/*
* Copyright (C) 2012-2015 DataStax Inc.
*
* Licensed 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 com.datastax.driver.core;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.concurrent.atomic.AtomicReference;
import static java.util.concurrent.TimeUnit.*;
/**
* A small abstraction around system clock that aims to provide microsecond precision with the best accuracy possible.
*/
interface Clock {
/**
* Returns the current time in microseconds.
*
* @return the difference, measured in microseconds, between the current time and and the Epoch
* (that is, midnight, January 1, 1970 UTC).
*/
long currentTimeMicros();
}
/**
* Factory that returns the best Clock implementation depending on what native libraries are available in the system.
* If LibC is available through JNR, and if the system property {@code com.datastax.driver.USE_NATIVE_CLOCK} is set to {@code true}
* (which is the default value), then {@link NativeClock} is returned, otherwise {@link SystemClock} is returned.
*/
class ClockFactory {
private static final Logger LOGGER = LoggerFactory.getLogger(ClockFactory.class);
private static final String USE_NATIVE_CLOCK_SYSTEM_PROPERTY = "com.datastax.driver.USE_NATIVE_CLOCK";
static Clock newInstance() {
if (SystemProperties.getBoolean(USE_NATIVE_CLOCK_SYSTEM_PROPERTY, true) && Native.isGettimeofdayAvailable()) {
LOGGER.info("Using native clock to generate timestamps.");
return new NativeClock();
} else {
LOGGER.info("Using java.lang.System clock to generate timestamps.");
return new SystemClock();
}
}
}
/**
* Default implementation of a clock that delegates its calls to the system clock.
*
* @see System#currentTimeMillis()
*/
class SystemClock implements Clock {
@Override
public long currentTimeMicros() {
return System.currentTimeMillis() * 1000;
}
}
/**
* Provides the current time with microseconds precision with some reasonable accuracy through
* the use of {@link Native#currentTimeMicros()}.
* <p/>
* Because calling JNR methods is slightly expensive,
* we only call it once per second and add the number of nanoseconds since the last call
* to get the current time, which is good enough an accuracy for our purpose (see CASSANDRA-6106).
* <p/>
* This reduces the cost of the call to {@link NativeClock#currentTimeMicros()} to levels comparable
* to those of a call to {@link System#nanoTime()}.
*/
class NativeClock implements Clock {
private static final long ONE_SECOND_NS = NANOSECONDS.convert(1, SECONDS);
private static final long ONE_MILLISECOND_NS = NANOSECONDS.convert(1, MILLISECONDS);
/**
* Records a time in micros along with the System.nanoTime() value at the time the
* time is fetched.
*/
private static class FetchedTime {
private final long timeInMicros;
private final long nanoTimeAtCheck;
private FetchedTime(long timeInMicros, long nanoTimeAtCheck) {
this.timeInMicros = timeInMicros;
this.nanoTimeAtCheck = nanoTimeAtCheck;
}
}
private final AtomicReference<FetchedTime> lastFetchedTime = new AtomicReference<FetchedTime>(fetchTimeMicros());
@Override
public long currentTimeMicros() {
FetchedTime spec = lastFetchedTime.get();
long curNano = System.nanoTime();
if (curNano > spec.nanoTimeAtCheck + ONE_SECOND_NS) {
lastFetchedTime.compareAndSet(spec, spec = fetchTimeMicros());
}
return spec.timeInMicros + ((curNano - spec.nanoTimeAtCheck) / 1000);
}
private static FetchedTime fetchTimeMicros() {
// To compensate for the fact that the Native.currentTimeMicros call could take
// some time, instead of picking the nano time before the call or after the
// call, we take the average of both.
long start = System.nanoTime();
long micros = Native.currentTimeMicros();
long end = System.nanoTime();
// If it turns out the call took us more than 1 millisecond (can happen while
// the JVM warms up, unlikely otherwise, but no reasons to take risks), fall back
// to System.currentTimeMillis() temporarily
if ((end - start) > ONE_MILLISECOND_NS)
return new FetchedTime(System.currentTimeMillis() * 1000, System.nanoTime());
return new FetchedTime(micros, (end + start) / 2);
}
}