now = time.get();
}
}
@Override
public String toString() {
return "Clock.Virtual";
}
};
}
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