因此。我有一个爬网的例程。该例程以IP列表播种，并跟踪在爬行设备时找到的IP。当它找到新的IP时，它也会抓取它们。
这是我的问题。我在并行foreach中运行种子IP的初始扫描，并且还在平行foreach中启动我在每个设备上找到的IP，因此我最终会产生10个线程，每个线程产生10个线程，总共100个线程（或更多）如果那些线程找到自己的设备）。我想限制整个过程使用的线程总数（比如25）。
可以在C＃的任务库中完成吗？
我知道foreach循环上的MaxDegreeOfParallelism属性，但它可以共享吗？
答案 0 :(得分：3)
如何将这些任务推送到共享任务工厂？ How to: Create a Task Scheduler That Limits Concurrency
答案 1 :(得分：0)
特工Shark的回答就是这个伎俩。我想我会分享我的工作实例，并讨论我遇到的一些事情。
首先，我使用嵌套的Parallel.ForEach循环。但是我想到，如果我将线程/任务计数限制为小于第一个循环中的总数，那么就没有线程来处理第二个循环，因此例程永远不会完成。所以这根本不起作用。
这让Agent Shark想到了一个共享队列的想法，你可以把任务推到那个就可以在线程被释放时运行。
以下是我的解决方案。
internal class Program
{
private static void Main(string[] args)
{
// setup the factory
LimitedConcurrencyLevelTaskScheduler lcts = new LimitedConcurrencyLevelTaskScheduler(9);
TaskFactory f = new TaskFactory(lcts);
// create my shared task queue
ConcurrentDictionary<string, Task> waiting = new ConcurrentDictionary<string, Task>();
ConcurrentBag<Task> finished = new ConcurrentBag<Task>();
// some numbers....
List<int> nums = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 };
foreach (int n in nums)
{
int i = n; // if you don't do this, n is 0 when it writes to the Debug console....
Task t = f.StartNew(() =>
{
Debug.WriteLine(i);
// some more numbers....
List<int> other = new List<int>() { 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
foreach (int nm in other)
{
int j = i;
int w = nm;
Task tk = f.StartNew(() =>
{
Debug.WriteLine(j + "," + w);
Thread.Sleep(1000);
});
waiting.TryAdd(j + "," + w, tk);
}
Thread.Sleep(500);
});
waiting.TryAdd(i.ToString(), t);
}
// loop until no further tasks are waiting.
while (waiting.Count > 0)
{
// run the tasks...
Task.WaitAll(waiting.Values.ToArray());
// remove the finised tasks from the waiting list.
foreach (KeyValuePair<string, Task> pair in waiting)
{
if (pair.Value.IsCompleted)
{
finished.Add(pair.Value);
Task o;
waiting.TryRemove(pair.Key, out o);
}
}
Thread.Sleep(100);
}
}
}
/// <summary>
/// Provides a task scheduler that ensures a maximum concurrency level while
/// running on top of the ThreadPool.
/// </summary>
public 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);
}
}
}