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Diffstat (limited to 'src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs')
-rw-r--r-- | src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs | 1486 |
1 files changed, 824 insertions, 662 deletions
diff --git a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs index 7aa5971690..90ada007dd 100644 --- a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs +++ b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs @@ -1,67 +1,100 @@ // Licensed to the .NET Foundation under one or more agreements. // The .NET Foundation licenses this file to you under the MIT license. // See the LICENSE file in the project root for more information. -#pragma warning disable 0420 - -// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ -// -// -// -// A lock-free, concurrent queue primitive, and its associated debugger view type. -// -// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- - -using System; -using System.Collections; using System.Collections.Generic; using System.Diagnostics; -using System.Diagnostics.Contracts; -using System.Runtime.ConstrainedExecution; using System.Runtime.InteropServices; using System.Runtime.Serialization; -using System.Security; -using System.Security.Permissions; using System.Threading; namespace System.Collections.Concurrent { - /// <summary> /// Represents a thread-safe first-in, first-out collection of objects. /// </summary> /// <typeparam name="T">Specifies the type of elements in the queue.</typeparam> /// <remarks> - /// All public and protected members of <see cref="ConcurrentQueue{T}"/> are thread-safe and may be used + /// All public and protected members of <see cref="ConcurrentQueue{T}"/> are thread-safe and may be used /// concurrently from multiple threads. /// </remarks> - [ComVisible(false)] [DebuggerDisplay("Count = {Count}")] [DebuggerTypeProxy(typeof(SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<>))] [Serializable] - public class ConcurrentQueue<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T> + internal class ConcurrentQueue<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T> { - //fields of ConcurrentQueue - [NonSerialized] - private volatile Segment m_head; + // This implementation provides an unbounded, multi-producer multi-consumer queue + // that supports the standard Enqueue/TryDequeue operations, as well as support for + // snapshot enumeration (GetEnumerator, ToArray, CopyTo), peeking, and Count/IsEmpty. + // It is composed of a linked list of bounded ring buffers, each of which has a head + // and a tail index, isolated from each other to minimize false sharing. As long as + // the number of elements in the queue remains less than the size of the current + // buffer (Segment), no additional allocations are required for enqueued items. When + // the number of items exceeds the size of the current segment, the current segment is + // "frozen" to prevent further enqueues, and a new segment is linked from it and set + // as the new tail segment for subsequent enqueues. As old segments are consumed by + // dequeues, the head reference is updated to point to the segment that dequeuers should + // try next. To support snapshot enumeration, segments also support the notion of + // preserving for observation, whereby they avoid overwriting state as part of dequeues. + // Any operation that requires a snapshot results in all current segments being + // both frozen for enqueues and preserved for observation: any new enqueues will go + // to new segments, and dequeuers will consume from the existing segments but without + // overwriting the existing data. + + /// <summary>Initial length of the segments used in the queue.</summary> + private const int InitialSegmentLength = 32; + /// <summary> + /// Maximum length of the segments used in the queue. This is a somewhat arbitrary limit: + /// larger means that as long as we don't exceed the size, we avoid allocating more segments, + /// but if we do exceed it, then the segment becomes garbage. + /// </summary> + private const int MaxSegmentLength = 1024 * 1024; + /// <summary> + /// Lock used to protect cross-segment operations, including any updates to <see cref="_tail"/> or <see cref="_head"/> + /// and any operations that need to get a consistent view of them. + /// </summary> [NonSerialized] - private volatile Segment m_tail; - - private T[] m_serializationArray; // Used for custom serialization. - - private const int SEGMENT_SIZE = 32; - - //number of snapshot takers, GetEnumerator(), ToList() and ToArray() operations take snapshot. + private object _crossSegmentLock; + /// <summary>The current tail segment.</summary> [NonSerialized] - internal volatile int m_numSnapshotTakers = 0; + private volatile Segment _tail; + /// <summary>The current head segment.</summary> + [NonSerialized] + private volatile Segment _head; + /// <summary>Field used to temporarily store the contents of the queue for serialization.</summary> + private T[] _serializationArray; /// <summary> /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> class. /// </summary> public ConcurrentQueue() { - m_head = m_tail = new Segment(0, this); + _crossSegmentLock = new object(); + _tail = _head = new Segment(InitialSegmentLength); + } + + /// <summary>Set the data array to be serialized.</summary> + [OnSerializing] + private void OnSerializing(StreamingContext context) + { + _serializationArray = ToArray(); + } + + /// <summary>Clear the data array that was serialized.</summary> + [OnSerialized] + private void OnSerialized(StreamingContext context) + { + _serializationArray = null; + } + + /// <summary>Construct the queue from the deserialized <see cref="_serializationArray"/>.</summary> + [OnDeserialized] + private void OnDeserialized(StreamingContext context) + { + Debug.Assert(_serializationArray != null); + InitializeFromCollection(_serializationArray); + _serializationArray = null; } /// <summary> @@ -70,34 +103,39 @@ namespace System.Collections.Concurrent /// <param name="collection">A collection from which to copy elements.</param> private void InitializeFromCollection(IEnumerable<T> collection) { - Segment localTail = new Segment(0, this);//use this local variable to avoid the extra volatile read/write. this is safe because it is only called from ctor - m_head = localTail; - - int index = 0; - foreach (T element in collection) + _crossSegmentLock = new object(); + + // Determine the initial segment size. We'll use the default, + // unless the collection is known to be larger than than, in which + // case we round its length up to a power of 2, as all segments must + // be a power of 2 in length. + int length = InitialSegmentLength; + var c = collection as ICollection<T>; + if (c != null) { - Debug.Assert(index >= 0 && index < SEGMENT_SIZE); - localTail.UnsafeAdd(element); - index++; - - if (index >= SEGMENT_SIZE) + int count = c.Count; + if (count > length) { - localTail = localTail.UnsafeGrow(); - index = 0; + length = RoundUpToPowerOf2(count); } } - m_tail = localTail; + // Initialize the segment and add all of the data to it. + _tail = _head = new Segment(length); + foreach (T item in collection) + { + Enqueue(item); + } } /// <summary> - /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> - /// class that contains elements copied from the specified collection + /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> class that contains elements copied + /// from the specified collection. /// </summary> - /// <param name="collection">The collection whose elements are copied to the new <see - /// cref="ConcurrentQueue{T}"/>.</param> - /// <exception cref="T:System.ArgumentNullException">The <paramref name="collection"/> argument is - /// null.</exception> + /// <param name="collection"> + /// The collection whose elements are copied to the new <see cref="ConcurrentQueue{T}"/>. + /// </param> + /// <exception cref="System.ArgumentNullException">The <paramref name="collection"/> argument is null.</exception> public ConcurrentQueue(IEnumerable<T> collection) { if (collection == null) @@ -109,37 +147,15 @@ namespace System.Collections.Concurrent } /// <summary> - /// Get the data array to be serialized - /// </summary> - [OnSerializing] - private void OnSerializing(StreamingContext context) - { - // save the data into the serialization array to be saved - m_serializationArray = ToArray(); - } - - /// <summary> - /// Construct the queue from a previously seiralized one - /// </summary> - [OnDeserialized] - private void OnDeserialized(StreamingContext context) - { - Debug.Assert(m_serializationArray != null); - InitializeFromCollection(m_serializationArray); - m_serializationArray = null; - } - - /// <summary> - /// Copies the elements of the <see cref="T:System.Collections.ICollection"/> to an <see - /// cref="T:System.Array"/>, starting at a particular - /// <see cref="T:System.Array"/> index. + /// Copies the elements of the <see cref="ICollection"/> to an <see + /// cref="Array"/>, starting at a particular <see cref="Array"/> index. /// </summary> - /// <param name="array">The one-dimensional <see cref="T:System.Array">Array</see> that is the - /// destination of the elements copied from the - /// <see cref="T:System.Collections.Concurrent.ConcurrentBag"/>. The <see - /// cref="T:System.Array">Array</see> must have zero-based indexing.</param> - /// <param name="index">The zero-based index in <paramref name="array"/> at which copying - /// begins.</param> + /// <param name="array"> + /// The one-dimensional <see cref="Array">Array</see> that is the destination of the + /// elements copied from the <see cref="ConcurrentQueue{T}"/>. <paramref name="array"/> must have + /// zero-based indexing. + /// </param> + /// <param name="index">The zero-based index in <paramref name="array"/> at which copying begins.</param> /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in /// Visual Basic).</exception> /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than @@ -148,100 +164,52 @@ namespace System.Collections.Concurrent /// <paramref name="array"/> is multidimensional. -or- /// <paramref name="array"/> does not have zero-based indexing. -or- /// <paramref name="index"/> is equal to or greater than the length of the <paramref name="array"/> - /// -or- The number of elements in the source <see cref="T:System.Collections.ICollection"/> is + /// -or- The number of elements in the source <see cref="ICollection"/> is /// greater than the available space from <paramref name="index"/> to the end of the destination /// <paramref name="array"/>. -or- The type of the source <see - /// cref="T:System.Collections.ICollection"/> cannot be cast automatically to the type of the + /// cref="ICollection"/> cannot be cast automatically to the type of the /// destination <paramref name="array"/>. /// </exception> void ICollection.CopyTo(Array array, int index) { + // Special-case when the Array is actually a T[], taking a faster path + T[] szArray = array as T[]; + if (szArray != null) + { + CopyTo(szArray, index); + return; + } + // Validate arguments. if (array == null) { throw new ArgumentNullException(nameof(array)); } - // We must be careful not to corrupt the array, so we will first accumulate an - // internal list of elements that we will then copy to the array. This requires - // some extra allocation, but is necessary since we don't know up front whether - // the array is sufficiently large to hold the stack's contents. - ((ICollection)ToList()).CopyTo(array, index); + // Otherwise, fall back to the slower path that first copies the contents + // to an array, and then uses that array's non-generic CopyTo to do the copy. + ToArray().CopyTo(array, index); } /// <summary> - /// Gets a value indicating whether access to the <see cref="T:System.Collections.ICollection"/> is + /// Gets a value indicating whether access to the <see cref="ICollection"/> is /// synchronized with the SyncRoot. /// </summary> - /// <value>true if access to the <see cref="T:System.Collections.ICollection"/> is synchronized + /// <value>true if access to the <see cref="ICollection"/> is synchronized /// with the SyncRoot; otherwise, false. For <see cref="ConcurrentQueue{T}"/>, this property always /// returns false.</value> - bool ICollection.IsSynchronized - { - // Gets a value indicating whether access to this collection is synchronized. Always returns - // false. The reason is subtle. While access is in face thread safe, it's not the case that - // locking on the SyncRoot would have prevented concurrent pushes and pops, as this property - // would typically indicate; that's because we internally use CAS operations vs. true locks. - get { return false; } - } - + bool ICollection.IsSynchronized => false; // always false, as true implies synchronization via SyncRoot /// <summary> /// Gets an object that can be used to synchronize access to the <see - /// cref="T:System.Collections.ICollection"/>. This property is not supported. - /// </summary> - /// <exception cref="T:System.NotSupportedException">The SyncRoot property is not supported.</exception> - object ICollection.SyncRoot - { - get - { - throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported")); - } - } - - /// <summary> - /// Returns an enumerator that iterates through a collection. - /// </summary> - /// <returns>An <see cref="T:System.Collections.IEnumerator"/> that can be used to iterate through the collection.</returns> - IEnumerator IEnumerable.GetEnumerator() - { - return ((IEnumerable<T>)this).GetEnumerator(); - } - - /// <summary> - /// Attempts to add an object to the <see - /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. + /// cref="ICollection"/>. This property is not supported. /// </summary> - /// <param name="item">The object to add to the <see - /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. The value can be a null - /// reference (Nothing in Visual Basic) for reference types. - /// </param> - /// <returns>true if the object was added successfully; otherwise, false.</returns> - /// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will always add the object to the - /// end of the <see cref="ConcurrentQueue{T}"/> - /// and return true.</remarks> - bool IProducerConsumerCollection<T>.TryAdd(T item) - { - Enqueue(item); - return true; - } + /// <exception cref="NotSupportedException">The SyncRoot property is not supported.</exception> + object ICollection.SyncRoot { get { throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported")); } } - /// <summary> - /// Attempts to remove and return an object from the <see - /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. - /// </summary> - /// <param name="item"> - /// When this method returns, if the operation was successful, <paramref name="item"/> contains the - /// object removed. If no object was available to be removed, the value is unspecified. - /// </param> - /// <returns>true if an element was removed and returned succesfully; otherwise, false.</returns> - /// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will attempt to remove the object - /// from the beginning of the <see cref="ConcurrentQueue{T}"/>. - /// </remarks> - bool IProducerConsumerCollection<T>.TryTake(out T item) - { - return TryDequeue(out item); - } + /// <summary>Returns an enumerator that iterates through a collection.</summary> + /// <returns>An <see cref="IEnumerator"/> that can be used to iterate through the collection.</returns> + IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator(); /// <summary> /// Gets a value that indicates whether the <see cref="ConcurrentQueue{T}"/> is empty. @@ -258,126 +226,43 @@ namespace System.Collections.Concurrent { get { - Segment head = m_head; - if (!head.IsEmpty) - //fast route 1: - //if current head is not empty, then queue is not empty - return false; - else if (head.Next == null) - //fast route 2: - //if current head is empty and it's the last segment - //then queue is empty - return true; - else - //slow route: - //current head is empty and it is NOT the last segment, - //it means another thread is growing new segment - { - SpinWait spin = new SpinWait(); - while (head.IsEmpty) - { - if (head.Next == null) - return true; - - spin.SpinOnce(); - head = m_head; - } - return false; - } + // IsEmpty == !TryPeek. We use a "resultUsed:false" peek in order to avoid marking + // segments as preserved for observation, making IsEmpty a cheaper way than either + // TryPeek(out T) or Count == 0 to check whether any elements are in the queue. + T ignoredResult; + return !TryPeek(out ignoredResult, resultUsed: false); } } - /// <summary> - /// Copies the elements stored in the <see cref="ConcurrentQueue{T}"/> to a new array. - /// </summary> - /// <returns>A new array containing a snapshot of elements copied from the <see - /// cref="ConcurrentQueue{T}"/>.</returns> + /// <summary>Copies the elements stored in the <see cref="ConcurrentQueue{T}"/> to a new array.</summary> + /// <returns>A new array containing a snapshot of elements copied from the <see cref="ConcurrentQueue{T}"/>.</returns> public T[] ToArray() { - return ToList().ToArray(); - } + // Snap the current contents for enumeration. + Segment head, tail; + int headHead, tailTail; + SnapForObservation(out head, out headHead, out tail, out tailTail); - /// <summary> - /// Copies the <see cref="ConcurrentQueue{T}"/> elements to a new <see - /// cref="T:System.Collections.Generic.List{T}"/>. - /// </summary> - /// <returns>A new <see cref="T:System.Collections.Generic.List{T}"/> containing a snapshot of - /// elements copied from the <see cref="ConcurrentQueue{T}"/>.</returns> - private List<T> ToList() - { - // Increments the number of active snapshot takers. This increment must happen before the snapshot is - // taken. At the same time, Decrement must happen after list copying is over. Only in this way, can it - // eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0. - Interlocked.Increment(ref m_numSnapshotTakers); + // Count the number of items in that snapped set, and use it to allocate an + // array of the right size. + long count = GetCount(head, headHead, tail, tailTail); + T[] arr = new T[count]; - List<T> list = new List<T>(); - try + // Now enumerate the contents, copying each element into the array. + using (IEnumerator<T> e = Enumerate(head, headHead, tail, tailTail)) { - //store head and tail positions in buffer, - Segment head, tail; - int headLow, tailHigh; - GetHeadTailPositions(out head, out tail, out headLow, out tailHigh); - - if (head == tail) - { - head.AddToList(list, headLow, tailHigh); - } - else + int i = 0; + while (e.MoveNext()) { - head.AddToList(list, headLow, SEGMENT_SIZE - 1); - Segment curr = head.Next; - while (curr != tail) - { - curr.AddToList(list, 0, SEGMENT_SIZE - 1); - curr = curr.Next; - } - //Add tail segment - tail.AddToList(list, 0, tailHigh); + arr[i++] = e.Current; } + Debug.Assert(count == i); } - finally - { - // This Decrement must happen after copying is over. - Interlocked.Decrement(ref m_numSnapshotTakers); - } - return list; - } - /// <summary> - /// Store the position of the current head and tail positions. - /// </summary> - /// <param name="head">return the head segment</param> - /// <param name="tail">return the tail segment</param> - /// <param name="headLow">return the head offset, value range [0, SEGMENT_SIZE]</param> - /// <param name="tailHigh">return the tail offset, value range [-1, SEGMENT_SIZE-1]</param> - private void GetHeadTailPositions(out Segment head, out Segment tail, - out int headLow, out int tailHigh) - { - head = m_head; - tail = m_tail; - headLow = head.Low; - tailHigh = tail.High; - SpinWait spin = new SpinWait(); - - //we loop until the observed values are stable and sensible. - //This ensures that any update order by other methods can be tolerated. - while ( - //if head and tail changed, retry - head != m_head || tail != m_tail - //if low and high pointers, retry - || headLow != head.Low || tailHigh != tail.High - //if head jumps ahead of tail because of concurrent grow and dequeue, retry - || head.m_index > tail.m_index) - { - spin.SpinOnce(); - head = m_head; - tail = m_tail; - headLow = head.Low; - tailHigh = tail.High; - } + // And return it. + return arr; } - /// <summary> /// Gets the number of elements contained in the <see cref="ConcurrentQueue{T}"/>. /// </summary> @@ -391,38 +276,140 @@ namespace System.Collections.Concurrent { get { - //store head and tail positions in buffer, Segment head, tail; - int headLow, tailHigh; - GetHeadTailPositions(out head, out tail, out headLow, out tailHigh); - - if (head == tail) + int headHead, headTail, tailHead, tailTail; + var spinner = new SpinWait(); + while (true) { - return tailHigh - headLow + 1; + // Capture the head and tail, as well as the head's head and tail. + head = _head; + tail = _tail; + headHead = Volatile.Read(ref head._headAndTail.Head); + headTail = Volatile.Read(ref head._headAndTail.Tail); + + if (head == tail) + { + // There was a single segment in the queue. If the captured + // values still (or again) represent reality, return the segment's + // count. A single segment should be the most common case once the + // queue's size has stabilized after segments have grown to + // the point where growing is no longer needed. + if (head == _head && + head == _tail && + headHead == Volatile.Read(ref head._headAndTail.Head) && + headTail == Volatile.Read(ref head._headAndTail.Tail)) + { + return GetCount(head, headHead, headTail); + } + } + else if (head._nextSegment == tail) + { + // There were two segments in the queue. Get the positions + // from the tail, and if the captured values still (or again) match + // reality, return the sum of the counts from both segments. + tailHead = Volatile.Read(ref tail._headAndTail.Head); + tailTail = Volatile.Read(ref tail._headAndTail.Tail); + if (head == _head && + tail == _tail && + headHead == Volatile.Read(ref head._headAndTail.Head) && + headTail == Volatile.Read(ref head._headAndTail.Tail) && + tailHead == Volatile.Read(ref tail._headAndTail.Head) && + tailTail == Volatile.Read(ref tail._headAndTail.Tail)) + { + // We got stable values, so we can just compute the sizes based on those + // values and return the sum of the counts of the segments. + return GetCount(head, headHead, headTail) + GetCount(tail, tailHead, tailTail); + } + } + else + { + // There were more than two segments. Take the slower path, where we freeze the + // queue and then count the now stable segments. + SnapForObservation(out head, out headHead, out tail, out tailTail); + return unchecked((int)GetCount(head, headHead, tail, tailTail)); + } + + // We raced with enqueues/dequeues and captured an inconsistent picture of the queue. + // Spin and try again. + spinner.SpinOnce(); } + } + } - //head segment - int count = SEGMENT_SIZE - headLow; + /// <summary>Computes the number of items in a segment based on a fixed head and tail in that segment.</summary> + private static int GetCount(Segment s, int head, int tail) + { + if (head != tail && head != tail - s.FreezeOffset) + { + head &= s._slotsMask; + tail &= s._slotsMask; + return head < tail ? tail - head : s._slots.Length - head + tail; + } + return 0; + } - //middle segment(s), if any, are full. - //We don't deal with overflow to be consistent with the behavior of generic types in CLR. - count += SEGMENT_SIZE * ((int)(tail.m_index - head.m_index - 1)); + /// <summary>Gets the number of items in snapped region.</summary> + private static long GetCount(Segment head, int headHead, Segment tail, int tailTail) + { + // All of the segments should have been both frozen for enqueues and preserved for observation. + // Validate that here for head and tail; we'll validate it for intermediate segments later. + Debug.Assert(head._preservedForObservation); + Debug.Assert(head._frozenForEnqueues); + Debug.Assert(tail._preservedForObservation); + Debug.Assert(tail._frozenForEnqueues); + + long count = 0; + + // Head segment. We've already marked it as frozen for enqueues, so its tail position is fixed, + // and we've already marked it as preserved for observation (before we grabbed the head), so we + // can safely enumerate from its head to its tail and access its elements. + int headTail = (head == tail ? tailTail : Volatile.Read(ref head._headAndTail.Tail)) - head.FreezeOffset; + if (headHead < headTail) + { + // Mask the head and tail for the head segment + headHead &= head._slotsMask; + headTail &= head._slotsMask; + + // Increase the count by either the one or two regions, based on whether tail + // has wrapped to be less than head. + count += headHead < headTail ? + headTail - headHead : + head._slots.Length - headHead + headTail; + } - //tail segment - count += tailHigh + 1; + // We've enumerated the head. If the tail is different from the head, we need to + // enumerate the remaining segments. + if (head != tail) + { + // Count the contents of each segment between head and tail, not including head and tail. + // Since there were segments before these, for our purposes we consider them to start at + // the 0th element, and since there is at least one segment after each, each was frozen + // by the time we snapped it, so we can iterate until each's frozen tail. + for (Segment s = head._nextSegment; s != tail; s = s._nextSegment) + { + Debug.Assert(s._preservedForObservation); + Debug.Assert(s._frozenForEnqueues); + count += s._headAndTail.Tail - s.FreezeOffset; + } - return count; + // Finally, enumerate the tail. As with the intermediate segments, there were segments + // before this in the snapped region, so we can start counting from the beginning. Unlike + // the intermediate segments, we can't just go until the Tail, as that could still be changing; + // instead we need to go until the tail we snapped for observation. + count += tailTail - tail.FreezeOffset; } - } + // Return the computed count. + return count; + } /// <summary> /// Copies the <see cref="ConcurrentQueue{T}"/> elements to an existing one-dimensional <see - /// cref="T:System.Array">Array</see>, starting at the specified array index. + /// cref="Array">Array</see>, starting at the specified array index. /// </summary> - /// <param name="array">The one-dimensional <see cref="T:System.Array">Array</see> that is the + /// <param name="array">The one-dimensional <see cref="Array">Array</see> that is the /// destination of the elements copied from the - /// <see cref="ConcurrentQueue{T}"/>. The <see cref="T:System.Array">Array</see> must have zero-based + /// <see cref="ConcurrentQueue{T}"/>. The <see cref="Array">Array</see> must have zero-based /// indexing.</param> /// <param name="index">The zero-based index in <paramref name="array"/> at which copying /// begins.</param> @@ -442,19 +429,36 @@ namespace System.Collections.Concurrent { throw new ArgumentNullException(nameof(array)); } + if (index < 0) + { + throw new ArgumentOutOfRangeException(nameof(index)); + } - // We must be careful not to corrupt the array, so we will first accumulate an - // internal list of elements that we will then copy to the array. This requires - // some extra allocation, but is necessary since we don't know up front whether - // the array is sufficiently large to hold the stack's contents. - ToList().CopyTo(array, index); - } + // Snap for enumeration + Segment head, tail; + int headHead, tailTail; + SnapForObservation(out head, out headHead, out tail, out tailTail); + // Get the number of items to be enumerated + long count = GetCount(head, headHead, tail, tailTail); + if (index > array.Length - count) + { + throw new ArgumentException(Environment.GetResourceString("Arg_ArrayPlusOffTooSmall")); + } - /// <summary> - /// Returns an enumerator that iterates through the <see - /// cref="ConcurrentQueue{T}"/>. - /// </summary> + // Copy the items to the target array + int i = index; + using (IEnumerator<T> e = Enumerate(head, headHead, tail, tailTail)) + { + while (e.MoveNext()) + { + array[i++] = e.Current; + } + } + Debug.Assert(count == i - index); + } + + /// <summary>Returns an enumerator that iterates through the <see cref="ConcurrentQueue{T}"/>.</summary> /// <returns>An enumerator for the contents of the <see /// cref="ConcurrentQueue{T}"/>.</returns> /// <remarks> @@ -465,124 +469,195 @@ namespace System.Collections.Concurrent /// </remarks> public IEnumerator<T> GetEnumerator() { - // Increments the number of active snapshot takers. This increment must happen before the snapshot is - // taken. At the same time, Decrement must happen after the enumeration is over. Only in this way, can it - // eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0. - Interlocked.Increment(ref m_numSnapshotTakers); - - // Takes a snapshot of the queue. - // A design flaw here: if a Thread.Abort() happens, we cannot decrement m_numSnapshotTakers. But we cannot - // wrap the following with a try/finally block, otherwise the decrement will happen before the yield return - // statements in the GetEnumerator (head, tail, headLow, tailHigh) method. Segment head, tail; - int headLow, tailHigh; - GetHeadTailPositions(out head, out tail, out headLow, out tailHigh); - - //If we put yield-return here, the iterator will be lazily evaluated. As a result a snapshot of - // the queue is not taken when GetEnumerator is initialized but when MoveNext() is first called. - // This is inconsistent with existing generic collections. In order to prevent it, we capture the - // value of m_head in a buffer and call out to a helper method. - //The old way of doing this was to return the ToList().GetEnumerator(), but ToList() was an - // unnecessary perfomance hit. - return GetEnumerator(head, tail, headLow, tailHigh); + int headHead, tailTail; + SnapForObservation(out head, out headHead, out tail, out tailTail); + return Enumerate(head, headHead, tail, tailTail); } /// <summary> - /// Helper method of GetEnumerator to seperate out yield return statement, and prevent lazy evaluation. + /// Gets the head and tail information of the current contents of the queue. + /// After this call returns, the specified region can be enumerated any number + /// of times and will not change. /// </summary> - private IEnumerator<T> GetEnumerator(Segment head, Segment tail, int headLow, int tailHigh) + private void SnapForObservation(out Segment head, out int headHead, out Segment tail, out int tailTail) { - try + lock (_crossSegmentLock) // _head and _tail may only change while the lock is held. { - SpinWait spin = new SpinWait(); + // Snap the head and tail + head = _head; + tail = _tail; + Debug.Assert(head != null); + Debug.Assert(tail != null); + Debug.Assert(tail._nextSegment == null); + + // Mark them and all segments in between as preserving, and ensure no additional items + // can be added to the tail. + for (Segment s = head; ; s = s._nextSegment) + { + s._preservedForObservation = true; + if (s == tail) break; + Debug.Assert(s._frozenForEnqueues); // any non-tail should already be marked + } + tail.EnsureFrozenForEnqueues(); // we want to prevent the tailTail from moving - if (head == tail) + // At this point, any dequeues from any segment won't overwrite the value, and + // none of the existing segments can have new items enqueued. + + headHead = Volatile.Read(ref head._headAndTail.Head); + tailTail = Volatile.Read(ref tail._headAndTail.Tail); + } + } + + /// <summary>Gets the item stored in the <paramref name="i"/>th entry in <paramref name="segment"/>.</summary> + private T GetItemWhenAvailable(Segment segment, int i) + { + Debug.Assert(segment._preservedForObservation); + + // Get the expected value for the sequence number + int expectedSequenceNumberAndMask = (i + 1) & segment._slotsMask; + + // If the expected sequence number is not yet written, we're still waiting for + // an enqueuer to finish storing it. Spin until it's there. + if ((segment._slots[i].SequenceNumber & segment._slotsMask) != expectedSequenceNumberAndMask) + { + var spinner = new SpinWait(); + while ((Volatile.Read(ref segment._slots[i].SequenceNumber) & segment._slotsMask) != expectedSequenceNumberAndMask) { - for (int i = headLow; i <= tailHigh; i++) - { - // If the position is reserved by an Enqueue operation, but the value is not written into, - // spin until the value is available. - spin.Reset(); - while (!head.m_state[i].m_value) - { - spin.SpinOnce(); - } - yield return head.m_array[i]; - } + spinner.SpinOnce(); + } + } + + // Return the value from the slot. + return segment._slots[i].Item; + } + + private IEnumerator<T> Enumerate(Segment head, int headHead, Segment tail, int tailTail) + { + Debug.Assert(head._preservedForObservation); + Debug.Assert(head._frozenForEnqueues); + Debug.Assert(tail._preservedForObservation); + Debug.Assert(tail._frozenForEnqueues); + + // Head segment. We've already marked it as not accepting any more enqueues, + // so its tail position is fixed, and we've already marked it as preserved for + // enumeration (before we grabbed its head), so we can safely enumerate from + // its head to its tail. + int headTail = (head == tail ? tailTail : Volatile.Read(ref head._headAndTail.Tail)) - head.FreezeOffset; + if (headHead < headTail) + { + headHead &= head._slotsMask; + headTail &= head._slotsMask; + + if (headHead < headTail) + { + for (int i = headHead; i < headTail; i++) yield return GetItemWhenAvailable(head, i); } else { - //iterate on head segment - for (int i = headLow; i < SEGMENT_SIZE; i++) - { - // If the position is reserved by an Enqueue operation, but the value is not written into, - // spin until the value is available. - spin.Reset(); - while (!head.m_state[i].m_value) - { - spin.SpinOnce(); - } - yield return head.m_array[i]; - } - //iterate on middle segments - Segment curr = head.Next; - while (curr != tail) - { - for (int i = 0; i < SEGMENT_SIZE; i++) - { - // If the position is reserved by an Enqueue operation, but the value is not written into, - // spin until the value is available. - spin.Reset(); - while (!curr.m_state[i].m_value) - { - spin.SpinOnce(); - } - yield return curr.m_array[i]; - } - curr = curr.Next; - } + for (int i = headHead; i < head._slots.Length; i++) yield return GetItemWhenAvailable(head, i); + for (int i = 0; i < headTail; i++) yield return GetItemWhenAvailable(head, i); + } + } - //iterate on tail segment - for (int i = 0; i <= tailHigh; i++) + // We've enumerated the head. If the tail is the same, we're done. + if (head != tail) + { + // Each segment between head and tail, not including head and tail. Since there were + // segments before these, for our purposes we consider it to start at the 0th element. + for (Segment s = head._nextSegment; s != tail; s = s._nextSegment) + { + Debug.Assert(s._preservedForObservation, "Would have had to been preserved as a segment part of enumeration"); + Debug.Assert(s._frozenForEnqueues, "Would have had to be frozen for enqueues as it's intermediate"); + + int sTail = s._headAndTail.Tail - s.FreezeOffset; + for (int i = 0; i < sTail; i++) { - // If the position is reserved by an Enqueue operation, but the value is not written into, - // spin until the value is available. - spin.Reset(); - while (!tail.m_state[i].m_value) - { - spin.SpinOnce(); - } - yield return tail.m_array[i]; + yield return GetItemWhenAvailable(s, i); } } - } - finally - { - // This Decrement must happen after the enumeration is over. - Interlocked.Decrement(ref m_numSnapshotTakers); + + // Enumerate the tail. Since there were segments before this, we can just start at + // its beginning, and iterate until the tail we already grabbed. + tailTail -= tail.FreezeOffset; + for (int i = 0; i < tailTail; i++) + { + yield return GetItemWhenAvailable(tail, i); + } } } - /// <summary> - /// Adds an object to the end of the <see cref="ConcurrentQueue{T}"/>. - /// </summary> - /// <param name="item">The object to add to the end of the <see - /// cref="ConcurrentQueue{T}"/>. The value can be a null reference - /// (Nothing in Visual Basic) for reference types. + /// <summary>Round the specified value up to the next power of 2, if it isn't one already.</summary> + private static int RoundUpToPowerOf2(int i) + { + --i; + i |= i >> 1; + i |= i >> 2; + i |= i >> 4; + i |= i >> 8; + i |= i >> 16; + return i + 1; + } + + /// <summary>Adds an object to the end of the <see cref="ConcurrentQueue{T}"/>.</summary> + /// <param name="item"> + /// The object to add to the end of the <see cref="ConcurrentQueue{T}"/>. + /// The value can be a null reference (Nothing in Visual Basic) for reference types. /// </param> public void Enqueue(T item) { - SpinWait spin = new SpinWait(); + // Try to enqueue to the current tail. + if (!_tail.TryEnqueue(item)) + { + // If we're unable to, we need to take a slow path that will + // try to add a new tail segment. + EnqueueSlow(item); + } + } + + /// <summary>Adds to the end of the queue, adding a new segment if necessary.</summary> + private void EnqueueSlow(T item) + { while (true) { - Segment tail = m_tail; - if (tail.TryAppend(item)) + Segment tail = _tail; + + // Try to append to the existing tail. + if (tail.TryEnqueue(item)) + { return; - spin.SpinOnce(); + } + + // If we were unsuccessful, take the lock so that we can compare and manipulate + // the tail. Assuming another enqueuer hasn't already added a new segment, + // do so, then loop around to try enqueueing again. + lock (_crossSegmentLock) + { + if (tail == _tail) + { + // Make sure no one else can enqueue to this segment. + tail.EnsureFrozenForEnqueues(); + + // We determine the new segment's length based on the old length. + // In general, we double the size of the segment, to make it less likely + // that we'll need to grow again. However, if the tail segment is marked + // as preserved for observation, something caused us to avoid reusing this + // segment, and if that happens a lot and we grow, we'll end up allocating + // lots of wasted space. As such, in such situations we reset back to the + // initial segment length; if these observations are happening frequently, + // this will help to avoid wasted memory, and if they're not, we'll + // relatively quickly grow again to a larger size. + int nextSize = tail._preservedForObservation ? InitialSegmentLength : tail.Capacity * 2; + var newTail = new Segment(nextSize); + + // Hook up the new tail. + tail._nextSegment = newTail; + _tail = newTail; + } + } } } - /// <summary> /// Attempts to remove and return the object at the beginning of the <see /// cref="ConcurrentQueue{T}"/>. @@ -591,369 +666,456 @@ namespace System.Collections.Concurrent /// When this method returns, if the operation was successful, <paramref name="result"/> contains the /// object removed. If no object was available to be removed, the value is unspecified. /// </param> - /// <returns>true if an element was removed and returned from the beggining of the <see - /// cref="ConcurrentQueue{T}"/> - /// succesfully; otherwise, false.</returns> - public bool TryDequeue(out T result) + /// <returns> + /// true if an element was removed and returned from the beginning of the + /// <see cref="ConcurrentQueue{T}"/> successfully; otherwise, false. + /// </returns> + public bool TryDequeue(out T result) => + _head.TryDequeue(out result) || // fast-path that operates just on the head segment + TryDequeueSlow(out result); // slow path that needs to fix up segments + + /// <summary>Tries to dequeue an item, removing empty segments as needed.</summary> + private bool TryDequeueSlow(out T item) { - while (!IsEmpty) + while (true) { - Segment head = m_head; - if (head.TryRemove(out result)) + // Get the current head + Segment head = _head; + + // Try to take. If we're successful, we're done. + if (head.TryDequeue(out item)) + { + return true; + } + + // Check to see whether this segment is the last. If it is, we can consider + // this to be a moment-in-time empty condition (even though between the TryDequeue + // check and this check, another item could have arrived). + if (head._nextSegment == null) + { + item = default(T); + return false; + } + + // At this point we know that head.Next != null, which means + // this segment has been frozen for additional enqueues. But between + // the time that we ran TryDequeue and checked for a next segment, + // another item could have been added. Try to dequeue one more time + // to confirm that the segment is indeed empty. + Debug.Assert(head._frozenForEnqueues); + if (head.TryDequeue(out item)) + { return true; - //since method IsEmpty spins, we don't need to spin in the while loop + } + + // This segment is frozen (nothing more can be added) and empty (nothing is in it). + // Update head to point to the next segment in the list, assuming no one's beat us to it. + lock (_crossSegmentLock) + { + if (head == _head) + { + _head = head._nextSegment; + } + } } - result = default(T); - return false; } /// <summary> /// Attempts to return an object from the beginning of the <see cref="ConcurrentQueue{T}"/> /// without removing it. /// </summary> - /// <param name="result">When this method returns, <paramref name="result"/> contains an object from - /// the beginning of the <see cref="T:System.Collections.Concurrent.ConccurrentQueue{T}"/> or an - /// unspecified value if the operation failed.</param> + /// <param name="result"> + /// When this method returns, <paramref name="result"/> contains an object from + /// the beginning of the <see cref="Concurrent.ConcurrentQueue{T}"/> or default(T) + /// if the operation failed. + /// </param> /// <returns>true if and object was returned successfully; otherwise, false.</returns> - public bool TryPeek(out T result) - { - Interlocked.Increment(ref m_numSnapshotTakers); + /// <remarks> + /// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/> + /// property is recommended rather than peeking. + /// </remarks> + public bool TryPeek(out T result) => TryPeek(out result, resultUsed: true); - while (!IsEmpty) + /// <summary>Attempts to retrieve the value for the first element in the queue.</summary> + /// <param name="result">The value of the first element, if found.</param> + /// <param name="resultUsed">true if the result is neede; otherwise false if only the true/false outcome is needed.</param> + /// <returns>true if an element was found; otherwise, false.</returns> + private bool TryPeek(out T result, bool resultUsed) + { + // Starting with the head segment, look through all of the segments + // for the first one we can find that's not empty. + Segment s = _head; + while (true) { - Segment head = m_head; - if (head.TryPeek(out result)) + // Grab the next segment from this one, before we peek. + // This is to be able to see whether the value has changed + // during the peek operation. + Segment next = Volatile.Read(ref s._nextSegment); + + // Peek at the segment. If we find an element, we're done. + if (s.TryPeek(out result, resultUsed)) { - Interlocked.Decrement(ref m_numSnapshotTakers); return true; } - //since method IsEmpty spins, we don't need to spin in the while loop + + // The current segment was empty at the moment we checked. + + if (next != null) + { + // If prior to the peek there was already a next segment, then + // during the peek no additional items could have been enqueued + // to it and we can just move on to check the next segment. + Debug.Assert(next == s._nextSegment); + s = next; + } + else if (Volatile.Read(ref s._nextSegment) == null) + { + // The next segment is null. Nothing more to peek at. + break; + } + + // The next segment was null before we peeked but non-null after. + // That means either when we peeked the first segment had + // already been frozen but the new segment not yet added, + // or that the first segment was empty and between the time + // that we peeked and then checked _nextSegment, so many items + // were enqueued that we filled the first segment and went + // into the next. Since we need to peek in order, we simply + // loop around again to peek on the same segment. The next + // time around on this segment we'll then either successfully + // peek or we'll find that next was non-null before peeking, + // and we'll traverse to that segment. } + result = default(T); - Interlocked.Decrement(ref m_numSnapshotTakers); return false; } - /// <summary> - /// private class for ConcurrentQueue. - /// a queue is a linked list of small arrays, each node is called a segment. - /// A segment contains an array, a pointer to the next segment, and m_low, m_high indices recording - /// the first and last valid elements of the array. + /// Removes all objects from the <see cref="ConcurrentQueue{T}"/>. /// </summary> - private class Segment + public void Clear() { - //we define two volatile arrays: m_array and m_state. Note that the accesses to the array items - //do not get volatile treatment. But we don't need to worry about loading adjacent elements or - //store/load on adjacent elements would suffer reordering. - // - Two stores: these are at risk, but CLRv2 memory model guarantees store-release hence we are safe. - // - Two loads: because one item from two volatile arrays are accessed, the loads of the array references - // are sufficient to prevent reordering of the loads of the elements. - internal volatile T[] m_array; - - // For each entry in m_array, the corresponding entry in m_state indicates whether this position contains - // a valid value. m_state is initially all false. - internal volatile VolatileBool[] m_state; - - //pointer to the next segment. null if the current segment is the last segment - private volatile Segment m_next; - - //We use this zero based index to track how many segments have been created for the queue, and - //to compute how many active segments are there currently. - // * The number of currently active segments is : m_tail.m_index - m_head.m_index + 1; - // * m_index is incremented with every Segment.Grow operation. We use Int64 type, and we can safely - // assume that it never overflows. To overflow, we need to do 2^63 increments, even at a rate of 4 - // billion (2^32) increments per second, it takes 2^31 seconds, which is about 64 years. - internal readonly long m_index; - - //indices of where the first and last valid values - // - m_low points to the position of the next element to pop from this segment, range [0, infinity) - // m_low >= SEGMENT_SIZE implies the segment is disposable - // - m_high points to the position of the latest pushed element, range [-1, infinity) - // m_high == -1 implies the segment is new and empty - // m_high >= SEGMENT_SIZE-1 means this segment is ready to grow. - // and the thread who sets m_high to SEGMENT_SIZE-1 is responsible to grow the segment - // - Math.Min(m_low, SEGMENT_SIZE) > Math.Min(m_high, SEGMENT_SIZE-1) implies segment is empty - // - initially m_low =0 and m_high=-1; - private volatile int m_low; - private volatile int m_high; - - private volatile ConcurrentQueue<T> m_source; - - /// <summary> - /// Create and initialize a segment with the specified index. - /// </summary> - internal Segment(long index, ConcurrentQueue<T> source) + lock (_crossSegmentLock) { - m_array = new T[SEGMENT_SIZE]; - m_state = new VolatileBool[SEGMENT_SIZE]; //all initialized to false - m_high = -1; - Debug.Assert(index >= 0); - m_index = index; - m_source = source; - } - - /// <summary> - /// return the next segment - /// </summary> - internal Segment Next - { - get { return m_next; } - } - - - /// <summary> - /// return true if the current segment is empty (doesn't have any element available to dequeue, - /// false otherwise - /// </summary> - internal bool IsEmpty - { - get { return (Low > High); } - } - - /// <summary> - /// Add an element to the tail of the current segment - /// exclusively called by ConcurrentQueue.InitializedFromCollection - /// InitializeFromCollection is responsible to guaratee that there is no index overflow, - /// and there is no contention - /// </summary> - /// <param name="value"></param> - internal void UnsafeAdd(T value) - { - Debug.Assert(m_high < SEGMENT_SIZE - 1); - m_high++; - m_array[m_high] = value; - m_state[m_high].m_value = true; + // Simply substitute a new segment for the existing head/tail, + // as is done in the constructor. Operations currently in flight + // may still read from or write to an existing segment that's + // getting dropped, meaning that in flight operations may not be + // linear with regards to this clear operation. To help mitigate + // in-flight operations enqueuing onto the tail that's about to + // be dropped, we first freeze it; that'll force enqueuers to take + // this lock to synchronize and see the new tail. + _tail.EnsureFrozenForEnqueues(); + _tail = _head = new Segment(InitialSegmentLength); } + } - /// <summary> - /// Create a new segment and append to the current one - /// Does not update the m_tail pointer - /// exclusively called by ConcurrentQueue.InitializedFromCollection - /// InitializeFromCollection is responsible to guaratee that there is no index overflow, - /// and there is no contention - /// </summary> - /// <returns>the reference to the new Segment</returns> - internal Segment UnsafeGrow() + /// <summary> + /// Provides a multi-producer, multi-consumer thread-safe bounded segment. When the queue is full, + /// enqueues fail and return false. When the queue is empty, dequeues fail and return null. + /// These segments are linked together to form the unbounded <see cref="ConcurrentQueue{T}"/>. + /// </summary> + [DebuggerDisplay("Capacity = {Capacity}")] + private sealed class Segment + { + // Segment design is inspired by the algorithm outlined at: + // http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue + + /// <summary>The array of items in this queue. Each slot contains the item in that slot and its "sequence number".</summary> + internal readonly Slot[] _slots; + /// <summary>Mask for quickly accessing a position within the queue's array.</summary> + internal readonly int _slotsMask; + /// <summary>The head and tail positions, with padding to help avoid false sharing contention.</summary> + /// <remarks>Dequeueing happens from the head, enqueueing happens at the tail.</remarks> + internal PaddedHeadAndTail _headAndTail; // mutable struct: do not make this readonly + + /// <summary>Indicates whether the segment has been marked such that dequeues don't overwrite the removed data.</summary> + internal bool _preservedForObservation; + /// <summary>Indicates whether the segment has been marked such that no additional items may be enqueued.</summary> + internal bool _frozenForEnqueues; + /// <summary>The segment following this one in the queue, or null if this segment is the last in the queue.</summary> + internal Segment _nextSegment; + + /// <summary>Creates the segment.</summary> + /// <param name="boundedLength"> + /// The maximum number of elements the segment can contain. Must be a power of 2. + /// </param> + public Segment(int boundedLength) { - Debug.Assert(m_high >= SEGMENT_SIZE - 1); - Segment newSegment = new Segment(m_index + 1, m_source); //m_index is Int64, we don't need to worry about overflow - m_next = newSegment; - return newSegment; + // Validate the length + Debug.Assert(boundedLength >= 2, $"Must be >= 2, got {boundedLength}"); + Debug.Assert((boundedLength & (boundedLength - 1)) == 0, $"Must be a power of 2, got {boundedLength}"); + + // Initialize the slots and the mask. The mask is used as a way of quickly doing "% _slots.Length", + // instead letting us do "& _slotsMask". + _slots = new Slot[boundedLength]; + _slotsMask = boundedLength - 1; + + // Initialize the sequence number for each slot. The sequence number provides a ticket that + // allows dequeuers to know whether they can dequeue and enqueuers to know whether they can + // enqueue. An enqueuer at position N can enqueue when the sequence number is N, and a dequeuer + // for position N can dequeue when the sequence number is N + 1. When an enqueuer is done writing + // at position N, it sets the sequence number to N so that a dequeuer will be able to dequeue, + // and when a dequeuer is done dequeueing at position N, it sets the sequence number to N + _slots.Length, + // so that when an enqueuer loops around the slots, it'll find that the sequence number at + // position N is N. This also means that when an enqueuer finds that at position N the sequence + // number is < N, there is still a value in that slot, i.e. the segment is full, and when a + // dequeuer finds that the value in a slot is < N + 1, there is nothing currently available to + // dequeue. (It is possible for multiple enqueuers to enqueue concurrently, writing into + // subsequent slots, and to have the first enqueuer take longer, so that the slots for 1, 2, 3, etc. + // may have values, but the 0th slot may still be being filled... in that case, TryDequeue will + // return false.) + for (int i = 0; i < _slots.Length; i++) + { + _slots[i].SequenceNumber = i; + } } - /// <summary> - /// Create a new segment and append to the current one - /// Update the m_tail pointer - /// This method is called when there is no contention - /// </summary> - internal void Grow() - { - //no CAS is needed, since there is no contention (other threads are blocked, busy waiting) - Segment newSegment = new Segment(m_index + 1, m_source); //m_index is Int64, we don't need to worry about overflow - m_next = newSegment; - Debug.Assert(m_source.m_tail == this); - m_source.m_tail = m_next; - } + /// <summary>Gets the number of elements this segment can store.</summary> + internal int Capacity => _slots.Length; + /// <summary>Gets the "freeze offset" for this segment.</summary> + internal int FreezeOffset => _slots.Length * 2; /// <summary> - /// Try to append an element at the end of this segment. + /// Ensures that the segment will not accept any subsequent enqueues that aren't already underway. /// </summary> - /// <param name="value">the element to append</param> - /// <param name="tail">The tail.</param> - /// <returns>true if the element is appended, false if the current segment is full</returns> - /// <remarks>if appending the specified element succeeds, and after which the segment is full, - /// then grow the segment</remarks> - internal bool TryAppend(T value) + /// <remarks> + /// When we mark a segment as being frozen for additional enqueues, + /// we set the <see cref="_frozenForEnqueues"/> bool, but that's mostly + /// as a small helper to avoid marking it twice. The real marking comes + /// by modifying the Tail for the segment, increasing it by this + /// <see cref="FreezeOffset"/>. This effectively knocks it off the + /// sequence expected by future enqueuers, such that any additional enqueuer + /// will be unable to enqueue due to it not lining up with the expected + /// sequence numbers. This value is chosen specially so that Tail will grow + /// to a value that maps to the same slot but that won't be confused with + /// any other enqueue/dequeue sequence number. + /// </remarks> + internal void EnsureFrozenForEnqueues() // must only be called while queue's segment lock is held { - //quickly check if m_high is already over the boundary, if so, bail out - if (m_high >= SEGMENT_SIZE - 1) + if (!_frozenForEnqueues) // flag used to ensure we don't increase the Tail more than once if frozen more than once { - return false; - } + _frozenForEnqueues = true; - //Now we will use a CAS to increment m_high, and store the result in newhigh. - //Depending on how many free spots left in this segment and how many threads are doing this Increment - //at this time, the returning "newhigh" can be - // 1) < SEGMENT_SIZE - 1 : we took a spot in this segment, and not the last one, just insert the value - // 2) == SEGMENT_SIZE - 1 : we took the last spot, insert the value AND grow the segment - // 3) > SEGMENT_SIZE - 1 : we failed to reserve a spot in this segment, we return false to - // Queue.Enqueue method, telling it to try again in the next segment. - - int newhigh = SEGMENT_SIZE; //initial value set to be over the boundary - - //We need do Interlocked.Increment and value/state update in a finally block to ensure that they run - //without interuption. This is to prevent anything from happening between them, and another dequeue - //thread maybe spinning forever to wait for m_state[] to be true; - try - { } - finally - { - newhigh = Interlocked.Increment(ref m_high); - if (newhigh <= SEGMENT_SIZE - 1) + // Increase the tail by FreezeOffset, spinning until we're successful in doing so. + var spinner = new SpinWait(); + while (true) { - m_array[newhigh] = value; - m_state[newhigh].m_value = true; - } - - //if this thread takes up the last slot in the segment, then this thread is responsible - //to grow a new segment. Calling Grow must be in the finally block too for reliability reason: - //if thread abort during Grow, other threads will be left busy spinning forever. - if (newhigh == SEGMENT_SIZE - 1) - { - Grow(); + int tail = Volatile.Read(ref _headAndTail.Tail); + if (Interlocked.CompareExchange(ref _headAndTail.Tail, tail + FreezeOffset, tail) == tail) + { + break; + } + spinner.SpinOnce(); } } - - //if newhigh <= SEGMENT_SIZE-1, it means the current thread successfully takes up a spot - return newhigh <= SEGMENT_SIZE - 1; } - - /// <summary> - /// try to remove an element from the head of current segment - /// </summary> - /// <param name="result">The result.</param> - /// <param name="head">The head.</param> - /// <returns>return false only if the current segment is empty</returns> - internal bool TryRemove(out T result) + /// <summary>Tries to dequeue an element from the queue.</summary> + public bool TryDequeue(out T item) { - SpinWait spin = new SpinWait(); - int lowLocal = Low, highLocal = High; - while (lowLocal <= highLocal) + // Loop in case of contention... + var spinner = new SpinWait(); + while (true) { - //try to update m_low - if (Interlocked.CompareExchange(ref m_low, lowLocal + 1, lowLocal) == lowLocal) - { - //if the specified value is not available (this spot is taken by a push operation, - // but the value is not written into yet), then spin - SpinWait spinLocal = new SpinWait(); - while (!m_state[lowLocal].m_value) - { - spinLocal.SpinOnce(); - } - result = m_array[lowLocal]; + // Get the head at which to try to dequeue. + int currentHead = Volatile.Read(ref _headAndTail.Head); + int slotsIndex = currentHead & _slotsMask; - // If there is no other thread taking snapshot (GetEnumerator(), ToList(), etc), reset the deleted entry to null. - // It is ok if after this conditional check m_numSnapshotTakers becomes > 0, because new snapshots won't include - // the deleted entry at m_array[lowLocal]. - if (m_source.m_numSnapshotTakers <= 0) - { - m_array[lowLocal] = default(T); //release the reference to the object. - } + // Read the sequence number for the head position. + int sequenceNumber = Volatile.Read(ref _slots[slotsIndex].SequenceNumber); - //if the current thread sets m_low to SEGMENT_SIZE, which means the current segment becomes - //disposable, then this thread is responsible to dispose this segment, and reset m_head - if (lowLocal + 1 >= SEGMENT_SIZE) + // We can dequeue from this slot if it's been filled by an enqueuer, which + // would have left the sequence number at pos+1. + int diff = sequenceNumber - (currentHead + 1); + if (diff == 0) + { + // We may be racing with other dequeuers. Try to reserve the slot by incrementing + // the head. Once we've done that, no one else will be able to read from this slot, + // and no enqueuer will be able to read from this slot until we've written the new + // sequence number. WARNING: The next few lines are not reliable on a runtime that + // supports thread aborts. If a thread abort were to sneak in after the CompareExchange + // but before the Volatile.Write, enqueuers trying to enqueue into this slot would + // spin indefinitely. If this implementation is ever used on such a platform, this + // if block should be wrapped in a finally / prepared region. + if (Interlocked.CompareExchange(ref _headAndTail.Head, currentHead + 1, currentHead) == currentHead) { - // Invariant: we only dispose the current m_head, not any other segment - // In usual situation, disposing a segment is simply seting m_head to m_head.m_next - // But there is one special case, where m_head and m_tail points to the same and ONLY - //segment of the queue: Another thread A is doing Enqueue and finds that it needs to grow, - //while the *current* thread is doing *this* Dequeue operation, and finds that it needs to - //dispose the current (and ONLY) segment. Then we need to wait till thread A finishes its - //Grow operation, this is the reason of having the following while loop - spinLocal = new SpinWait(); - while (m_next == null) + // Successfully reserved the slot. Note that after the above CompareExchange, other threads + // trying to dequeue from this slot will end up spinning until we do the subsequent Write. + item = _slots[slotsIndex].Item; + if (!Volatile.Read(ref _preservedForObservation)) { - spinLocal.SpinOnce(); + // If we're preserving, though, we don't zero out the slot, as we need it for + // enumerations, peeking, ToArray, etc. And we don't update the sequence number, + // so that an enqueuer will see it as full and be forced to move to a new segment. + _slots[slotsIndex].Item = default(T); + Volatile.Write(ref _slots[slotsIndex].SequenceNumber, currentHead + _slots.Length); } - Debug.Assert(m_source.m_head == this); - m_source.m_head = m_next; + return true; } - return true; } - else + else if (diff < 0) { - //CAS failed due to contention: spin briefly and retry - spin.SpinOnce(); - lowLocal = Low; highLocal = High; + // The sequence number was less than what we needed, which means this slot doesn't + // yet contain a value we can dequeue, i.e. the segment is empty. Technically it's + // possible that multiple enqueuers could have written concurrently, with those + // getting later slots actually finishing first, so there could be elements after + // this one that are available, but we need to dequeue in order. So before declaring + // failure and that the segment is empty, we check the tail to see if we're actually + // empty or if we're just waiting for items in flight or after this one to become available. + bool frozen = _frozenForEnqueues; + int currentTail = Volatile.Read(ref _headAndTail.Tail); + if (currentTail - currentHead <= 0 || (frozen && (currentTail - FreezeOffset - currentHead <= 0))) + { + item = default(T); + return false; + } + + // It's possible it could have become frozen after we checked _frozenForEnqueues + // and before reading the tail. That's ok: in that rare race condition, we just + // loop around again. } - }//end of while - result = default(T); - return false; + + // Lost a race. Spin a bit, then try again. + spinner.SpinOnce(); + } } - /// <summary> - /// try to peek the current segment - /// </summary> - /// <param name="result">holds the return value of the element at the head position, - /// value set to default(T) if there is no such an element</param> - /// <returns>true if there are elements in the current segment, false otherwise</returns> - internal bool TryPeek(out T result) + /// <summary>Tries to peek at an element from the queue, without removing it.</summary> + public bool TryPeek(out T result, bool resultUsed) { - result = default(T); - int lowLocal = Low; - if (lowLocal > High) - return false; - SpinWait spin = new SpinWait(); - while (!m_state[lowLocal].m_value) + if (resultUsed) { - spin.SpinOnce(); + // In order to ensure we don't get a torn read on the value, we mark the segment + // as preserving for observation. Additional items can still be enqueued to this + // segment, but no space will be freed during dequeues, such that the segment will + // no longer be reusable. + _preservedForObservation = true; + Interlocked.MemoryBarrier(); } - result = m_array[lowLocal]; - return true; - } - /// <summary> - /// Adds part or all of the current segment into a List. - /// </summary> - /// <param name="list">the list to which to add</param> - /// <param name="start">the start position</param> - /// <param name="end">the end position</param> - internal void AddToList(List<T> list, int start, int end) - { - for (int i = start; i <= end; i++) + // Loop in case of contention... + var spinner = new SpinWait(); + while (true) { - SpinWait spin = new SpinWait(); - while (!m_state[i].m_value) + // Get the head at which to try to peek. + int currentHead = Volatile.Read(ref _headAndTail.Head); + int slotsIndex = currentHead & _slotsMask; + + // Read the sequence number for the head position. + int sequenceNumber = Volatile.Read(ref _slots[slotsIndex].SequenceNumber); + + // We can peek from this slot if it's been filled by an enqueuer, which + // would have left the sequence number at pos+1. + int diff = sequenceNumber - (currentHead + 1); + if (diff == 0) { - spin.SpinOnce(); + result = resultUsed ? _slots[slotsIndex].Item : default(T); + return true; + } + else if (diff < 0) + { + // The sequence number was less than what we needed, which means this slot doesn't + // yet contain a value we can peek, i.e. the segment is empty. Technically it's + // possible that multiple enqueuers could have written concurrently, with those + // getting later slots actually finishing first, so there could be elements after + // this one that are available, but we need to peek in order. So before declaring + // failure and that the segment is empty, we check the tail to see if we're actually + // empty or if we're just waiting for items in flight or after this one to become available. + bool frozen = _frozenForEnqueues; + int currentTail = Volatile.Read(ref _headAndTail.Tail); + if (currentTail - currentHead <= 0 || (frozen && (currentTail - FreezeOffset - currentHead <= 0))) + { + result = default(T); + return false; + } + + // It's possible it could have become frozen after we checked _frozenForEnqueues + // and before reading the tail. That's ok: in that rare race condition, we just + // loop around again. } - list.Add(m_array[i]); + + // Lost a race. Spin a bit, then try again. + spinner.SpinOnce(); } } /// <summary> - /// return the position of the head of the current segment - /// Value range [0, SEGMENT_SIZE], if it's SEGMENT_SIZE, it means this segment is exhausted and thus empty + /// Attempts to enqueue the item. If successful, the item will be stored + /// in the queue and true will be returned; otherwise, the item won't be stored, and false + /// will be returned. /// </summary> - internal int Low + public bool TryEnqueue(T item) { - get + // Loop in case of contention... + var spinner = new SpinWait(); + while (true) { - return Math.Min(m_low, SEGMENT_SIZE); + // Get the tail at which to try to return. + int currentTail = Volatile.Read(ref _headAndTail.Tail); + int slotsIndex = currentTail & _slotsMask; + + // Read the sequence number for the tail position. + int sequenceNumber = Volatile.Read(ref _slots[slotsIndex].SequenceNumber); + + // The slot is empty and ready for us to enqueue into it if its sequence + // number matches the slot. + int diff = sequenceNumber - currentTail; + if (diff == 0) + { + // We may be racing with other enqueuers. Try to reserve the slot by incrementing + // the tail. Once we've done that, no one else will be able to write to this slot, + // and no dequeuer will be able to read from this slot until we've written the new + // sequence number. WARNING: The next few lines are not reliable on a runtime that + // supports thread aborts. If a thread abort were to sneak in after the CompareExchange + // but before the Volatile.Write, other threads will spin trying to access this slot. + // If this implementation is ever used on such a platform, this if block should be + // wrapped in a finally / prepared region. + if (Interlocked.CompareExchange(ref _headAndTail.Tail, currentTail + 1, currentTail) == currentTail) + { + // Successfully reserved the slot. Note that after the above CompareExchange, other threads + // trying to return will end up spinning until we do the subsequent Write. + _slots[slotsIndex].Item = item; + Volatile.Write(ref _slots[slotsIndex].SequenceNumber, currentTail + 1); + return true; + } + } + else if (diff < 0) + { + // The sequence number was less than what we needed, which means this slot still + // contains a value, i.e. the segment is full. Technically it's possible that multiple + // dequeuers could have read concurrently, with those getting later slots actually + // finishing first, so there could be spaces after this one that are available, but + // we need to enqueue in order. + return false; + } + + // Lost a race. Spin a bit, then try again. + spinner.SpinOnce(); } } - /// <summary> - /// return the logical position of the tail of the current segment - /// Value range [-1, SEGMENT_SIZE-1]. When it's -1, it means this is a new segment and has no elemnet yet - /// </summary> - internal int High + /// <summary>Represents a slot in the queue.</summary> + [StructLayout(LayoutKind.Auto)] + [DebuggerDisplay("Item = {Item}, SequenceNumber = {SequenceNumber}")] + internal struct Slot { - get - { - //if m_high > SEGMENT_SIZE, it means it's out of range, we should return - //SEGMENT_SIZE-1 as the logical position - return Math.Min(m_high, SEGMENT_SIZE - 1); - } + /// <summary>The item.</summary> + public T Item; + /// <summary>The sequence number for this slot, used to synchronize between enqueuers and dequeuers.</summary> + public int SequenceNumber; } - } - }//end of class Segment + } - /// <summary> - /// A wrapper struct for volatile bool, please note the copy of the struct it self will not be volatile - /// for example this statement will not include in volatilness operation volatileBool1 = volatileBool2 the jit will copy the struct and will ignore the volatile - /// </summary> - struct VolatileBool + /// <summary>Padded head and tail indices, to avoid false sharing between producers and consumers.</summary> + [DebuggerDisplay("Head = {Head}, Tail = {Tail}")] + [StructLayout(LayoutKind.Explicit, Size = 192)] // padding before/between/after fields based on typical cache line size of 64 + internal struct PaddedHeadAndTail { - public VolatileBool(bool value) - { - m_value = value; - } - public volatile bool m_value; + [FieldOffset(64)] public int Head; + [FieldOffset(128)] public int Tail; } } |