一个简单的模拟实例说明Task及其调度问题
Task对于.NET的重要性毋庸置疑。通过最近的一些面试经历，发现很多人对与Task及其调度机制，以及线程和线程池之间的关系并没有清晰的认识。本文采用最简单的方式模拟了Task的实现，旨在说明Task是什么？它是如何被调度执行的？
Task对于.NET的重要性毋庸置疑。通过最近的一些面试经历，发现很多人对与Task及其调度机制，以及线程和线程池之间的关系并没有清晰的认识。本文采用最简单的方式模拟了Task的实现，旨在说明Task是什么？它是如何被调度执行的？源代码从这里下载。
一、Task（Job）
二、TaskScheduler（JobScheduler）
三、基于线程池的调度
四、使用指定线程进行调度
五、异步等待
六、await关键字的运用
七、状态机
一、Task（Job）
Task代表一项具有某种状态的操作，我们使用如下这个Job类型来模拟Task。Job封装的操作体现为一个Action委托，状态则通过JobStatus枚举来表示（对应TaskStatus枚举）。简单起见，我们仅仅定义了四种状态（创建、调度、执行和完成）。Invoke方法负责执行封装的Action委托，并对状态进行相应设置。
public class Job
{
private readonly Action _work;
public Job(Action work)=> _work = work;
public JobStatus Status { get; internal set; }
internal protected virtual void Invoke()
{
Status = JobStatus.Running;
\_work();
Status = JobStatus.Completed;
}
}
public enum JobStatus
{
Created,
Scheduled,
Running,
Completed
}
二、TaskScheduler（JobScheduler）
Task承载的操作通过调度得以执行，具体的调度策略取决于调度器的选择。Task调度器通过TaskScheduler表示，我们利用如下这个JobScheduler类型对它进行模拟。如下面的代码片段所示，我们只为抽象类JobScheduler定义了唯一的QueueJob方法来调度作为参数的Job对象。静态Current属性表示当前默认实现的调度器。
public abstract class JobScheduler
{
public abstract void QueueJob(Job job);
public static JobScheduler Current { get; set; } = new ThreadPoolJobScheduler ();
}
对于开发者来说，执行Task就是将它提交给调度器，这一操作体现在我们为Job类型定义的静态Start方法中。该方法通过参数指定具体的调度器，如果没有显式指定，默认采用JobScheduler的Current静态属性设置的默认调度器。为了方便后面的演示，我们还定义了一个静态的Run方法，该方法会将指定的Action对象封装成Job，并调用Start方法利用默认的调度器进行调度。
public class Job
{
private readonly Action _work;
public Job(Action work)=> _work = work;
public JobStatus Status { get; internal set; }
internal protected virtual void Invoke()
{
Status = JobStatus.Running;
\_work();
Status = JobStatus.Completed;
}
** public void Start(JobScheduler? scheduler = null) => (scheduler ?? JobScheduler.Current).QueueJob(this);
public static Job Run(Action work)
{
var job = new Job(work);
job.Start();
return job;
}**
}
三、基于线程池的调度
Task如何执行取决于选择怎样的调度器，.NET默认采用基于线程池的调度策略，这一策略体现在ThreadPoolTaskScheduler类型上，我们使用如下这个ThreadPoolJobScheduler 进行模拟。如下面的代码片段所示，重写的QueueJob方法通过调用ThreadPool.QueueUserWorkItem方法执行指定Job对象封装的Action委托。JobScheduler的Current属性设置的默认调度器就是这么一个ThreadPoolJobScheduler 对象。
public class ThreadPoolJobScheduler : JobScheduler
{
public override void QueueJob(Job job)
{
job.Status = JobStatus.Scheduled;
var executionContext = ExecutionContext.Capture();
ThreadPool.QueueUserWorkItem(_ => ExecutionContext.Run(executionContext!, _ => job.Invoke(), null));
}
}
我们按照如下的方式调用Job的静态Run方法创建并执行了三个Job，每个Job封装的Action委托在执行的时候会将当前线程ID打印出来。
_ = Job.Run(() => Console.WriteLine($”Job1 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job2 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job3 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
Console.ReadLine();
由于采用默认的基于线程池的调度策略，所以三个Job会在三个不同的线程上执行。
四、使用指定线程进行调度
我们知道.NET进程只有一个全局的线程池，对于一些需要长时间运行且具有较高优先级的操作，采用基于线程池的调用未必是好的选择。比如在一个Web应用中，线程池的工作线程会被用来处理请求，对于一个需要持续运行的Job可能会因为可用工作线程的不足而被阻塞。.NET对于这种情况具有不同的处理方式（启动Task的时候选择TaskCreationOptions.LongRunning选项），这里我们使用自定义调度器的方式来解决这个问题。如下这个DedicatedThreadJobScheduler 利用创建的“专有线程”来保证被调用的Job能够“立即”执行。线程的数量通过构造函数的参数指定，线程在无事可做的时候被“挂起”以及有新的Job被调度时被“复苏”通过一个ManualResetEvent对象来完成。
public class DedicatedThreadJobScheduler : JobScheduler
{
private readonly Queue
private readonly Thread[] _threads;
private readonly ManualResetEvent[] _events;
public DedicatedThreadJobScheduler (int threadCount)
{
_queues = new Queue
_threads = new Thread[threadCount];
_events = new ManualResetEvent[threadCount];
for (int index = 0; index < threadCount; index++)
{
var queue = \_queues\[index\] = new Queue<Job>();
var thread = \_threads\[index\] = new Thread(Invoke);
\_events\[index\] = new ManualResetEvent(true);
thread.Start(index);
}
void Invoke(object? state)
{
var index = (int)state!;
var @event = \_events\[index\];
while (true)
{
if (@event.WaitOne())
{
while (true)
{
if (!\_queues\[index\].TryDequeue(out var job))
{
Suspend(index);
break;
}
job.Invoke();
}
}
}
}
}
public override void QueueJob(Job job)
{
job.Status = JobStatus.Scheduled;
var (queue, index) = \_queues.Select((queue, index) => (queue, index)).OrderBy(it => it.queue.Count).First();
queue.Enqueue(job);
Resume(index);
}
public void Suspend(int index) => \_events\[index\].Reset();
public void Resume(int index) => \_events\[index\].Set();
}
还是上面演示的程序，这次我们将当前调度器设置为上面这个DedicatedThreadJobScheduler ，并将使用的线程数设置为2。
JobScheduler.Current = new DedicatedThreadJobScheduler (2);
_ = Job.Run(() => Console.WriteLine($”Job1 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job2 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job3 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job4 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job5 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
_ = Job.Run(() => Console.WriteLine($”Job6 is excuted in thread {Thread.CurrentThread.ManagedThreadId}”));
Console.ReadLine();
我们会发现所有的操作只会在两个固定的线程中被执行。
五、异步等待
如果需要在某个Task执行之后接着执行后续的操作，我们可以调用其ContinueWith方法指定待执行的操作，现在我们将这个方法定义Job类型上。Job与Task的ContinueWith有些差异，在这里我们认为ContinueWith指定的也是一个Job，那么多个Job则可以按照预先编排的顺序构成一个链表。当前Job执行后，只需要将后续这个Job交付给调度器就可以了。如下面的代码片段所示，我们利用_continue字段来表示异步等待执行的Job，并利用它维持一个Job链表。ContinueWith方法会将指定的Action委托封装成Job并添加到链表末端。
public class Job
{
private readonly Action _work;
private Job? _continue;
public Job(Action work) => _work = work;
public JobStatus Status { get; internal set; }
public void Start(JobScheduler? scheduler = null) => (scheduler ?? JobScheduler.Current).QueueJob(this);
internal protected virtual void Invoke()
{
Status = JobStatus.Running;
_work();
Status = JobStatus.Completed;
_continue?.Start();
}
public static Job Run(Action work)
{
var job = new Job(work);
job.Start();
return job;
}
public Job ContinueWith(Action<Job> continuation)
{
if (\_continue == null)
{
var job = new Job(() => continuation(this));
\_continue = job;
}
else
{
\_continue.ContinueWith(continuation);
}
return this;
}
}
利用ContinueWith方法实现异步操作的按序执行体现在如下的程序中。
Job.Run(() =>
{
Thread.Sleep(1000);
Console.WriteLine(“Foo1”);
}).ContinueWith(_ =>
{
Thread.Sleep(100);
Console.WriteLine(“Bar1”);
}).ContinueWith(_ =>
{
Thread.Sleep(100);
Console.WriteLine(“Baz1”);
});
Job.Run(() =>
{
Thread.Sleep(100);
Console.WriteLine(“Foo2”);
}).ContinueWith(_ =>
{
Thread.Sleep(10);
Console.WriteLine(“Bar2”);
}).ContinueWith(_ =>
{
Thread.Sleep(10);
Console.WriteLine(“Baz2”);
});
Console.ReadLine();
输出结果
六、await关键字的运用
虽然ContinueWith方法能够解决“异步等待”的问题，但是我们更喜欢使用await关键字，接下来我们就为Job赋予这个能力。为此我们定义了如下这个实现了ICriticalNotifyCompletion接口的JobAwaiter结构体。顾名思义，该接口用来发送操作完成的通知。一个JobAwaiter对象由一个Job对象构建而成，当它自身执行完成之后，OnCompleted方法会被调用，我们利用它执行后续的操作。
public struct JobAwaiter: ICriticalNotifyCompletion
{
private readonly Job _job;
public bool IsCompleted => _job.Status == JobStatus.Completed;
public JobAwaiter(Job job)
{
_job = job;
if (job.Status == JobStatus.Created)
{
job.Start();
}
}
public void OnCompleted(Action continuation)
{
_job.ContinueWith(_ => continuation());
}
public void GetResult() { }
public void UnsafeOnCompleted(Action continuation)=>OnCompleted(continuation);
}
我们在Job类型上添加这个GetAwaiter方法返回根据自身创建的JobAwaiter对象。
public class Job
{
private readonly Action _work;
private Job? _continue;
public Job(Action work) => _work = work;
public JobStatus Status { get; internal set; }
public void Start(JobScheduler? scheduler = null) => (scheduler ?? JobScheduler.Current).QueueJob(this);
internal protected virtual void Invoke()
{
Status = JobStatus.Running;
_work();
Status = JobStatus.Completed;
_continue?.Start();
}
public static Job Run(Action work)
{
var job = new Job(work);
job.Start();
return job;
}
public Job ContinueWith(Action<Job> continuation)
{
if (\_continue == null)
{
var job = new Job(() => continuation(this));
\_continue = job;
}
else
{
\_continue.ContinueWith(continuation);
}
return this;
}
**public JobAwaiter GetAwaiter() => new(this);**
}
任何一个类型一旦拥有了这样一个GetAwaiter方法，我们就能将await关键词应用在对应的对象上面。
await Foo();
await Bar();
await Baz();
Console.ReadLine();
static Job Foo() => new Job(() =>
{
Thread.Sleep(1000);
Console.WriteLine(“Foo”);
});
static Job Bar() => new Job(() =>
{
Thread.Sleep(100);
Console.WriteLine(“Bar”);
});
static Job Baz() => new Job(() =>
{
Thread.Sleep(10);
Console.WriteLine(“Baz”);
});
输出结果：
七、状态机
我想你应该知道await关键字仅仅是编译器提供的语法糖，编译后的代码会利用一个“状态机”实现“异步等待”的功能，上面这段代码最终编译成如下的形式。值得一提的是，Debug和Release模式编译出来的代码是不同的，下面给出的是Release模式下的编译结果，上述的状态机体现为生成的<
// Program
using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Threading.Tasks;
using Jobs;
[CompilerGenerated]
internal class Program
{
[StructLayout(LayoutKind.Auto)]
[CompilerGenerated]
private struct <
{
public int <>1__state;
public AsyncTaskMethodBuilder <>t\_\_builder;
private JobAwaiter <>u\_\_1;
private void MoveNext()
{
int num = <>1\_\_state;
try
{
JobAwaiter awaiter;
switch (num)
{
default:
awaiter = <<Main>$>g\_\_Foo|0\_0().GetAwaiter();
if (!awaiter.IsCompleted)
{
num = (<>1\_\_state = 0);
<>u\_\_1 = awaiter;
<>t\_\_builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
return;
}
goto IL\_006c;
case 0:
awaiter = <>u\_\_1;
<>u\_\_1 = default(JobAwaiter);
num = (<>1\_\_state = -1);
goto IL\_006c;
case 1:
awaiter = <>u\_\_1;
<>u\_\_1 = default(JobAwaiter);
num = (<>1\_\_state = -1);
goto IL\_00c6;
case 2:
{
awaiter = <>u\_\_1;
<>u\_\_1 = default(JobAwaiter);
num = (<>1\_\_state = -1);
break;
}
IL\_00c6:
awaiter.GetResult();
awaiter = <<Main>$>g\_\_Baz|0\_2().GetAwaiter();
if (!awaiter.IsCompleted)
{
num = (<>1\_\_state = 2);
<>u\_\_1 = awaiter;
<>t\_\_builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
return;
}
break;
IL\_006c:
awaiter.GetResult();
awaiter = <<Main>$>g\_\_Bar|0\_1().GetAwaiter();
if (!awaiter.IsCompleted)
{
num = (<>1\_\_state = 1);
<>u\_\_1 = awaiter;
<>t\_\_builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
return;
}
goto IL\_00c6;
}
awaiter.GetResult();
Console.ReadLine();
}
catch (Exception exception)
{
<>1\_\_state = -2;
<>t\_\_builder.SetException(exception);
return;
}
<>1\_\_state = -2;
<>t\_\_builder.SetResult();
}
void IAsyncStateMachine.MoveNext()
{
//ILSpy generated this explicit interface implementation from .override directive in MoveNext
this.MoveNext();
}
\[DebuggerHidden\]
private void SetStateMachine(\[System.Runtime.CompilerServices.Nullable(1)\] IAsyncStateMachine stateMachine)
{
<>t\_\_builder.SetStateMachine(stateMachine);
}
void IAsyncStateMachine.SetStateMachine(\[System.Runtime.CompilerServices.Nullable(1)\] IAsyncStateMachine stateMachine)
{
//ILSpy generated this explicit interface implementation from .override directive in SetStateMachine
this.SetStateMachine(stateMachine);
}
}
\[AsyncStateMachine(typeof(<<Main>$>d\_\_0))\]
private static Task <Main>$(string\[\] args)
{
<<Main>$>d\_\_0 stateMachine = default(<<Main>$>d\_\_0);
stateMachine.<>t\_\_builder = AsyncTaskMethodBuilder.Create();
stateMachine.<>1\_\_state = -1;
stateMachine.<>t\_\_builder.Start(ref stateMachine);
return stateMachine.<>t\_\_builder.Task;
}
\[SpecialName\]
private static void <Main>(string\[\] args)
{
<Main>$(args).GetAwaiter().GetResult();
}
}
上面提到过，编译器生成的状态机代码在Debug和Release模式是不一样的。在Release模式下状态机是一个结构体，虽然是以接口ICriticalNotifyCompletion的方式使用它，但是由于使用了ref关键字，所以不会涉及装箱，所以不会对GC造成任何影响。但是Debug模式下生成的状态机则是一个类（如下所示），将会涉及针对堆内存的分配和回收。对于遍布await关键字的应用程序，两者之间的性能差异肯定是不同的。实际上针对Task的很多优化策略，比如使用ValueTask，对某些Task
// Program
using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
using Jobs;
[CompilerGenerated]
internal class Program
{
[CompilerGenerated]
private sealed class <
{
public int <>1__state;
public AsyncTaskMethodBuilder <>t\_\_builder;
public string\[\] args;
private JobAwaiter <>u\_\_1;
private void MoveNext()
{
int num = <>1\_\_state;
try
{
JobAwaiter awaiter3;
JobAwaiter awaiter2;
JobAwaiter awaiter;
switch (num)
{
default:
awaiter3 = <<Main>$>g\_\_Foo|0\_0().GetAwaiter();
if (!awaiter3.IsCompleted)
{
num = (<>1\_\_state = 0);
<>u\_\_1 = awaiter3;
<<Main>$>d\_\_0 stateMachine = this;
<>t\_\_builder.AwaitUnsafeOnCompleted(ref awaiter3, ref stateMachine);
return;
}
goto IL\_007e;
case 0:
awaiter3 = <>u\_\_1;
<>u\_\_1 = default(JobAwaiter);
num = (<>1\_\_state = -1);
goto IL\_007e;
case 1:
awaiter2 = <>u\_\_1;
<>u\_\_1 = default(JobAwaiter);
num = (<>1\_\_state = -1);
goto IL\_00dd;
case 2:
{
awaiter = <>u\_\_1;
<>u\_\_1 = default(JobAwaiter);
num = (<>1\_\_state = -1);
break;
}
IL\_00dd:
awaiter2.GetResult();
awaiter = <<Main>$>g\_\_Baz|0\_2().GetAwaiter();
if (!awaiter.IsCompleted)
{
num = (<>1\_\_state = 2);
<>u\_\_1 = awaiter;
<<Main>$>d\_\_0 stateMachine = this;
<>t\_\_builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
return;
}
break;
IL\_007e:
awaiter3.GetResult();
awaiter2 = <<Main>$>g\_\_Bar|0\_1().GetAwaiter();
if (!awaiter2.IsCompleted)
{
num = (<>1\_\_state = 1);
<>u\_\_1 = awaiter2;
<<Main>$>d\_\_0 stateMachine = this;
<>t\_\_builder.AwaitUnsafeOnCompleted(ref awaiter2, ref stateMachine);
return;
}
goto IL\_00dd;
}
awaiter.GetResult();
Console.ReadLine();
}
catch (Exception exception)
{
<>1\_\_state = -2;
<>t\_\_builder.SetException(exception);
return;
}
<>1\_\_state = -2;
<>t\_\_builder.SetResult();
}
void IAsyncStateMachine.MoveNext()
{
//ILSpy generated this explicit interface implementation from .override directive in MoveNext
this.MoveNext();
}
\[DebuggerHidden\]
private void SetStateMachine(\[System.Runtime.CompilerServices.Nullable(1)\] IAsyncStateMachine stateMachine)
{
}
void IAsyncStateMachine.SetStateMachine(\[System.Runtime.CompilerServices.Nullable(1)\] IAsyncStateMachine stateMachine)
{
//ILSpy generated this explicit interface implementation from .override directive in SetStateMachine
this.SetStateMachine(stateMachine);
}
}
\[AsyncStateMachine(typeof(<<Main>$>d\_\_0))\]
\[DebuggerStepThrough\]
private static Task <Main>$(string\[\] args)
{
<<Main>$>d\_\_0 stateMachine = new <<Main>$>d\_\_0();
stateMachine.<>t\_\_builder = AsyncTaskMethodBuilder.Create();
stateMachine.args = args;
stateMachine.<>1\_\_state = -1;
stateMachine.<>t\_\_builder.Start(ref stateMachine);
return stateMachine.<>t\_\_builder.Task;
}
\[SpecialName\]
\[DebuggerStepThrough\]
private static void <Main>(string\[\] args)
{
<Main>$(args).GetAwaiter().GetResult();
}
}