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一个简单的模拟实例说明Task及其调度问题
fuyao-w
2022-06-22 11:57:54 2022-06-22
13
0

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。

image

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<Job>[] _queues;
private readonly Thread[] _threads;
private readonly ManualResetEvent[] _events;
public DedicatedThreadJobScheduler (int threadCount)
{
_queues = new Queue<Job>[threadCount];
_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。

image

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

image

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:

image

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<

$>d__0这个结构体。它的实现其实很简单:如果个方法出现了N个await关键字,它们相当于将整个方法的执行流程切割成N+1段,状态机的状态体现为当前应该执行那段,具体的执行体现在MoveNext方法上。GetAwaiter方法返回的ICriticalNotifyCompletion对象用来确定当前操作是否结束,如果结束则可以直接指定后续操作,否则需要调用AwaitUnsafeOnCompleted对后续操作进行处理。

// 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 <<Main>$>d__0 : IAsyncStateMachine
{
public int <>1__state;
public AsyncTaskMethodBuilder <>t__builder;
private JobAwaiter <>u__1;
private void MoveNext()
{
int num = <>1__state;
try
{
JobAwaiter awaiter;
switch (num)
{
default:
awaiter = <<Main>$>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对象(比如状态为Completed的Task对象)的复用,以及使用IValueTaskSource等,都是为了解决内存分配的问题。

// Program
using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
using Jobs;
[CompilerGenerated]
internal class Program
{
[CompilerGenerated]
private sealed class <<Main>$>d__0 : IAsyncStateMachine
{
public int <>1__state;
public AsyncTaskMethodBuilder <>t__builder;
public string[] args;
private JobAwaiter <>u__1;
private void MoveNext()
{
int num = <>1__state;
try
{
JobAwaiter awaiter3;
JobAwaiter awaiter2;
JobAwaiter awaiter;
switch (num)
{
default:
awaiter3 = <<Main>$>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();
}
}
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