SingleWriterRecorder.java
/**
* Written by Gil Tene of Azul Systems, and released to the public domain,
* as explained at http://creativecommons.org/publicdomain/zero/1.0/
*
* @author Gil Tene
*/
package org.HdrHistogram;
import java.util.concurrent.atomic.AtomicLong;
/**
* Records integer values, and provides stable interval {@link Histogram} samples from
* live recorded data without interrupting or stalling active recording of values. Each interval
* histogram provided contains all value counts accumulated since the previous interval histogram
* was taken.
* <p>
* This pattern is commonly used in logging interval histogram information while recording is ongoing.
* <p>
* {@link SingleWriterRecorder} expects only a single thread (the "single writer") to
* call {@link SingleWriterRecorder#recordValue} or
* {@link SingleWriterRecorder#recordValueWithExpectedInterval} at any point in time.
* It DOES NOT safely support concurrent recording calls.
* Recording calls are wait-free on architectures that support atomic increment operations, and
* re lock-free on architectures that do not.
* * <p>
* A common pattern for using a {@link SingleWriterRecorder} looks like this:
* <br><pre><code>
* SingleWriterRecorder recorder = new SingleWriterRecorder(2); // Two decimal point accuracy
* Histogram intervalHistogram = null;
* ...
* [start of some loop construct that periodically wants to grab an interval histogram]
* ...
* // Get interval histogram, recycling previous interval histogram:
* intervalHistogram = recorder.getIntervalHistogram(intervalHistogram);
* histogramLogWriter.outputIntervalHistogram(intervalHistogram);
* ...
* [end of loop construct]
* </code></pre>
*/
public class SingleWriterRecorder implements ValueRecorder, IntervalHistogramProvider<Histogram> {
private static AtomicLong instanceIdSequencer = new AtomicLong(1);
private final long instanceId = instanceIdSequencer.getAndIncrement();
private final WriterReaderPhaser recordingPhaser = new WriterReaderPhaser();
private volatile Histogram activeHistogram;
private Histogram inactiveHistogram;
/**
* Construct an auto-resizing {@link SingleWriterRecorder} with a lowest discernible value of
* 1 and an auto-adjusting highestTrackableValue. Can auto-resize up to track values up to (Long.MAX_VALUE / 2).
* <p>
* Depending on the valuer of the <b><code>packed</code></b> parameter {@link SingleWriterRecorder} can be configured to
* track value counts in a packed internal representation optimized for typical histogram recoded values are
* sparse in the value range and tend to be incremented in small unit counts. This packed representation tends
* to require significantly smaller amounts of storage when compared to unpacked representations, but can incur
* additional recording cost due to resizing and repacking operations that may
* occur as previously unrecorded values are encountered.
*
* @param numberOfSignificantValueDigits Specifies the precision to use. This is the number of significant
* decimal digits to which the histogram will maintain value resolution
* and separation. Must be a non-negative integer between 0 and 5.
* @param packed Specifies whether the recorder will uses a packed internal representation or not.
*/
public SingleWriterRecorder(final int numberOfSignificantValueDigits, final boolean packed) {
activeHistogram = packed ?
new PackedInternalHistogram(instanceId, numberOfSignificantValueDigits) :
new InternalHistogram(instanceId, numberOfSignificantValueDigits);
inactiveHistogram = null;
activeHistogram.setStartTimeStamp(System.currentTimeMillis());
}
/**
* Construct an auto-resizing {@link SingleWriterRecorder} with a lowest discernible value of
* 1 and an auto-adjusting highestTrackableValue. Can auto-resize up to track values up to (Long.MAX_VALUE / 2).
*
* @param numberOfSignificantValueDigits Specifies the precision to use. This is the number of significant
* decimal digits to which the histogram will maintain value resolution
* and separation. Must be a non-negative integer between 0 and 5.
*/
public SingleWriterRecorder(final int numberOfSignificantValueDigits) {
this(numberOfSignificantValueDigits, false);
}
/**
* Construct a {@link SingleWriterRecorder} given the highest value to be tracked and a number
* of significant decimal digits. The histogram will be constructed to implicitly track (distinguish from 0)
* values as low as 1.
*
* @param highestTrackableValue The highest value to be tracked by the histogram. Must be a positive
* integer that is {@literal >=} 2.
* @param numberOfSignificantValueDigits Specifies the precision to use. This is the number of significant
* decimal digits to which the histogram will maintain value resolution
* and separation. Must be a non-negative integer between 0 and 5.
*/
public SingleWriterRecorder(final long highestTrackableValue,
final int numberOfSignificantValueDigits) {
this(1, highestTrackableValue, numberOfSignificantValueDigits);
}
/**
* Construct a {@link SingleWriterRecorder} given the Lowest and highest values to be tracked
* and a number of significant decimal digits. Providing a lowestDiscernibleValue is useful is situations where
* the units used for the histogram's values are much smaller that the minimal accuracy required. E.g. when
* tracking time values stated in nanosecond units, where the minimal accuracy required is a microsecond, the
* proper value for lowestDiscernibleValue would be 1000.
*
* @param lowestDiscernibleValue The lowest value that can be tracked (distinguished from 0) by the histogram.
* Must be a positive integer that is {@literal >=} 1. May be internally rounded
* down to nearest power of 2.
* @param highestTrackableValue The highest value to be tracked by the histogram. Must be a positive
* integer that is {@literal >=} (2 * lowestDiscernibleValue).
* @param numberOfSignificantValueDigits Specifies the precision to use. This is the number of significant
* decimal digits to which the histogram will maintain value resolution
* and separation. Must be a non-negative integer between 0 and 5.
*/
public SingleWriterRecorder(final long lowestDiscernibleValue,
final long highestTrackableValue,
final int numberOfSignificantValueDigits) {
activeHistogram = new InternalHistogram(
instanceId, lowestDiscernibleValue, highestTrackableValue, numberOfSignificantValueDigits);
inactiveHistogram = null;
activeHistogram.setStartTimeStamp(System.currentTimeMillis());
}
/**
* Record a value
* @param value the value to record
* @throws ArrayIndexOutOfBoundsException (may throw) if value is exceeds highestTrackableValue
*/
@Override
public void recordValue(final long value) {
long criticalValueAtEnter = recordingPhaser.writerCriticalSectionEnter();
try {
activeHistogram.recordValue(value);
} finally {
recordingPhaser.writerCriticalSectionExit(criticalValueAtEnter);
}
}
/**
* Record a value in the histogram (adding to the value's current count)
*
* @param value The value to be recorded
* @param count The number of occurrences of this value to record
* @throws ArrayIndexOutOfBoundsException (may throw) if value is exceeds highestTrackableValue
*/
@Override
public void recordValueWithCount(final long value, final long count) throws ArrayIndexOutOfBoundsException {
long criticalValueAtEnter = recordingPhaser.writerCriticalSectionEnter();
try {
activeHistogram.recordValueWithCount(value, count);
} finally {
recordingPhaser.writerCriticalSectionExit(criticalValueAtEnter);
}
}
/**
* Record a value
* <p>
* To compensate for the loss of sampled values when a recorded value is larger than the expected
* interval between value samples, Histogram will auto-generate an additional series of decreasingly-smaller
* (down to the expectedIntervalBetweenValueSamples) value records.
* <p>
* See related notes {@link AbstractHistogram#recordValueWithExpectedInterval(long, long)}
* for more explanations about coordinated omission and expected interval correction.
* *
* @param value The value to record
* @param expectedIntervalBetweenValueSamples If expectedIntervalBetweenValueSamples is larger than 0, add
* auto-generated value records as appropriate if value is larger
* than expectedIntervalBetweenValueSamples
* @throws ArrayIndexOutOfBoundsException (may throw) if value is exceeds highestTrackableValue
*/
@Override
public void recordValueWithExpectedInterval(final long value, final long expectedIntervalBetweenValueSamples)
throws ArrayIndexOutOfBoundsException {
long criticalValueAtEnter = recordingPhaser.writerCriticalSectionEnter();
try {
activeHistogram.recordValueWithExpectedInterval(value, expectedIntervalBetweenValueSamples);
} finally {
recordingPhaser.writerCriticalSectionExit(criticalValueAtEnter);
}
}
@Override
public synchronized Histogram getIntervalHistogram() {
return getIntervalHistogram(null);
}
@Override
public synchronized Histogram getIntervalHistogram(Histogram histogramToRecycle) {
return getIntervalHistogram(histogramToRecycle, true);
}
@Override
public synchronized Histogram getIntervalHistogram(Histogram histogramToRecycle,
boolean enforceContainingInstance) {
// Verify that replacement histogram can validly be used as an inactive histogram replacement:
validateFitAsReplacementHistogram(histogramToRecycle, enforceContainingInstance);
inactiveHistogram = histogramToRecycle;
performIntervalSample();
Histogram sampledHistogram = inactiveHistogram;
inactiveHistogram = null; // Once we expose the sample, we can't reuse it internally until it is recycled
return sampledHistogram;
}
@Override
public synchronized void getIntervalHistogramInto(Histogram targetHistogram) {
performIntervalSample();
inactiveHistogram.copyInto(targetHistogram);
}
/**
* Reset any value counts accumulated thus far.
*/
@Override
public synchronized void reset() {
// the currently inactive histogram is reset each time we flip. So flipping twice resets both:
performIntervalSample();
performIntervalSample();
}
private void performIntervalSample() {
try {
recordingPhaser.readerLock();
// Make sure we have an inactive version to flip in:
if (inactiveHistogram == null) {
if (activeHistogram instanceof InternalHistogram) {
inactiveHistogram = new InternalHistogram((InternalHistogram) activeHistogram);
} else if (activeHistogram instanceof PackedInternalHistogram) {
inactiveHistogram = new PackedInternalHistogram(
instanceId, activeHistogram.getNumberOfSignificantValueDigits());
} else {
throw new IllegalStateException("Unexpected internal histogram type for activeHistogram");
}
}
inactiveHistogram.reset();
// Swap active and inactive histograms:
final Histogram tempHistogram = inactiveHistogram;
inactiveHistogram = activeHistogram;
activeHistogram = tempHistogram;
// Mark end time of previous interval and start time of new one:
long now = System.currentTimeMillis();
activeHistogram.setStartTimeStamp(now);
inactiveHistogram.setEndTimeStamp(now);
// Make sure we are not in the middle of recording a value on the previously active histogram:
// Flip phase to make sure no recordings that were in flight pre-flip are still active:
recordingPhaser.flipPhase(500000L /* yield in 0.5 msec units if needed */);
} finally {
recordingPhaser.readerUnlock();
}
}
private static class InternalHistogram extends Histogram {
private final long containingInstanceId;
private InternalHistogram(long id, int numberOfSignificantValueDigits) {
super(numberOfSignificantValueDigits);
this.containingInstanceId = id;
}
private InternalHistogram(long id,
long lowestDiscernibleValue,
long highestTrackableValue,
int numberOfSignificantValueDigits) {
super(lowestDiscernibleValue, highestTrackableValue, numberOfSignificantValueDigits);
this.containingInstanceId = id;
}
private InternalHistogram(InternalHistogram source) {
super(source);
this.containingInstanceId = source.containingInstanceId;
}
}
private static class PackedInternalHistogram extends PackedHistogram {
private final long containingInstanceId;
private PackedInternalHistogram(long id, int numberOfSignificantValueDigits) {
super(numberOfSignificantValueDigits);
this.containingInstanceId = id;
}
}
private void validateFitAsReplacementHistogram(Histogram replacementHistogram,
boolean enforceContainingInstance) {
boolean bad = true;
if (replacementHistogram == null) {
bad = false;
} else if ((replacementHistogram instanceof InternalHistogram)
&&
((!enforceContainingInstance) ||
(((InternalHistogram) replacementHistogram).containingInstanceId ==
((InternalHistogram) activeHistogram).containingInstanceId)
)) {
bad = false;
} else if ((replacementHistogram instanceof PackedInternalHistogram)
&&
((!enforceContainingInstance) ||
(((PackedInternalHistogram) replacementHistogram).containingInstanceId ==
((PackedInternalHistogram) activeHistogram).containingInstanceId)
)) {
bad = false;
}
if (bad) {
throw new IllegalArgumentException("replacement histogram must have been obtained via a previous " +
"getIntervalHistogram() call from this " + this.getClass().getName() +
(enforceContainingInstance ? " instance" : " class"));
}
}
}