.NET6中的await原理浅析
前言看过不少关于 await 的原理的文章,也知道背后是编译器给转成了状态机实现的,但是具体是怎么完成的,回调又是如何衔接的,一直都没有搞清楚,这次下定决心把源码自己跑了下,终于豁然开朗了
本文的演示代码基于 VS2022 + .NET 6
示例
public class Program
{
static int Work()
{
Console.WriteLine("In Task.Run");
return 1;
}
static async Task TestAsync()
{
Console.WriteLine("Before Task.Run");
await Task.Run(Work);
Console.WriteLine("After Task.Run");
}
static void Main()
{
_ = TestAsync();
Console.WriteLine("End");
Console.ReadKey();
}
}
[*]很简单的异步代码,我们来看下,编译器把它变成了啥
class Program
{
static int Work()
{
Console.WriteLine("In Task.Run");
return 1;
}
static Task TestAsync()
{
var stateMachine = new StateMachine()
{
_builder = AsyncTaskMethodBuilder.Create(),
_state = -1
};
stateMachine._builder.Start(ref stateMachine);
return stateMachine._builder.Task;
}
static void Main()
{
_ = TestAsync();
Console.WriteLine("End");
Console.ReadKey();
}
class StateMachine : IAsyncStateMachine
{
public int _state;
public AsyncTaskMethodBuilder _builder;
private TaskAwaiter<int> _awaiter;
void IAsyncStateMachine.MoveNext()
{
int num = _state;
try
{
TaskAwaiter<int> awaiter;
if (num != 0)
{
Console.WriteLine("Before Task.Run");
awaiter = Task.Run(Work).GetAwaiter();
if (!awaiter.IsCompleted)
{
_state = 0;
_awaiter = awaiter;
StateMachine stateMachine = this;
_builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
return;
}
}
else
{
awaiter = _awaiter;
_awaiter = default;
_state = -1;
}
awaiter.GetResult();
Console.WriteLine("After Task.Run");
}
catch (Exception exception)
{
_state = -2;
_builder.SetException(exception);
return;
}
_state = -2;
_builder.SetResult();
}
void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine) { }
}
}
[*]编译后的代码经过我的整理,命名简化了,更容易理解
状态机实现
[*]我们看到实际是生成了一个隐藏的状态机类 StateMachine
[*]把状态机的初始状态 _state 设置 -1
[*]stateMachine._builder.Start(ref stateMachine); 启动状态机,内部实际调用的就是状态机的 MoveNext 方法
[*]Task.Run 创建一个任务, 把委托放在 Task.m_action 字段,丢到线程池,等待调度
[*]任务在线程池内被调度完成后,是怎么回到这个状态机继续执行后续代码的呢?
_builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine); 就是关键了, 跟下去,到了如下的代码:
if (!this.AddTaskContinuation(stateMachineBox, false))
{
ThreadPool.UnsafeQueueUserWorkItemInternal(stateMachineBox, true);
}
bool AddTaskContinuation(object tc, bool addBeforeOthers)
{
return !this.IsCompleted && ((this.m_continuationObject == null && Interlocked.CompareExchange(ref this.m_continuationObject, tc, null) == null) || this.AddTaskContinuationComplex(tc, addBeforeOthers));
}
[*]这里很清楚的看到,尝试把状态机对象(实际是状态机的包装类),赋值到 Task.m_continuationObject, 如果操作失败,则把状态机对象丢进线程池等待调度,这里为什么这么实现,看一下线程池是怎么执行的就清楚了
线程池实现
[*].NET6 的线程池实现,实际是放到了 PortableThreadPool, 具体调试步骤我就不放了,直接说结果就是, 线程池线程从任务队列中拿到任务后都执行了 DispatchWorkItem 方法
static void DispatchWorkItem(object workItem, Thread currentThread)
{
Task task = workItem as Task;
if (task != null)
{
task.ExecuteFromThreadPool(currentThread);
return;
}
Unsafe.As<IThreadPoolWorkItem>(workItem).Execute();
}
virtual void ExecuteFromThreadPool(Thread threadPoolThread)
{
this.ExecuteEntryUnsafe(threadPoolThread);
}
[*]我们看到, 线程池队列中的任务都是 object 类型的, 这里进行了类型判断, 如果是 Task , 直接执行 task.ExecuteFromThreadPool, 更有意思的这个方法是个虚方法,后面说明
[*]ExecuteFromThreadPool 继续追下去,我们来到了这里,代码做了简化
private void ExecuteWithThreadLocal(ref Task currentTaskSlot, Thread threadPoolThread = null)
{
this.InnerInvoke();
this.Finish(true);
}
virtual void InnerInvoke()
{
Action action = this.m_action as Action;
if (action != null)
{
action();
return;
}
}
[*]很明显 this.InnerInvoke 就是执行了最开始 Task.Run(Work) 封装的委托了, 在 m_action 字段
[*]this.Finish(true); 跟下去会发现会调用 FinishStageTwo 设置任务的完成状态,异常等, 继续调用 FinishStageThree 就来了重点: FinishContinuations 这个方法就是衔接后续回调的核心
internal void FinishContinuations()
{
object obj = Interlocked.Exchange(ref this.m_continuationObject, Task.s_taskCompletionSentinel);
if (obj != null)
{
this.RunContinuations(obj);
}
}
[*]还记得状态机实现么, Task.m_continuationObject 字段实际存储的就是状态机的包装类,这里线程池线程也会判断这个字段有值的话,就直接使用它执行后续代码了
void RunContinuations(object continuationObject)
{
var asyncStateMachineBox = continuationObject as IAsyncStateMachineBox;
if (asyncStateMachineBox != null)
{
AwaitTaskContinuation.RunOrScheduleAction(asyncStateMachineBox, flag2);
return;
}
}
static void RunOrScheduleAction(IAsyncStateMachineBox box, bool allowInlining)
{
if (allowInlining && AwaitTaskContinuation.IsValidLocationForInlining)
{
box.MoveNext();
return;
}
}
总结
[*]Task.Run 创建 Task, 把委托放在 m_action 字段, 把 Task 压入线程池队列,等待调度
[*]_builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine); 尝试把状态机对象放在 Task.m_continuationObject 字段上,等待线程池线程调度完成任务后使用(用来执行后续),若操作失败,直接把状态机对象压入线程池队列,等待调度
[*]线程池线程调度任务完成后,会判断 Task.m_continuationObject 有值,直接执行它的 MoveNext
备注
[*]状态机实现中,尝试修改 Task.m_continuationObject,可能会失败,
就会直接把状态机对象压入线程池, 但是线程池调度,不都是判断是不是 Task 类型么, 其实状态机的包装类是 Task 的子类,哈哈,是不是明白了
class AsyncStateMachineBox<TStateMachine> : Task<TResult>, IAsyncStateMachineBox where TStateMachine : IAsyncStateMachine
static void DispatchWorkItem(object workItem, Thread currentThread)
{
Task task = workItem as Task;
if (task != null)
{
task.ExecuteFromThreadPool(currentThread);
return;
}
Unsafe.As<IThreadPoolWorkItem>(workItem).Execute();
}
[*]还有就是状态机包装类,重写了 Task.ExecuteFromThreadPool,所以线程池调用 task.ExecuteFromThreadPool 就是直接调用了状态机的 MoveNext 了, Soga ^_^override void ExecuteFromThreadPool(Thread threadPoolThread)
{
this.MoveNext(threadPoolThread);
}
参考链接
[*]关于线程池和异步的更深刻的原理,大家可以参考下面的文章
概述 .NET 6 ThreadPool 实现: https://www.cnblogs.com/eventhorizon/p/15316955.html
.NET Task 揭秘（2）：Task 的回调执行与 await: https://www.cnblogs.com/eventhorizon/p/15912383.html
来源:https://www.cnblogs.com/broadm/archive/2023/11/15/17833442.html
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