C#教程之.NET Task揭秘（一）

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C#教程之.NET Task揭秘（一）

Task为.NET提供了基于任务的异步模式，它不是线程，它运行在线程池的线程上。本着开源的精神，本文以解读基于.NET4.5 Task源码的方式来揭秘Task的实现原理。

Task的创建

Task的创建方式主要有2种：Task.Run 和Task.Factory.StartNew，各自有不同的overload，这里只解读其中的一种方式，其他有兴趣的请自行解读。

先来看看Task.Run源码：

1 public static Task Run(Action action, CancellationToken cancellationToken)
2 {
3 StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
4 return Task.InternalStartNew((Task) null, (Delegate) action, (object) null, cancellationToken, TaskScheduler.Default, TaskCreationOptions.DenyChildAttach, InternalTaskOptions.None, ref stackMark);
5 }

调用了Task.InternalStartNew，第一个参数为null，并传入TaskScheduler.Default和TaskCreationOptions.DenyChildAttach.

再来看看Task.Factory.StartNew源码：

1 public Task StartNew(Action<object> action, object state, CancellationToken cancellationToken)
2 {
3 StackCrawlMark stackMark = StackCrawlMark.LookForMyCaller;
4 Task internalCurrent = Task.InternalCurrent;
5 return Task.InternalStartNew(internalCurrent, (Delegate) action, state, cancellationToken, this.GetDefaultScheduler(internalCurrent), this.m_defaultCreationOptions, InternalTaskOptions.None, ref stackMark);
6 }

也是调用Task.InternalStartNew，第一个参数为internalCurrent，当前为null，并传入GetDefaultScheduler(internalCurrent)和m_defaultCreationOptions。

1 private TaskScheduler GetDefaultScheduler(Task currTask)
2 {
3 if (this.m_defaultScheduler != null)
4 return this.m_defaultScheduler;
5 if (currTask != null && (currTask.CreationOptions & TaskCreationOptions.HideScheduler) == TaskCreationOptions.None)
6 return currTask.ExecutingTaskScheduler;
7 return TaskScheduler.Default;
8 }

如果internalCurrent不为空而且options是TaskCreationOptions.HideScheduler，那么启用internalCurrent的TaskScheduler。可惜internalCurrent为null，所以启用默认的TaskScheduler，跟入代码发现默认的TaskScheduler是ThreadPoolTaskScheduler，看名字就知道用的是线程池的任务调度，跟“黑盒”传说的一样的。m_defaultCreationOptions在Task.Factory的默认无参构造函数里被赋值TaskCreationOptions.None。

1 public abstract class TaskScheduler
2 {
3 private static readonly ConditionalWeakTable<TaskScheduler, object> s_activeTaskSchedulers = new ConditionalWeakTable<TaskScheduler, object>();
4 private static readonly TaskScheduler s_defaultTaskScheduler = (TaskScheduler) new ThreadPoolTaskScheduler();
5 ...
6 }

目前来看两个方法最大的区别在于TaskCreationOption的不同，一个是DenyChildAttach，另一个是None。

接着往下看InternalStartNew：

1 internal static Task InternalStartNew(Task creatingTask, Delegate action, object state, CancellationToken cancellationToken, TaskScheduler scheduler, TaskCreationOptions options, InternalTaskOptions internalOptions, ref StackCrawlMark stackMark)
2 {
3 if (scheduler == null)
4 throw new ArgumentNullException("scheduler");
5 Task task = new Task(action, state, creatingTask, cancellationToken, options, internalOptions | InternalTaskOptions.QueuedByRuntime, scheduler);
6 task.PossiblyCaptureContext(ref stackMark);
7 task.ScheduleAndStart(false);
8 return task;
9 }

首先实例化一个Task：

1 internal Task(Delegate action, object state, Task parent, CancellationToken cancellationToken, TaskCreationOptions creationOptions, InternalTaskOptions internalOptions, TaskScheduler scheduler)
2 {
3 if (action == null)
4 throw new ArgumentNullException("action");
5 if ((creationOptions & TaskCreationOptions.AttachedToParent) != TaskCreationOptions.None || (internalOptions & InternalTaskOptions.SelfReplicating) != InternalTaskOptions.None)
6 this.m_parent = parent;
7 this.TaskConstructorCore((object) action, state, cancellationToken, creationOptions, internalOptions, scheduler);
8 }

如果option是AttachToParent，那么internalCurrent就赋值给m_parent，目前为null，SelfReplicating是用来做并行计算的，会在TPL里详解。随后调用TaskConstructorCore。

 1 internal void TaskConstructorCore(object action, object state, CancellationToken cancellationToken, TaskCreationOptions creationOptions, InternalTaskOptions internalOptions, TaskScheduler scheduler)
 2 {
 3 this.m_action = action;
 4 this.m_stateObject = state;
 5 this.m_taskScheduler = scheduler;
 6 if ((creationOptions & ~(TaskCreationOptions.PreferFairness | TaskCreationOptions.LongRunning | TaskCreationOptions.AttachedToParent | TaskCreationOptions.DenyChildAttach | TaskCreationOptions.HideScheduler | TaskCreationOptions.RunContinuationsAsynchronously)) != TaskCreationOptions.None)
 7 throw new ArgumentOutOfRangeException("creationOptions");
 8 if ((creationOptions & TaskCreationOptions.LongRunning) != TaskCreationOptions.None && (internalOptions & InternalTaskOptions.SelfReplicating) != InternalTaskOptions.None)
 9 throw new InvalidOperationException(Environment.GetResourceString("Task_ctor_LRandSR"));
10 int num = (int) (creationOptions | (TaskCreationOptions) internalOptions);
11 if (this.m_action == null || (internalOptions & InternalTaskOptions.ContinuationTask) != InternalTaskOptions.None)
12 num |= 33554432;
13 this.m_stateFlags = num;
14 if (this.m_parent != null && (creationOptions & TaskCreationOptions.AttachedToParent) != TaskCreationOptions.None && (this.m_parent.CreationOptions & TaskCreationOptions.DenyChildAttach) == TaskCreationOptions.None)
15 this.m_parent.AddNewChild();
16 if (!cancellationToken.CanBeCanceled)
17 return;
18 this.AssignCancellationToken(cancellationToken, (Task) null, (TaskContinuation) null);
19 }

如果options不为DenyChildAttach而且m_parent不为空，则把当前task作为child添加到m_parent。也就是说Task.Run不允许把要执行的task作为当前task的child。

Task已创建，接着调用PossiblyCaptureContext来获取execution context。

 1 internal static ExecutionContext Capture(ref StackCrawlMark stackMark, ExecutionContext.CaptureOptions options)
 2 {
 3 ExecutionContext.Reader executionContextReader = Thread.CurrentThread.GetExecutionContextReader();
 4 if (executionContextReader.IsFlowSuppressed)
 5 return (ExecutionContext) null;
 6 SecurityContext securityContext = SecurityContext.Capture(executionContextReader, ref stackMark);
 7 HostExecutionContext executionContext1 = HostExecutionContextManager.CaptureHostExecutionContext();
 8 SynchronizationContext synchronizationContext = (SynchronizationContext) null;
 9 LogicalCallContext logicalCallContext = (LogicalCallContext) null;
10 if (!executionContextReader.IsNull)
11 {
12 if ((options & ExecutionContext.CaptureOptions.IgnoreSyncCtx) == ExecutionContext.CaptureOptions.None)
13 synchronizationContext = executionContextReader.SynchronizationContext == null ? (SynchronizationContext) null : executionContextReader.SynchronizationContext.CreateCopy();
14 if (executionContextReader.LogicalCallContext.HasInfo)
15 logicalCallContext = executionContextReader.LogicalCallContext.Clone();
16 }
17 Dictionary<IAsyncLocal, object> dictionary = (Dictionary<IAsyncLocal, object>) null;
18 List<IAsyncLocal> asyncLocalList = (List<IAsyncLocal>) null;
19 if (!executionContextReader.IsNull)
20 {
21 dictionary = executionContextReader.DangerousGetRawExecutionContext()._localValues;
22 asyncLocalList = executionContextReader.DangerousGetRawExecutionContext()._localChangeNotifications;
23 }
24 if ((options & ExecutionContext.CaptureOptions.OptimizeDefaultCase) != ExecutionContext.CaptureOptions.None && securityContext == null && (executionContext1 == null && synchronizationContext == null) && ((logicalCallContext == null || !logicalCallContext.HasInfo) && (dictionary == null && asyncLocalList == null)))
25 return ExecutionContext.s_dummyDefaultEC;
26 ExecutionContext executionContext2 = new ExecutionContext();
27 executionContext2.SecurityContext = securityContext;
28 if (executionContext2.SecurityContext != null)
29 executionContext2.SecurityContext.ExecutionContext = executionContext2;
30 executionContext2._hostExecutionContext = executionContext1;
31 executionContext2._syncContext = synchronizationContext;
32 executionContext2.LogicalCallContext = logicalCallContext;
33 executionContext2._localValues = dictionary;
34 executionContext2._localChangeNotifications = asyncLocalList;
35 executionContext2.isNewCapture = true;
36 return executionContext2;
37 }

ExecutionContext包含了SecurityContext，SynchronizationContext以及LogicalCallContext，其中SynchronizationContext需要做CreateCopy，LogicalCallContext需要做clone，所有这一切都是用户态的，不涉及内核，性能棒棒哒！

接着调用ScheduleAndStart:

 1 internal void ScheduleAndStart(bool needsProtection)
 2 {
 3 if (needsProtection)
 4 {
 5 if (!this.MarkStarted())
 6 return;
 7 }
 8 else
 9 this.m_stateFlags = this.m_stateFlags | 65536;
10 if (Task.s_asyncDebuggingEnabled)
11 Task.AddToActiveTasks(this);
12 if (AsyncCausalityTracer.LoggingOn && (this.Options & (TaskCreationOptions) 512) == TaskCreationOptions.None)
13 AsyncCausalityTracer.TraceOperationCreation(CausalityTraceLevel.Required, this.Id, "Task: " + ((Delegate) this.m_action).Method.Name, 0UL);
14 try
15 {
16 this.m_taskScheduler.InternalQueueTask(this);
17 }
18 catch (ThreadAbortException ex)
19 {
20 this.AddException((object) ex);
21 this.FinishThreadAbortedTask(true, false);
22 }
23 catch (System.Exception ex)
24 {
25 TaskSchedulerException schedulerException = new TaskSchedulerException(ex);
26 this.AddException((object) schedulerException);
27 this.Finish(false);
28 if ((this.Options & (TaskCreationOptions) 512) == TaskCreationOptions.None)
29 this.m_contingentProperties.m_exceptionsHolder.MarkAsHandled(false);
30 throw schedulerException;
31 }
32 }
33  
34 internal void InternalQueueTask(Task task)
35 {
36 task.FireTaskScheduledIfNeeded(this);
37 this.QueueTask(task);
38 }

FireTaskScheduledIfNeeded判断是否开启EWT Trace，接着调用ThreadPoolTaskScheduler.QueueTask。

 1 private static readonly ParameterizedThreadStart s_longRunningThreadWork = new ParameterizedThreadStart(ThreadPoolTaskScheduler.LongRunningThreadWork);
 2 private static void LongRunningThreadWork(object obj)
 3 {
 4 (obj as Task).ExecuteEntry(false);
 5 }
 6 protected internal override void QueueTask(Task task)
 7 {
 8 if ((task.Options & TaskCreationOptions.LongRunning) != TaskCreationOptions.None)
 9 {
10 new Thread(ThreadPoolTaskScheduler.s_longRunningThreadWork)
11 {
12 IsBackground = true
13 }.Start((object) task);
14 }
15 else
16 {
17 bool forceGlobal = (uint) (task.Options & TaskCreationOptions.PreferFairness) > 0U;
18 ThreadPool.UnsafeQueueCustomWorkItem((IThreadPoolWorkItem) task, forceGlobal);
19 }
20 }

如果options是LongRunning，那么单独创建一个线程执行该任务(ExecuteEntry)，否则就调用ThreadPool.UnsafeQueueCustomWorkItem，这个方法我们熟，还记得在.net线程池内幕里有讲到的global work queue和local work queue吗？给ThreadPool添加一个任务实际上是在global work queue添加一个任务，而task就是往local work queue里添加任务。

ThreadPoolWorkQueue源码：

 1 public void Enqueue(IThreadPoolWorkItem callback, bool forceGlobal)
 2 {
 3 ThreadPoolWorkQueueThreadLocals queueThreadLocals = (ThreadPoolWorkQueueThreadLocals) null;
 4 if (!forceGlobal)
 5 queueThreadLocals = ThreadPoolWorkQueueThreadLocals.threadLocals;
 6 if (this.loggingEnabled)
 7 FrameworkEventSource.Log.ThreadPoolEnqueueWorkObject((object) callback);
 8 if (queueThreadLocals != null)
 9 {
10 queueThreadLocals.workStealingQueue.LocalPush(callback);
11 }
12 else
13 {
14 ThreadPoolWorkQueue.QueueSegment comparand = this.queueHead;
15 while (!comparand.TryEnqueue(callback))
16 {
17 Interlocked.CompareExchange<ThreadPoolWorkQueue.QueueSegment>(ref comparand.Next, new ThreadPoolWorkQueue.QueueSegment(), (ThreadPoolWorkQueue.QueueSegment) null);
18 for (; comparand.Next != null; comparand = this.queueHead)
19 Interlocked.CompareExchange<ThreadPoolWorkQueue.QueueSegment>(ref this.queueHead, comparand.Next, comparand);
20 }
21 }
22 this.EnsureThreadRequested();
23 }

由于线程已经执行过任务（global的也有可能是local的），所以代码会走到queueThreadLocals.workStealingQueue.LocalPush(callback)。

 1 internal volatile IThreadPoolWorkItem[] m_array = new IThreadPoolWorkItem[32];
 2 private SpinLock m_foreignLock = new SpinLock(false);
 3 public void LocalPush(IThreadPoolWorkItem obj)
 4 {
 5 int num1 = this.m_tailIndex;
 6 if (num1 == int.MaxValue)
 7 {
 8 bool lockTaken = false;
 9 try
10 {
11 this.m_foreignLock.Enter(ref lockTaken);
12 if (this.m_tailIndex == int.MaxValue)
13 {
14 this.m_headIndex = this.m_headIndex & this.m_mask;
15 this.m_tailIndex = num1 = this.m_tailIndex & this.m_mask;
16 }
17 }
18 finally
19 {
20 if (lockTaken)
21 this.m_foreignLock.Exit(true);
22 }
23 }
24 if (num1 < this.m_headIndex + this.m_mask)
25 {
26 Volatile.Write<IThreadPoolWorkItem>(ref this.m_array[num1 & this.m_mask], obj);
27 this.m_tailIndex = num1 + 1;
28 }
29 else
30 {
31 bool lockTaken = false;
32 try
33 {
34 this.m_foreignLock.Enter(ref lockTaken);
35 int num2 = this.m_headIndex;
36 int num3 = this.m_tailIndex - this.m_headIndex;
37 if (num3 >= this.m_mask)
38 {
39 IThreadPoolWorkItem[] threadPoolWorkItemArray = new IThreadPoolWorkItem[this.m_array.Length << 1];
40 for (int index = 0; index < this.m_array.Length; ++index)
41 threadPoolWorkItemArray[index] = this.m_array[index + num2 & this.m_mask];
42 this.m_array = threadPoolWorkItemArray;
43 this.m_headIndex = 0;
44 this.m_tailIndex = num1 = num3;
45 this.m_mask = this.m_mask << 1 | 1;
46 }
47 Volatile.Write<IThreadPoolWorkItem>(ref this.m_array[num1 & this.m_mask], obj);
48 this.m_tailIndex = num1 + 1;
49 }
50 finally
51 {
52 if (lockTaken)
53 this.m_foreignLock.Exit(false);
54 }
55 }
56 }

Local work queue(m_array)首先被限死为32，如果queue超过最大数了，则扩大为原来的2倍，以此类推。这里也使用了自旋锁和内存写屏障来代替同步锁提高性能。

至此，task已被创建好，并加入到了ThreadPool的local work queue。那么task是如何被调度的呢？为什么LongRunning就要单独起一个线程去做？请听下回分解!

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