nebula.plugin.metrics.time.MonotonicClock Maven / Gradle / Ivy
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/*
* Copyright 2016 the original author or authors.
*
* 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 nebula.plugin.metrics.time;
import com.google.common.annotations.VisibleForTesting;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import static com.google.common.base.Preconditions.checkNotNull;
/**
* A clock that is guaranteed to not go backwards.
*
* It aims to strike a balance between never going backwards (allowing timestamps to represent causality)
* and keeping in sync with the system wall clock so that time values make sense in comparison with the system wall clock,
* including timestamps generated from other processes.
*
* This clock effectively measures time by duration (according to System.nanoTime()),
* in between syncs with the system wall clock.
* When issuing the first timestamp after the sync interval has expired,
* The system wall clock will be read, and the current time set to the max of wall clock time or the most recently issued timestamp.
* All other timestamps are calculated as the wall clock time at last sync + elapsed time since.
*
* This clock deals relatively well when the system wall clock shift is adjusted by small amounts.
* It also deals relatively well when the system wall clock jumps forward by large amounts (this clock will jump with it).
* It does not deal as well with large jumps back in time.
*
* When the system wall clock jumps back in time, this clock will effectively slow down until it is back in sync.
* All syncing timestamps will be the same as the previously issued timestamp.
* The rate by which this clock slows, and therefore the time it takes to resync,
* is determined by how frequently the clock is read.
* If timestamps are only requested at a rate greater than the sync interval,
* all timestamps will have the same value until the clocks synchronize (i.e. this clock will pause).
* If timestamps are requested more frequently than the sync interval,
* timestamps before and after the sync point will under represent the actual elapsed time,
* gradually bringing the clocks back into sync.
*/
public class MonotonicClock implements Clock {
private static final long SYNC_INTERVAL_MILLIS = TimeUnit.SECONDS.toMillis(3);
private final long syncIntervalMillis;
private final TimeSource timeSource;
private final AtomicLong syncMillisRef;
private final AtomicLong syncNanosRef;
private final AtomicLong max = new AtomicLong(0);
public MonotonicClock() {
this(TimeSource.SYSTEM, SYNC_INTERVAL_MILLIS);
}
@VisibleForTesting
public MonotonicClock(TimeSource timeSource, long syncIntervalMillis) {
checkNotNull(timeSource);
long nanoTime = timeSource.nanoTime();
long currentTimeMillis = timeSource.currentTimeMillis();
this.timeSource = timeSource;
this.syncIntervalMillis = syncIntervalMillis;
this.syncNanosRef = new AtomicLong(nanoTime);
this.syncMillisRef = new AtomicLong(currentTimeMillis);
this.max.set(currentTimeMillis);
}
public long getCurrentTime() {
long nowNanos = timeSource.nanoTime();
long syncNanos = syncNanosRef.get();
long syncMillis = syncMillisRef.get();
long sinceSyncNanos = nowNanos - syncNanos;
long sinceSyncMillis = TimeUnit.NANOSECONDS.toMillis(sinceSyncNanos);
if (syncIsDue(nowNanos, syncNanos, sinceSyncMillis)) {
return sync(syncMillis);
} else {
return advance(syncMillis + sinceSyncMillis);
}
}
private boolean syncIsDue(long nowNanos, long syncNanos, long sinceSyncMillis) {
return sinceSyncMillis >= syncIntervalMillis && syncNanosRef.compareAndSet(syncNanos, nowNanos);
}
private long sync(long syncMillis) {
long newSyncMillis = advance(timeSource.currentTimeMillis());
// CAS due to potentially a later, but overlapping, sync having already completed
syncMillisRef.compareAndSet(syncMillis, newSyncMillis);
return newSyncMillis;
}
private long advance(long timestamp) {
long prev;
long next;
do {
prev = max.get();
next = Math.max(prev, timestamp);
} while (!max.compareAndSet(prev, next));
return next;
}
}