//--------------------------------------------------------------------------
|
//
|
// Copyright (c) Microsoft Corporation. All rights reserved.
|
//
|
// File: LimitedConcurrencyTaskScheduler.cs
|
//
|
//--------------------------------------------------------------------------
|
|
using System.Collections.Generic;
|
using System.Linq;
|
|
namespace System.Threading.Tasks.Schedulers
|
{
|
/// <summary>
|
/// Provides a task scheduler that ensures a maximum concurrency level while
|
/// running on top of the ThreadPool.
|
/// </summary>
|
internal sealed class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
|
{
|
/// <summary>Whether the current thread is processing work items.</summary>
|
[ThreadStatic]
|
private static bool _currentThreadIsProcessingItems;
|
/// <summary>The list of tasks to be executed.</summary>
|
private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks)
|
/// <summary>The maximum concurrency level allowed by this scheduler.</summary>
|
private readonly int _maxDegreeOfParallelism;
|
/// <summary>Whether the scheduler is currently processing work items.</summary>
|
private int _delegatesQueuedOrRunning = 0; // protected by lock(_tasks)
|
|
/// <summary>
|
/// Initializes an instance of the LimitedConcurrencyLevelTaskScheduler class with the
|
/// specified degree of parallelism.
|
/// </summary>
|
/// <param name="maxDegreeOfParallelism">The maximum degree of parallelism provided by this scheduler.</param>
|
public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism)
|
{
|
if (maxDegreeOfParallelism < 1)
|
{
|
throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
|
}
|
|
_maxDegreeOfParallelism = maxDegreeOfParallelism;
|
}
|
|
/// <summary>Queues a task to the scheduler.</summary>
|
/// <param name="task">The task to be queued.</param>
|
protected sealed override void QueueTask(Task task)
|
{
|
// Add the task to the list of tasks to be processed. If there aren't enough
|
// delegates currently queued or running to process tasks, schedule another.
|
lock (_tasks)
|
{
|
_tasks.AddLast(task);
|
if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
|
{
|
++_delegatesQueuedOrRunning;
|
NotifyThreadPoolOfPendingWork();
|
}
|
}
|
}
|
|
/// <summary>
|
/// Informs the ThreadPool that there's work to be executed for this scheduler.
|
/// </summary>
|
private void NotifyThreadPoolOfPendingWork()
|
{
|
ThreadPool.UnsafeQueueUserWorkItem(_ =>
|
{
|
// Note that the current thread is now processing work items.
|
// This is necessary to enable inlining of tasks into this thread.
|
_currentThreadIsProcessingItems = true;
|
try
|
{
|
// Process all available items in the queue.
|
while (true)
|
{
|
Task item;
|
lock (_tasks)
|
{
|
// When there are no more items to be processed,
|
// note that we're done processing, and get out.
|
if (_tasks.Count == 0)
|
{
|
--_delegatesQueuedOrRunning;
|
break;
|
}
|
|
// Get the next item from the queue
|
item = _tasks.First.Value;
|
_tasks.RemoveFirst();
|
}
|
|
// Execute the task we pulled out of the queue
|
base.TryExecuteTask(item);
|
}
|
}
|
// We're done processing items on the current thread
|
finally { _currentThreadIsProcessingItems = false; }
|
}, null);
|
}
|
|
/// <summary>Attempts to execute the specified task on the current thread.</summary>
|
/// <param name="task">The task to be executed.</param>
|
/// <param name="taskWasPreviouslyQueued"></param>
|
/// <returns>Whether the task could be executed on the current thread.</returns>
|
protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
|
{
|
// If this thread isn't already processing a task, we don't support inlining
|
if (!_currentThreadIsProcessingItems)
|
{
|
return false;
|
}
|
|
// If the task was previously queued, remove it from the queue
|
if (taskWasPreviouslyQueued)
|
{
|
TryDequeue(task);
|
}
|
|
// Try to run the task.
|
return base.TryExecuteTask(task);
|
}
|
|
/// <summary>Attempts to remove a previously scheduled task from the scheduler.</summary>
|
/// <param name="task">The task to be removed.</param>
|
/// <returns>Whether the task could be found and removed.</returns>
|
protected sealed override bool TryDequeue(Task task)
|
{
|
lock (_tasks)
|
{
|
return _tasks.Remove(task);
|
}
|
}
|
|
/// <summary>Gets the maximum concurrency level supported by this scheduler.</summary>
|
public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }
|
|
/// <summary>Gets an enumerable of the tasks currently scheduled on this scheduler.</summary>
|
/// <returns>An enumerable of the tasks currently scheduled.</returns>
|
protected sealed override IEnumerable<Task> GetScheduledTasks()
|
{
|
bool lockTaken = false;
|
try
|
{
|
Monitor.TryEnter(_tasks, ref lockTaken);
|
if (lockTaken)
|
{
|
return _tasks.ToArray();
|
}
|
else
|
{
|
throw new NotSupportedException();
|
}
|
}
|
finally
|
{
|
if (lockTaken)
|
{
|
Monitor.Exit(_tasks);
|
}
|
}
|
}
|
}
|
}
|
