/*
* 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 org.jctools_voltpatches.queues;
import static org.jctools_voltpatches.util.JvmInfo.CPUs;
import static org.jctools_voltpatches.util.UnsafeAccess.UNSAFE;
import static org.jctools_voltpatches.util.UnsafeRefArrayAccess.lpElement;
import static org.jctools_voltpatches.util.UnsafeRefArrayAccess.soElement;
import java.util.Objects;
abstract class MpmcArrayQueueL1Pad<E> extends ConcurrentSequencedCircularArrayQueue<E> {
long p00, p01, p02, p03, p04, p05, p06, p07;
long p10, p11, p12, p13, p14, p15, p16;
public MpmcArrayQueueL1Pad(int capacity) {
super(capacity);
}
}
abstract class MpmcArrayQueueProducerField<E> extends MpmcArrayQueueL1Pad<E> {
private final static long P_INDEX_OFFSET;
static {
try {
P_INDEX_OFFSET = UNSAFE.objectFieldOffset(MpmcArrayQueueProducerField.class
.getDeclaredField("producerIndex"));
} catch (NoSuchFieldException e) {
throw new RuntimeException(e);
}
}
private volatile long producerIndex;
public MpmcArrayQueueProducerField(int capacity) {
super(capacity);
}
protected final long lvProducerIndex() {
return producerIndex;
}
protected final boolean casProducerIndex(long expect, long newValue) {
return UNSAFE.compareAndSwapLong(this, P_INDEX_OFFSET, expect, newValue);
}
}
abstract class MpmcArrayQueueL2Pad<E> extends MpmcArrayQueueProducerField<E> {
long p01, p02, p03, p04, p05, p06, p07;
long p10, p11, p12, p13, p14, p15, p16, p17;
public MpmcArrayQueueL2Pad(int capacity) {
super(capacity);
}
}
abstract class MpmcArrayQueueConsumerField<E> extends MpmcArrayQueueL2Pad<E> {
private final static long C_INDEX_OFFSET;
static {
try {
C_INDEX_OFFSET = UNSAFE.objectFieldOffset(MpmcArrayQueueConsumerField.class
.getDeclaredField("consumerIndex"));
} catch (NoSuchFieldException e) {
throw new RuntimeException(e);
}
}
private volatile long consumerIndex;
public MpmcArrayQueueConsumerField(int capacity) {
super(capacity);
}
protected final long lvConsumerIndex() {
return consumerIndex;
}
protected final boolean casConsumerIndex(long expect, long newValue) {
return UNSAFE.compareAndSwapLong(this, C_INDEX_OFFSET, expect, newValue);
}
}
/**
* A Multi-Producer-Multi-Consumer queue based on a {@link ConcurrentCircularArrayQueue}. This implies that
* any and all threads may call the offer/poll/peek methods and correctness is maintained. <br>
* This implementation follows patterns documented on the package level for False Sharing protection.<br>
* The algorithm for offer/poll is an adaptation of the one put forward by D. Vyukov (See <a
* href="http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue">here</a>). The original
* algorithm uses an array of structs which should offer nice locality properties but is sadly not possible in
* Java (waiting on Value Types or similar). The alternative explored here utilizes 2 arrays, one for each
* field of the struct. There is a further alternative in the experimental project which uses iteration phase
* markers to achieve the same algo and is closer structurally to the original, but sadly does not perform as
* well as this implementation.<br>
*
* Tradeoffs to keep in mind:
* <ol>
* <li>Padding for false sharing: counter fields and queue fields are all padded as well as either side of
* both arrays. We are trading memory to avoid false sharing(active and passive).
* <li>2 arrays instead of one: The algorithm requires an extra array of longs matching the size of the
* elements array. This is doubling/tripling the memory allocated for the buffer.
* <li>Power of 2 capacity: Actual elements buffer (and sequence buffer) is the closest power of 2 larger or
* equal to the requested capacity.
* </ol>
*
* @param <E>
* type of the element stored in the {@link java.util.Queue}
*/
public class MpmcArrayQueue<E> extends MpmcArrayQueueConsumerField<E> implements QueueProgressIndicators {
long p01, p02, p03, p04, p05, p06, p07;
long p10, p11, p12, p13, p14, p15, p16, p17;
final static int RECOMENDED_POLL_BATCH = CPUs * 4;
final static int RECOMENDED_OFFER_BATCH = CPUs * 4;
public MpmcArrayQueue(final int capacity) {
super(validateCapacity(capacity));
}
private static int validateCapacity(int capacity) {
if(capacity < 2)
throw new IllegalArgumentException("Minimum size is 2");
return capacity;
}
@Override
public boolean offer(final E e) {
if (null == e) {
throw new NullPointerException();
}
final long mask = this.mask;
final long capacity = mask + 1;
final long[] sBuffer = sequenceBuffer;
long pIndex;
long seqOffset;
long seq;
long cIndex = Long.MAX_VALUE;// start with bogus value, hope we don't need it
do {
pIndex = lvProducerIndex();
seqOffset = calcSequenceOffset(pIndex, mask);
seq = lvSequence(sBuffer, seqOffset);
if (seq < pIndex) { // consumer has not moved this value forward
if (pIndex - capacity <= cIndex && // test against cached cIndex
pIndex - capacity <= (cIndex = lvConsumerIndex())) { // test against latest cIndex
// Extra check required to ensure [Queue.offer == false iff queue is full]
return false;
} else {
seq = pIndex + 1; // hack to make it go around again
}
}
} while (seq > pIndex || // another producer has moved the sequence
!casProducerIndex(pIndex, pIndex + 1)); // failed to increment
assert null == lpElement(buffer, calcElementOffset(pIndex, mask));
soElement(buffer, calcElementOffset(pIndex, mask), e);
soSequence(sBuffer, seqOffset, pIndex + 1); // seq++;
return true;
}
/**
* {@inheritDoc}
* <p>
* Because return null indicates queue is empty we cannot simply rely on next element visibility for poll
* and must test producer index when next element is not visible.
*/
@Override
public E poll() {
// local load of field to avoid repeated loads after volatile reads
final long[] sBuffer = sequenceBuffer;
final long mask = this.mask;
long cIndex;
long seq;
long seqOffset;
long expectedSeq;
long pIndex = -1; // start with bogus value, hope we don't need it
do {
cIndex = lvConsumerIndex();
seqOffset = calcSequenceOffset(cIndex, mask);
seq = lvSequence(sBuffer, seqOffset);
expectedSeq = cIndex + 1;
if (seq < expectedSeq) { // slot has not been moved by producer
if (cIndex >= pIndex && // test against cached pIndex
cIndex == (pIndex = lvProducerIndex())) { // update pIndex if we must
// strict empty check, this ensures [Queue.poll() == null iff isEmpty()]
return null;
} else {
seq = expectedSeq + 1; // trip another go around
}
}
} while (seq > expectedSeq || // another consumer beat us to it
!casConsumerIndex(cIndex, cIndex + 1)); // failed the CAS
final long offset = calcElementOffset(cIndex, mask);
final E e = lpElement(buffer, offset);
assert e != null;
soElement(buffer, offset, null);
soSequence(sBuffer, seqOffset, cIndex + mask + 1);// i.e. seq += capacity
return e;
}
@Override
public E peek() {
long cIndex;
E e;
do {
cIndex = lvConsumerIndex();
// other consumers may have grabbed the element, or queue might be empty
e = lpElement(buffer, calcElementOffset(cIndex));
// only return null if queue is empty
} while (e == null && cIndex != lvProducerIndex());
return e;
}
@Override
public int size() {
/*
* It is possible for a thread to be interrupted or reschedule between the read of the producer and
* consumer indices, therefore protection is required to ensure size is within valid range. In the
* event of concurrent polls/offers to this method the size is OVER estimated as we read consumer
* index BEFORE the producer index.
*/
long after = lvConsumerIndex();
while (true) {
final long before = after;
final long currentProducerIndex = lvProducerIndex();
after = lvConsumerIndex();
if (before == after) {
return (int) (currentProducerIndex - after);
}
}
}
@Override
public boolean isEmpty() {
// Order matters!
// Loading consumer before producer allows for producer increments after consumer index is read.
// This ensures this method is conservative in it's estimate. Note that as this is an MPMC there is
// nothing we can do to make this an exact method.
return (lvConsumerIndex() == lvProducerIndex());
}
@Override
public long currentProducerIndex() {
return lvProducerIndex();
}
@Override
public long currentConsumerIndex() {
return lvConsumerIndex();
}
@Override
public boolean relaxedOffer(E e) {
if (null == e) {
throw new NullPointerException();
}
final long mask = this.mask;
final long[] sBuffer = sequenceBuffer;
long pIndex;
long seqOffset;
long seq;
do {
pIndex = lvProducerIndex();
seqOffset = calcSequenceOffset(pIndex, mask);
seq = lvSequence(sBuffer, seqOffset);
if (seq < pIndex) { // slot not cleared by consumer yet
return false;
}
} while (seq > pIndex || // another producer has moved the sequence
!casProducerIndex(pIndex, pIndex + 1)); // failed to increment
soElement(buffer, calcElementOffset(pIndex, mask), e);
soSequence(sBuffer, seqOffset, pIndex + 1);
return true;
}
@Override
public E relaxedPoll() {
final long[] sBuffer = sequenceBuffer;
final long mask = this.mask;
long cIndex;
long seqOffset;
long seq;
long expectedSeq;
do {
cIndex = lvConsumerIndex();
seqOffset = calcSequenceOffset(cIndex, mask);
seq = lvSequence(sBuffer, seqOffset);
expectedSeq = cIndex + 1;
if (seq < expectedSeq) {
return null;
}
} while (seq > expectedSeq || // another consumer beat us to it
!casConsumerIndex(cIndex, cIndex + 1)); // failed the CAS
final long offset = calcElementOffset(cIndex, mask);
final E e = lpElement(buffer, offset);
soElement(buffer, offset, null);
soSequence(sBuffer, seqOffset, cIndex + mask + 1);
return e;
}
@Override
public E relaxedPeek() {
long currConsumerIndex = lvConsumerIndex();
return lpElement(buffer, calcElementOffset(currConsumerIndex));
}
@Override
public int drain(Consumer<E> c) {
final int capacity = capacity();
int sum = 0;
while (sum < capacity) {
int drained = 0;
if((drained = drain(c, MpmcArrayQueue.RECOMENDED_POLL_BATCH)) == 0) {
break;
}
sum+=drained;
}
return sum;
}
@Override
public int fill(Supplier<E> s) {
long result = 0;// result is a long because we want to have a safepoint check at regular intervals
final int capacity = capacity();
do {
final int filled = fill(s, RECOMENDED_OFFER_BATCH);
if (filled == 0) {
return (int) result;
}
result += filled;
} while (result <= capacity);
return (int) result;
}
@Override
public int drain(Consumer<E> c, int limit) {
final long[] sBuffer = sequenceBuffer;
final long mask = this.mask;
final E[] buffer = this.buffer;
long cIndex;
long seqOffset;
long seq;
long expectedSeq;
for (int i = 0; i < limit; i++) {
do {
cIndex = lvConsumerIndex();
seqOffset = calcSequenceOffset(cIndex, mask);
seq = lvSequence(sBuffer, seqOffset);
expectedSeq = cIndex + 1;
if (seq < expectedSeq) {
return i;
}
} while (seq > expectedSeq || // another consumer beat us to it
!casConsumerIndex(cIndex, cIndex + 1)); // failed the CAS
final long offset = calcElementOffset(cIndex, mask);
final E e = lpElement(buffer, offset);
soElement(buffer, offset, null);
soSequence(sBuffer, seqOffset, cIndex + mask + 1);
c.accept(e);
}
return limit;
}
@Override
public int fill(Supplier<E> s, int limit) {
final long[] sBuffer = sequenceBuffer;
final long mask = this.mask;
final E[] buffer = this.buffer;
long pIndex;
long seqOffset;
long seq;
for (int i = 0; i < limit; i++) {
do {
pIndex = lvProducerIndex();
seqOffset = calcSequenceOffset(pIndex, mask);
seq = lvSequence(sBuffer, seqOffset);
if (seq < pIndex) { // slot not cleared by consumer yet
return i;
}
} while (seq > pIndex || // another producer has moved the sequence
!casProducerIndex(pIndex, pIndex + 1)); // failed to increment
soElement(buffer, calcElementOffset(pIndex, mask), s.get());
soSequence(sBuffer, seqOffset, pIndex + 1);
}
return limit;
}
@Override
public void drain(Consumer<E> c,
WaitStrategy w,
ExitCondition exit) {
int idleCounter = 0;
while (exit.keepRunning()) {
if(drain(c, MpmcArrayQueue.RECOMENDED_POLL_BATCH) == 0) {
idleCounter = w.idle(idleCounter);
continue;
}
idleCounter = 0;
}
}
@Override
public void fill(Supplier<E> s,
WaitStrategy w,
ExitCondition exit) {
int idleCounter = 0;
while (exit.keepRunning()) {
if (fill(s, MpmcArrayQueue.RECOMENDED_OFFER_BATCH) == 0) {
idleCounter = w.idle(idleCounter);
continue;
}
idleCounter = 0;
}
}
}