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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* 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.cassandra.metrics;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import static java.lang.Math.exp;
import com.codahale.metrics.Clock;
/**
* A meter metric which measures mean throughput as well as fifteen-minute and two-hour
* exponentially-weighted moving average throughputs.
*
* This is based heavily on the Meter and EWMA classes from codahale/yammer metrics.
*
* @see EMA
*/
public class RestorableMeter
{
private static final long TICK_INTERVAL = TimeUnit.SECONDS.toNanos(5);
private static final double NANOS_PER_SECOND = TimeUnit.SECONDS.toNanos(1);
private final RestorableEWMA m15Rate;
private final RestorableEWMA m120Rate;
private final AtomicLong count = new AtomicLong();
private final long startTime;
private final AtomicLong lastTick;
private final Clock clock = Clock.defaultClock();
/**
* Creates a new, uninitialized RestorableMeter.
*/
public RestorableMeter()
{
this.m15Rate = new RestorableEWMA(TimeUnit.MINUTES.toSeconds(15));
this.m120Rate = new RestorableEWMA(TimeUnit.MINUTES.toSeconds(120));
this.startTime = this.clock.getTick();
this.lastTick = new AtomicLong(startTime);
}
/**
* Restores a RestorableMeter from the last seen 15m and 2h rates.
* @param lastM15Rate the last-seen 15m rate, in terms of events per second
* @param lastM120Rate the last seen 2h rate, in terms of events per second
*/
public RestorableMeter(double lastM15Rate, double lastM120Rate)
{
this.m15Rate = new RestorableEWMA(lastM15Rate, TimeUnit.MINUTES.toSeconds(15));
this.m120Rate = new RestorableEWMA(lastM120Rate, TimeUnit.MINUTES.toSeconds(120));
this.startTime = this.clock.getTick();
this.lastTick = new AtomicLong(startTime);
}
/**
* Updates the moving averages as needed.
*/
private void tickIfNecessary()
{
final long oldTick = lastTick.get();
final long newTick = clock.getTick();
final long age = newTick - oldTick;
if (age > TICK_INTERVAL)
{
final long newIntervalStartTick = newTick - age % TICK_INTERVAL;
if (lastTick.compareAndSet(oldTick, newIntervalStartTick))
{
final long requiredTicks = age / TICK_INTERVAL;
for (long i = 0; i < requiredTicks; i++)
{
m15Rate.tick();
m120Rate.tick();
}
}
}
}
/**
* Mark the occurrence of an event.
*/
public void mark()
{
mark(1);
}
/**
* Mark the occurrence of a given number of events.
*
* @param n the number of events
*/
public void mark(long n)
{
tickIfNecessary();
count.addAndGet(n);
m15Rate.update(n);
m120Rate.update(n);
}
/**
* Returns the 15-minute rate in terms of events per second. This carries the previous rate when restored.
*/
public double fifteenMinuteRate()
{
tickIfNecessary();
return m15Rate.rate();
}
/**
* Returns the two-hour rate in terms of events per second. This carries the previous rate when restored.
*/
public double twoHourRate()
{
tickIfNecessary();
return m120Rate.rate();
}
/**
* The total number of events that have occurred since this object was created. Note that the previous count
* is *not* carried over when a RestorableMeter is restored.
*/
public long count()
{
return count.get();
}
/**
* Returns the mean rate of events per second since this object was created. Note that the mean rate
* does *not* carry over when a RestorableMeter is restored, so the mean rate is only a measure since
* this object was created.
*/
public double meanRate()
{
if (count() == 0)
{
return 0.0;
} else {
final long elapsed = (clock.getTick() - startTime);
return (count() / (double) elapsed) * NANOS_PER_SECOND;
}
}
static class RestorableEWMA
{
private volatile boolean initialized = false;
private volatile double rate = 0.0; // average rate in terms of events per nanosecond
private final AtomicLong uncounted = new AtomicLong();
private final double alpha, interval;
/**
* Create a new, uninitialized EWMA with a given window.
*
* @param windowInSeconds the window of time this EWMA should average over, expressed as a number of seconds
*/
public RestorableEWMA(long windowInSeconds)
{
this.alpha = 1 - exp((-TICK_INTERVAL / NANOS_PER_SECOND) / windowInSeconds);
this.interval = TICK_INTERVAL;
}
/**
* Restore an EWMA from a last-seen rate and a given window.
*
* @param intervalInSeconds the window of time this EWMA should average over, expressed as a number of seconds
*/
public RestorableEWMA(double lastRate, long intervalInSeconds)
{
this(intervalInSeconds);
this.rate = lastRate / NANOS_PER_SECOND;
this.initialized = true;
}
/**
* Update the moving average with a new value.
*/
public void update(long n)
{
uncounted.addAndGet(n);
}
/**
* Mark the passage of time and decay the current rate accordingly.
*/
public void tick()
{
final long count = uncounted.getAndSet(0);
final double instantRate = count / interval;
if (initialized)
{
rate += (alpha * (instantRate - rate));
}
else
{
rate = instantRate;
initialized = true;
}
}
/**
* Returns the rate in terms of events per second.
*/
public double rate()
{
return rate * NANOS_PER_SECOND;
}
}
}