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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.InteropServices;
using System.Threading;
using Internal;
namespace System.Collections.Concurrent
{
/// <summary>
/// Provides a producer/consumer queue safe to be used by only one producer and one consumer concurrently.
/// </summary>
/// <typeparam name="T">Specifies the type of data contained in the queue.</typeparam>
[DebuggerDisplay("Count = {Count}")]
[DebuggerTypeProxy(typeof(SingleProducerSingleConsumerQueue<>.SingleProducerSingleConsumerQueue_DebugView))]
internal sealed class SingleProducerSingleConsumerQueue<T> : IProducerConsumerQueue<T>
{
// Design:
//
// SingleProducerSingleConsumerQueue (SPSCQueue) is a concurrent queue designed to be used
// by one producer thread and one consumer thread. SPSCQueue does not work correctly when used by
// multiple producer threads concurrently or multiple consumer threads concurrently.
//
// SPSCQueue is based on segments that behave like circular buffers. Each circular buffer is represented
// as an array with two indexes: _first and _last. _first is the index of the array slot for the consumer
// to read next, and _last is the slot for the producer to write next. The circular buffer is empty when
// (_first == _last), and full when ((_last+1) % _array.Length == _first).
//
// Since _first is only ever modified by the consumer thread and _last by the producer, the two indices can
// be updated without interlocked operations. As long as the queue size fits inside a single circular buffer,
// enqueues and dequeues simply advance the corresponding indices around the circular buffer. If an enqueue finds
// that there is no room in the existing buffer, however, a new circular buffer is allocated that is twice as big
// as the old buffer. From then on, the producer will insert values into the new buffer. The consumer will first
// empty out the old buffer and only then follow the producer into the new (larger) buffer.
//
// As described above, the enqueue operation on the fast path only modifies the _first field of the current segment.
// However, it also needs to read _last in order to verify that there is room in the current segment. Similarly, the
// dequeue operation on the fast path only needs to modify _last, but also needs to read _first to verify that the
// queue is non-empty. This results in true cache line sharing between the producer and the consumer.
//
// The cache line sharing issue can be mitigating by having a possibly stale copy of _first that is owned by the producer,
// and a possibly stale copy of _last that is owned by the consumer. So, the consumer state is described using
// (_first, _lastCopy) and the producer state using (_firstCopy, _last). The consumer state is separated from
// the producer state by padding, which allows fast-path enqueues and dequeues from hitting shared cache lines.
// _lastCopy is the consumer's copy of _last. Whenever the consumer can tell that there is room in the buffer
// simply by observing _lastCopy, the consumer thread does not need to read _last and thus encounter a cache miss. Only
// when the buffer appears to be empty will the consumer refresh _lastCopy from _last. _firstCopy is used by the producer
// in the same way to avoid reading _first on the hot path.
/// <summary>The initial size to use for segments (in number of elements).</summary>
private const int InitialSegmentSize = 32; // must be a power of 2
/// <summary>The maximum size to use for segments (in number of elements).</summary>
private const int MaxSegmentSize = 0x1000000; // this could be made as large as int.MaxValue / 2
/// <summary>The head of the linked list of segments.</summary>
private volatile Segment _head;
/// <summary>The tail of the linked list of segments.</summary>
private volatile Segment _tail;
/// <summary>Initializes the queue.</summary>
public SingleProducerSingleConsumerQueue()
{
// Validate constants in ctor rather than in an explicit cctor that would cause perf degradation
Debug.Assert(InitialSegmentSize > 0, "Initial segment size must be > 0.");
Debug.Assert((InitialSegmentSize & (InitialSegmentSize - 1)) == 0, "Initial segment size must be a power of 2");
Debug.Assert(InitialSegmentSize <= MaxSegmentSize, "Initial segment size should be <= maximum.");
Debug.Assert(MaxSegmentSize < int.MaxValue / 2, "Max segment size * 2 must be < int.MaxValue, or else overflow could occur.");
// Initialize the queue
_head = _tail = new Segment(InitialSegmentSize);
}
/// <summary>Enqueues an item into the queue.</summary>
/// <param name="item">The item to enqueue.</param>
public void Enqueue(T item)
{
Segment segment = _tail;
T[] array = segment._array;
int last = segment._state._last; // local copy to avoid multiple volatile reads
// Fast path: there's obviously room in the current segment
int tail2 = (last + 1) & (array.Length - 1);
if (tail2 != segment._state._firstCopy)
{
array[last] = item;
segment._state._last = tail2;
return;
}
// Slow path: there may not be room in the current segment.
EnqueueSlow(item, ref segment);
}
/// <summary>Enqueues an item into the queue.</summary>
/// <param name="item">The item to enqueue.</param>
/// <param name="segment">The segment in which to first attempt to store the item.</param>
private void EnqueueSlow(T item, ref Segment segment)
{
Debug.Assert(segment != null, "Expected a non-null segment.");
if (segment._state._firstCopy != segment._state._first)
{
segment._state._firstCopy = segment._state._first;
Enqueue(item); // will only recur once for this enqueue operation
return;
}
int newSegmentSize = Math.Min(_tail._array.Length * 2, MaxSegmentSize);
Debug.Assert(newSegmentSize > 0, "The max size should always be small enough that we don't overflow.");
var newSegment = new Segment(newSegmentSize);
newSegment._array[0] = item;
newSegment._state._last = 1;
newSegment._state._lastCopy = 1;
try { }
finally
{
// Finally block to protect against corruption due to a thread abort between
// setting _next and setting _tail (this is only relevant on .NET Framework).
Volatile.Write(ref _tail._next, newSegment); // ensure segment not published until item is fully stored
_tail = newSegment;
}
}
/// <summary>Attempts to dequeue an item from the queue.</summary>
/// <param name="result">The dequeued item.</param>
/// <returns>true if an item could be dequeued; otherwise, false.</returns>
public bool TryDequeue([MaybeNullWhen(false)] out T result)
{
Segment segment = _head;
T[] array = segment._array;
int first = segment._state._first; // local copy to avoid multiple volatile reads
// Fast path: there's obviously data available in the current segment
if (first != segment._state._lastCopy)
{
result = array[first];
array[first] = default!; // Clear the slot to release the element
segment._state._first = (first + 1) & (array.Length - 1);
return true;
}
// Slow path: there may not be data available in the current segment
return TryDequeueSlow(segment, array, peek: false, out result);
}
/// <summary>Attempts to peek at an item in the queue.</summary>
/// <param name="result">The peeked item.</param>
/// <returns>true if an item could be peeked; otherwise, false.</returns>
public bool TryPeek([MaybeNullWhen(false)] out T result)
{
Segment segment = _head;
T[] array = segment._array;
int first = segment._state._first; // local copy to avoid multiple volatile reads
// Fast path: there's obviously data available in the current segment
if (first != segment._state._lastCopy)
{
result = array[first];
return true;
}
// Slow path: there may not be data available in the current segment
return TryDequeueSlow(segment, array, peek: true, out result);
}
/// <summary>Attempts to dequeue an item from the queue.</summary>
/// <param name="segment">The segment from which the item was dequeued.</param>
/// <param name="array">The array from <paramref name="segment"/>.</param>
/// <param name="peek">true if this is only a peek operation; false if the item should be dequeued.</param>
/// <param name="result">The dequeued item.</param>
/// <returns>true if an item could be dequeued; otherwise, false.</returns>
private bool TryDequeueSlow(Segment segment, T[] array, bool peek, [MaybeNullWhen(false)] out T result)
{
Debug.Assert(segment != null, "Expected a non-null segment.");
Debug.Assert(array != null, "Expected a non-null item array.");
if (segment._state._last != segment._state._lastCopy)
{
segment._state._lastCopy = segment._state._last;
return peek ?
TryPeek(out result) :
TryDequeue(out result); // will only recur once for this operation
}
if (segment._next != null && segment._state._first == segment._state._last)
{
segment = segment._next;
array = segment._array;
_head = segment;
}
int first = segment._state._first; // local copy to avoid extraneous volatile reads
if (first == segment._state._last)
{
result = default;
return false;
}
result = array[first];
if (!peek)
{
array[first] = default!; // Clear the slot to release the element
segment._state._first = (first + 1) & (segment._array.Length - 1);
segment._state._lastCopy = segment._state._last; // Refresh _lastCopy to ensure that _first has not passed _lastCopy
}
return true;
}
/// <summary>Attempts to dequeue an item from the queue.</summary>
/// <param name="predicate">The predicate that must return true for the item to be dequeued. If null, all items implicitly return true.</param>
/// <param name="result">The dequeued item.</param>
/// <returns>true if an item could be dequeued; otherwise, false.</returns>
public bool TryDequeueIf(Predicate<T>? predicate, [MaybeNullWhen(false)] out T result)
{
Segment segment = _head;
T[] array = segment._array;
int first = segment._state._first; // local copy to avoid multiple volatile reads
// Fast path: there's obviously data available in the current segment
if (first != segment._state._lastCopy)
{
result = array[first];
if (predicate == null || predicate(result))
{
array[first] = default!; // Clear the slot to release the element
segment._state._first = (first + 1) & (array.Length - 1);
return true;
}
result = default;
return false;
}
// Slow path: there may not be data available in the current segment
return TryDequeueIfSlow(predicate, segment, array, out result);
}
/// <summary>Attempts to dequeue an item from the queue.</summary>
/// <param name="predicate">The predicate that must return true for the item to be dequeued. If null, all items implicitly return true.</param>
/// <param name="array">The array from which the item was dequeued.</param>
/// <param name="segment">The segment from which the item was dequeued.</param>
/// <param name="result">The dequeued item.</param>
/// <returns>true if an item could be dequeued; otherwise, false.</returns>
private bool TryDequeueIfSlow(Predicate<T>? predicate, Segment segment, T[] array, [MaybeNullWhen(false)] out T result)
{
Debug.Assert(segment != null, "Expected a non-null segment.");
Debug.Assert(array != null, "Expected a non-null item array.");
if (segment._state._last != segment._state._lastCopy)
{
segment._state._lastCopy = segment._state._last;
return TryDequeueIf(predicate, out result); // will only recur once for this dequeue operation
}
if (segment._next != null && segment._state._first == segment._state._last)
{
segment = segment._next;
array = segment._array;
_head = segment;
}
int first = segment._state._first; // local copy to avoid extraneous volatile reads
if (first == segment._state._last)
{
result = default;
return false;
}
result = array[first];
if (predicate == null || predicate(result))
{
array[first] = default!; // Clear the slot to release the element
segment._state._first = (first + 1) & (segment._array.Length - 1);
segment._state._lastCopy = segment._state._last; // Refresh _lastCopy to ensure that _first has not passed _lastCopy
return true;
}
result = default;
return false;
}
public void Clear()
{
while (TryDequeue(out _)) ;
}
/// <summary>Gets whether the collection is currently empty.</summary>
/// <remarks>WARNING: This should not be used concurrently without further vetting.</remarks>
public bool IsEmpty
{
get
{
// This implementation is optimized for calls from the consumer.
Segment head = _head;
if (head._state._first != head._state._lastCopy)
{
return false; // _first is volatile, so the read of _lastCopy cannot get reordered
}
if (head._state._first != head._state._last)
{
return false;
}
return head._next == null;
}
}
/// <summary>Gets an enumerable for the collection.</summary>
/// <remarks>This method is not safe to use concurrently with any other members that may mutate the collection.</remarks>
public IEnumerator<T> GetEnumerator()
{
for (Segment? segment = _head; segment != null; segment = segment._next)
{
for (int pt = segment._state._first;
pt != segment._state._last;
pt = (pt + 1) & (segment._array.Length - 1))
{
yield return segment._array[pt];
}
}
}
/// <summary>Gets an enumerable for the collection.</summary>
/// <remarks>This method is not safe to use concurrently with any other members that may mutate the collection.</remarks>
IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
/// <summary>Gets the number of items in the collection.</summary>
/// <remarks>This method is not safe to use concurrently with any other members that may mutate the collection.</remarks>
public int Count
{
get
{
int count = 0;
for (Segment? segment = _head; segment != null; segment = segment._next)
{
int arraySize = segment._array.Length;
int first, last;
while (true) // Count is not meant to be used concurrently, but this helps to avoid issues if it is
{
first = segment._state._first;
last = segment._state._last;
if (first == segment._state._first)
{
break;
}
}
count += (last - first) & (arraySize - 1);
}
return count;
}
}
/// <summary>A thread-safe way to get the number of items in the collection. May synchronize access by locking the provided synchronization object.</summary>
/// <remarks>The Count is not thread safe, so we need to acquire the lock.</remarks>
int IProducerConsumerQueue<T>.GetCountSafe(object syncObj)
{
Debug.Assert(syncObj != null, "The syncObj parameter is null.");
lock (syncObj)
{
return Count;
}
}
/// <summary>A segment in the queue containing one or more items.</summary>
[StructLayout(LayoutKind.Sequential)]
private sealed class Segment
{
/// <summary>The next segment in the linked list of segments.</summary>
internal Segment? _next;
/// <summary>The data stored in this segment.</summary>
internal readonly T[] _array;
/// <summary>Details about the segment.</summary>
internal SegmentState _state; // separated out to enable StructLayout attribute to take effect
/// <summary>Initializes the segment.</summary>
/// <param name="size">The size to use for this segment.</param>
internal Segment(int size)
{
Debug.Assert((size & (size - 1)) == 0, "Size must be a power of 2");
_array = new T[size];
}
}
/// <summary>Stores information about a segment.</summary>
[StructLayout(LayoutKind.Sequential)] // enforce layout so that padding reduces false sharing
private struct SegmentState
{
/// <summary>Padding to reduce false sharing between the segment's array and _first.</summary>
internal PaddingFor32 _pad0;
/// <summary>The index of the current head in the segment.</summary>
internal volatile int _first;
/// <summary>A copy of the current tail index.</summary>
internal int _lastCopy; // not volatile as read and written by the producer, except for IsEmpty, and there _lastCopy is only read after reading the volatile _first
/// <summary>Padding to reduce false sharing between the first and last.</summary>
internal PaddingFor32 _pad1;
/// <summary>A copy of the current head index.</summary>
internal int _firstCopy; // not volatile as only read and written by the consumer thread
/// <summary>The index of the current tail in the segment.</summary>
internal volatile int _last;
/// <summary>Padding to reduce false sharing with the last and what's after the segment.</summary>
internal PaddingFor32 _pad2;
}
/// <summary>Debugger type proxy for a SingleProducerSingleConsumerQueue of T.</summary>
private sealed class SingleProducerSingleConsumerQueue_DebugView
{
/// <summary>The queue being visualized.</summary>
private readonly SingleProducerSingleConsumerQueue<T> _queue;
/// <summary>Initializes the debug view.</summary>
/// <param name="queue">The queue being debugged.</param>
public SingleProducerSingleConsumerQueue_DebugView(SingleProducerSingleConsumerQueue<T> queue)
{
Debug.Assert(queue != null, "Expected a non-null queue.");
_queue = queue;
}
/// <summary>Gets the contents of the list.</summary>
[DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
public T[] Items => new List<T>(_queue).ToArray();
}
}
}
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