// 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>
{
- //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>
/// <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)
}
/// <summary>
- /// Get the data array to be serialized
+ /// Copies the elements of the <see cref="ICollection"/> to an <see
+ /// cref="Array"/>, starting at a particular <see cref="Array"/> index.
/// </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.
- /// </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
/// <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.
+ /// cref="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"));
- }
- }
+ /// <exception cref="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>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>
- /// Attempts to add an object to the <see
- /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
+ /// Attempts to add an object to the <see cref="Concurrent.IProducerConsumerCollection{T}"/>.
/// </summary>
/// <param name="item">The object to add to the <see
- /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. The value can be a null
+ /// cref="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>
}
/// <summary>
- /// Attempts to remove and return an object from the <see
- /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
+ /// Attempts to remove and return an object from the <see cref="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>
+ /// <returns>true if an element was removed and returned successfully; 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);
- }
+ bool IProducerConsumerCollection<T>.TryTake(out T item) => TryDequeue(out item);
/// <summary>
/// Gets a value that indicates whether the <see cref="ConcurrentQueue{T}"/> is empty.
{
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)
+ int i = 0;
+ while (e.MoveNext())
{
- head.AddToList(list, headLow, tailHigh);
- }
- else
- {
- 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>
{
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>
{
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(); // TODO: finish this
+ }
- /// <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>
/// </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)
+ {
+ lock (_crossSegmentLock) // _head and _tail may only change while the lock is held.
+ {
+ // 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
+
+ // 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)
{
- try
+ 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)
{
- SpinWait spin = new SpinWait();
+ var spinner = new SpinWait();
+ while ((Volatile.Read(ref segment._slots[i].SequenceNumber) & segment._slotsMask) != expectedSequenceNumberAndMask)
+ {
+ spinner.SpinOnce();
+ }
+ }
- if (head == tail)
+ // 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 = 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];
- }
+ 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);
+ }
+ }
+
+ // 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");
- //iterate on tail segment
- for (int i = 0; i <= tailHigh; i++)
+ 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}"/>.
/// 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)
- {
- 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
+ lock (_crossSegmentLock)
{
- 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.
+ if (sequenceNumber == currentHead + 1)
+ {
+ // 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 (sequenceNumber < currentHead + 1)
{
- //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.
+ if (sequenceNumber == currentHead + 1)
+ {
+ result = resultUsed ? _slots[slotsIndex].Item : default(T);
+ return true;
+ }
+ else if (sequenceNumber < currentHead + 1)
{
- spin.SpinOnce();
+ // 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.
+ if (sequenceNumber == currentTail)
+ {
+ // 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 (sequenceNumber < currentTail)
+ {
+ // 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;
}
}
projects / platform / upstream / dotnet / runtime.git / commitdiff