The Thread Pool in the .NET Framework
文/黃忠成
Threads一直以來都是程式設計中最有趣也是風險最高的技巧之一，.NET Framework的出現大幅降低了Threads應用的難度及風險，
其所內建的Thread Pool機制則有效的管理Threads的數量及建立的時間，在.NET Framework中會為每個CPU核心建立一個Thread Pool，
以i7 四核8緒來說，.NET Framework會預先建立8個Thread Pool，但其可容納的Threads數量則依據版本不同而改變。
在2.0 SP1之前，每個Thread Pool預設最大可容納25個Threads，而2.0 SP1 之後則放大為每個核心 250個Threads，在4.0則改變為視乎系統狀態而定，
一般來說4.0 32-bit為1023，64 bit則為32768。
要特別注意的是，在4.0中，當處於4核8緒的環境下，前8個Threads是立即建立並執行，超過的部分則以每個間隔不超過兩秒為單位逐步建立，這意思是說，
當你排入10個工作至Thread Pool中時，前8個會立即執行，而第9個則會間隔0.5秒後執行，接著再間隔0.5秒後執行第10個，你可以由下列的程式來得到驗證。
static void TestThreadPool()
{
for (int i = 0; i < 500; i++)
{
int i1 = i;
ThreadPool.QueueUserWorkItem((state) =>
{
Console.WriteLine("{0} on thread {1}", i1, Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(500000);
});
}
}
static void Main(string[] args)
{
TestThreadPool();
Console.ReadKey();
}
執行後會發現相對於你核心數量的Threads會立即建立並執行，之後則以0.5秒的間隔建立其它的，這個間隔最大不會超過2秒，順帶一提，在TPL的行為也是一樣的。
Thread Pool的風險
與一般的Threads建立一樣，Thread Pool依然存在當設計不當導致太多Threads同時存在而耗損資源的問題，只是相對於手動建立Threads來說少多了，當遭遇這類情況時，
可以透過設定其最大的Threads數量來限制(ThreadPool.SetMaxThreads)。這個技巧有個小陷阱，你不能設定比自己核心數少的數量，也就是說當4核8緒下，8以下的數字是不被接受的，
另外ThreadPool.SetMaxThreads是全域性的，有時當預期特定的行為可能會導致大量的Threads建立時，我們可以透過一些手段來為其提供獨立且受限於特定數量的Thread Pool(注意，
其仍然受限於ThreadPool MaxThreads的最大數量)，最簡單的方式就是透過TPL。
static void TestSequrenceScheduler()
{
ConcurrentExclusiveSchedulerPair exScheduler = new ConcurrentExclusiveSchedulerPair();
TaskFactory factory = new TaskFactory(exScheduler.ExclusiveScheduler);
for (int i = 0; i < 5; i++)
{
int i1 = i;
factory.StartNew(() =>
{
Console.WriteLine("{0} on thread {1}", i1, Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(5000);
});
}
}
執行這段程式碼會發現同一時間只會有一個Thread執行，如果需要更多則可以利用下面的程式碼。
static void TestSequrenceScheduler2()
{
ConcurrentExclusiveSchedulerPair exScheduler = new ConcurrentExclusiveSchedulerPair(TaskScheduler.Default, 2);
TaskFactory factory = new TaskFactory(exScheduler.ConcurrentScheduler);
for (int i = 0; i < 5; i++)
{
int i1 = i;
factory.StartNew(() =>
{
Console.WriteLine("{0} on thread {1}", i1, Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(5000);
});
}
}透過指定建立ConcurrentExclusiveSchedulerPair物件的第二個參數，可調節這個TaskFactory可同時執行的Threads數量，以本例而言是2個。
如果需要更細緻的來控制數量，那麼下面的程式碼可以完全接管TaskFactory中排程的部分。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApplication67
{
/// <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);
}
}
}
}使用方法很簡單。
static void TestLimitTaskScheduler()
{
LimitedConcurrencyLevelTaskScheduler lcts = new LimitedConcurrencyLevelTaskScheduler(20);
TaskFactory factory = new TaskFactory(lcts);
for (int i = 0; i < 500; i++)
{
int i1 = i;
factory.StartNew(() =>
{
Console.WriteLine("{0} on thread {1}", i1, Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(5000);
});
}
}