虚幻4与现代C++:使用TaskGraph实现Fork-Join模型

Fork-Join 模型简介

Fork-Join 是一种并行编程的设计模式,通过下面这个图片可以有一个直观的理解:
虚幻4与现代C++:使用TaskGraph实现Fork-Join模型_第1张图片

图片来自维基百科

在上图中,不同颜色的方块代表着可并行执行的“任务”,它们可以根据需要从主线程中“分叉(fork)”出来执行,在需要顺序执行的点上又“合并(join)”到主线程。

使用 TaskGraph 实现 Fork-Join 模型

虚幻4的 TaskGraph 可以为每个“任务”指定“一个或多个前置任务”,也就是组成所谓的 Graph 啦!在这种框架下,Fork-Join 也是一种常用的任务组织的手法。

下面我还是通过一个简单的例子,来看看具体的编程实现。

假定我们需要从一个 Json 格式的文本文件中读取过去 20 年的上证指数数据,然后需要统计出:

  • 最大值,最小值,平均值

最后把这个三个值显示到一个 UMG 的界面上!

咋选了这么个古怪的例子呢?呃,本来是想做一个异步加载 N 个 Static Mesh 模型之类的例子,但是异步加载资源的话,其实用 FStreamableManager 更合适。为了避免误解,就想弄个简单计算的例子。

分拆任务

首先需要把上述需求分拆成多个小的任务,看看哪些可以并行执行:

任务 执行线程 说明
加载并解析 Json Any Thread 加载和解析这两个动作就放在一起了
计算最大值 Any Thread 加载之后即可执行
计算最小值 Any Thread 同上
计算平均值 Any Thread 同上
完成通知 Game Thread 通知界面更新

看下面这个图可能更直观一点:
虚幻4与现代C++:使用TaskGraph实现Fork-Join模型_第2张图片

Task Context 对象

在正式开始编写任务之前,我们需要先解决数据在任务之间“传递”和“共享”的问题。

在这里,我打算使用一个 Context 对象存储所有数据,这种方式也是引擎中很多 TaskGraph 所使用的。

下面是一个任务数据的详细分析:

数据项 主线程:任务发起 异步任务:加载并解析 异步任务:计算X3 主线程:UI更新 完成通知
数据文件路径 写入 只读 NA NA NA
完成回调 写入 NA NA NA 只读
Json 数据对象 NA 写入 只读 NA NA
计算结果X3 NA NA 写入 只读 只读

经过上面的分析之后,我设计了下面的数据结构,这个对象将在主线程和几个异步任务之间共享。结合前面那个图片中的执行序列分析,我决定:不用给Context对象加锁

struct FStockAnalyzeContext
{
	bool bRunning = false;
	FString DataFilePath;
	FTaskDelegate_StockAnalyzeComplete CompletionDelegate;
	TArray<TSharedPtr<FJsonValue>> StockData;
	FVector Result;	// {X:max, Y:min, Z:average}
};

那个 Json 对象,使用“ TSharedPtr StockData”感觉更好一点,不过,引擎中的 JSON 代码的参数写死了,只支持上面那个指针类型。我只能非常谨慎的编码,保证这些Json智能指针在访问的时候,不产生指针的复制。 如果你有更好的写法,请留言告诉我!

我们将在一个测试用的 Actor 对象里面存储一个 FStockAnalyzeContext 实例,然后在不同的 Task 之间共享它。

决定了这个 Context 数据结构之后,下面就是挨个实现每个 Task 了!

任务实现:异步加载 JSON

这个 Task 很简单,基本上就是把前一篇博客:基于任务的并行编程与TaskGraph 中的 FTask_LoadFileToString 稍加改造,在 DoTask() 中加上 Json 解析,并去掉派发子任务逻辑即可:

class FTask_LoadFileToJson
{
	FStockAnalyzeContext* Context;
public:
	FTask_LoadFileToJson(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	TStatId GetStatId() const {
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTask_LoadFileToJson, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return CPrio_StockTasks.Get(); }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		TSharedPtr<FJsonObject> JsonObject;

		// load file from Content folder
		FString FilePath = Context->DataFilePath;
		FString FileContent;
		FString FullPath = FPaths::Combine(FPaths::ProjectContentDir(), FilePath);
		if (FPaths::FileExists(FullPath))
		{
			if (FFileHelper::LoadFileToString(FileContent, *FullPath))
			{
				TSharedRef< TJsonReader<> > Reader = TJsonReaderFactory<>::Create(FileContent);
				FJsonSerializer::Deserialize(Reader, JsonObject);
			}
		}

		// write resut to context
		if (JsonObject)
			Context->StockData = JsonObject->GetArrayField(TEXT("stock"));
	}
};

为了代码简单,我没有做什么错误处理啊~

任务实现:数据统计计算

对“上证指数”求最大值、最小值、平均值,就是从 Context 中读取数据, 进行个简单的计算啦:

class FTask_StockMax
{
	FStockAnalyzeContext* Context;

public:
	FTask_StockMax(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	TStatId GetStatId() const {
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTask_StockMax, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return CPrio_StockTasks.Get(); }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		// process data 
		float Result = TNumericLimits<float>::Min();
		int32 Count = Context->GetStockDataCount();
		for (int32 i = 0; i < Count; i++)
		{
			float Value = Context->GetStockData(i);
			if (Value > Result)
				Result = Value;
		}

		// write resut to context
		Context->Result.X = Result;
	}
};

任务实现:完成通知

和前一篇博客一样:基于任务的并行编程与TaskGraph 我还是使用一个指定在 Game Thread 执行的 Task 来调用蓝图实现的事件:

class FTaskCompletion_StockAnalyze
{
	FStockAnalyzeContext* Context;
public:
	FTaskCompletion_StockAnalyze(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	FORCEINLINE TStatId GetStatId() const	{
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTaskCompletion_StockAnalyze, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return ENamedThreads::GameThread; }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		check(IsInGameThread());

		Context->CompletionDelegate.ExecuteIfBound(Context->Result);
		Context->bRunning = false;
	}
};

派发所有任务

重点来了!我们需要把任务的执行组织成下面这个图片所示:
虚幻4与现代C++:使用TaskGraph实现Fork-Join模型_第3张图片
这个重点就是使用:TGraphTask::CreateTask() 函数的第一个个参数。

void AForkJoinDemo::AsyncAnalyzeStockData(const FString& FilePath)
{
	if (TaskContext.bRunning)
		return;

	FTaskDelegate_StockAnalyzeComplete CompletionDelegate;
	CompletionDelegate.BindUFunction(this, "OnAnalyzeComplete");

	TaskContext = {};
	TaskContext.bRunning = true;
	TaskContext.CompletionDelegate = CompletionDelegate;
	TaskContext.DataFilePath = FilePath;

	FGraphEventRef LoadJson = TGraphTask<FTask_LoadFileToJson>::CreateTask().
		ConstructAndDispatchWhenReady(&TaskContext);

	// data process tasks
	FGraphEventArray RootTasks = { LoadJson };
	FGraphEventRef CalMax = TGraphTask<FTask_StockMax>::CreateTask(&RootTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

	FGraphEventRef CalMin = TGraphTask<FTask_StockMin>::CreateTask(&RootTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

	FGraphEventRef CalAverage = TGraphTask<FTask_StockAverage>::CreateTask(&RootTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

	// compeletion
	FGraphEventArray CalTasks = { CalMax, CalMin, CalAverage };
	TGraphTask<FTaskCompletion_StockAnalyze>::CreateTask(&CalTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

}

小结

通过指定任务的依赖关系,可以很方便的使用 TaskGraph 实现 Fork-Join 模型。

相关的样例工程在我的 GitHub :https://github.com/neil3d/UnrealCookBook/tree/master/MakingUseOfTaskGraph 。
本文相关的 Demo 完整源代码也附上:

ForkJoinDemo.h

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "Dom/JsonObject.h"	// Json
#include "Dom/JsonValue.h"	// Json
#include "ForkJoinDemo.generated.h"

DECLARE_DELEGATE_OneParam(FTaskDelegate_StockAnalyzeComplete, FVector);

struct FStockAnalyzeContext
{
	bool bRunning = false;
	FString DataFilePath;
	FTaskDelegate_StockAnalyzeComplete CompletionDelegate;

	TArray<TSharedPtr<FJsonValue>> StockData;
	FVector Result;	// {X:max, Y:min, Z:average}

	int32 GetStockDataCount() const;
	float GetStockData(int32 Index) const;
};

UCLASS()
class MAKINGUSEOFTASKGRAPH_API AForkJoinDemo : public AActor
{
	GENERATED_BODY()
	
public:	
	// Sets default values for this actor's properties
	AForkJoinDemo();

	UFUNCTION(BlueprintCallable)
		void AsyncAnalyzeStockData(const FString& FilePath);

	UFUNCTION(BlueprintImplementableEvent)
		void OnAnalyzeComplete(FVector Result);

protected:
	FStockAnalyzeContext TaskContext;
};

ForkJoinDemo.cpp

FStockAnalyzeContext::GetStockData() 的效率有很大优化空间,这里请忽略,咱们是谈 TaskGraph 为主。

#include "ForkJoinDemo.h"
#include "Misc/Paths.h"
#include "Misc/FileHelper.h"
#include "Math/NumericLimits.h"
#include "Async/TaskGraphInterfaces.h"	// Core
#include "Serialization/JsonReader.h"	// Json
#include "Serialization/JsonSerializer.h" // Json

int32 FStockAnalyzeContext::GetStockDataCount() const
{
	return StockData.Num();
}

float FStockAnalyzeContext::GetStockData(int32 Index) const
{
	const TSharedPtr<FJsonValue>& Element = StockData[Index];
	const TSharedPtr<FJsonObject>& Stock = Element->AsObject();
	const TSharedPtr<FJsonValue>* FieldPtr = Stock->Values.Find(TEXT("close"));

	if (!FieldPtr)
		return 0.0f;

	const TSharedPtr<FJsonValue>& Field = *FieldPtr;

	check(Field && !Field->IsNull());
	return FCString::Atof(*(Field->AsString()));
}

FAutoConsoleTaskPriority CPrio_StockTasks(
	TEXT("TaskGraph.TaskPriorities.StockTasks"),
	TEXT("Task and thread priority for stock analyzation."),
	ENamedThreads::HighThreadPriority,
	ENamedThreads::NormalTaskPriority,
	ENamedThreads::HighTaskPriority
);

class FTaskCompletion_StockAnalyze
{
	FStockAnalyzeContext* Context;

public:
	FTaskCompletion_StockAnalyze(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	FORCEINLINE TStatId GetStatId() const
	{
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTaskCompletion_StockAnalyze, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return ENamedThreads::GameThread; }

	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		check(IsInGameThread());

		Context->CompletionDelegate.ExecuteIfBound(Context->Result);
		Context->bRunning = false;
	}
};

class FTask_StockMax
{
	FStockAnalyzeContext* Context;

public:
	FTask_StockMax(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	TStatId GetStatId() const {
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTask_StockMax, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return CPrio_StockTasks.Get(); }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		// process data 
		float Result = TNumericLimits<float>::Min();
		int32 Count = Context->GetStockDataCount();
		for (int32 i = 0; i < Count; i++)
		{
			float Value = Context->GetStockData(i);
			if (Value > Result)
				Result = Value;
		}

		// write resut to context
		Context->Result.X = Result;
	}
};


class FTask_StockMin
{
	FStockAnalyzeContext* Context;

public:
	FTask_StockMin(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	TStatId GetStatId() const {
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTask_StockMin, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return CPrio_StockTasks.Get(); }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		// process data 
		float Result = TNumericLimits<float>::Max();
		int32 Count = Context->GetStockDataCount();
		for (int32 i = 0; i < Count; i++)
		{
			float Value = Context->GetStockData(i);
			if (Value < Result)
				Result = Value;
		}

		// write resut to context
		Context->Result.Y = Result;
	}
};


class FTask_StockAverage
{
	FStockAnalyzeContext* Context;

public:
	FTask_StockAverage(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	TStatId GetStatId() const {
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTask_StockAverage, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return CPrio_StockTasks.Get(); }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		// process data 
		float Result = 0;
		int32 Count = Context->GetStockDataCount();
		for (int32 i = 0; i < Count; i++)
		{
			float Value = Context->GetStockData(i);
			Result += Value;
		}

		// write resut to context
		Context->Result.Z = Result / Count;
	}
};

class FTask_LoadFileToJson
{
	FStockAnalyzeContext* Context;

public:
	FTask_LoadFileToJson(FStockAnalyzeContext* InContext) : Context(InContext)
	{}

	TStatId GetStatId() const {
		RETURN_QUICK_DECLARE_CYCLE_STAT(FTask_LoadFileToJson, STATGROUP_TaskGraphTasks);
	}

	static ENamedThreads::Type GetDesiredThread() { return CPrio_StockTasks.Get(); }
	static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; }

	void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent)
	{
		TSharedPtr<FJsonObject> JsonObject;

		// load file from Content folder
		FString FilePath = Context->DataFilePath;
		FString FileContent;
		FString FullPath = FPaths::Combine(FPaths::ProjectContentDir(), FilePath);
		if (FPaths::FileExists(FullPath))
		{
			if (FFileHelper::LoadFileToString(FileContent, *FullPath))
			{
				TSharedRef< TJsonReader<> > Reader = TJsonReaderFactory<>::Create(FileContent);
				FJsonSerializer::Deserialize(Reader, JsonObject);
			}
		}

		// write resut to context
		if (JsonObject)
			Context->StockData = JsonObject->GetArrayField(TEXT("stock"));
	}
};

// Sets default values
AForkJoinDemo::AForkJoinDemo()
{
}

void AForkJoinDemo::AsyncAnalyzeStockData(const FString& FilePath)
{
	if (TaskContext.bRunning)
		return;

	FTaskDelegate_StockAnalyzeComplete CompletionDelegate;
	CompletionDelegate.BindUFunction(this, "OnAnalyzeComplete");

	TaskContext = {};
	TaskContext.bRunning = true;
	TaskContext.CompletionDelegate = CompletionDelegate;
	TaskContext.DataFilePath = FilePath;

	FGraphEventRef LoadJson = TGraphTask<FTask_LoadFileToJson>::CreateTask().
		ConstructAndDispatchWhenReady(&TaskContext);

	// data process tasks
	FGraphEventArray RootTasks = { LoadJson };
	FGraphEventRef CalMax = TGraphTask<FTask_StockMax>::CreateTask(&RootTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

	FGraphEventRef CalMin = TGraphTask<FTask_StockMin>::CreateTask(&RootTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

	FGraphEventRef CalAverage = TGraphTask<FTask_StockAverage>::CreateTask(&RootTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

	// compeletion
	FGraphEventArray CalTasks = { CalMax, CalMin, CalAverage };
	TGraphTask<FTaskCompletion_StockAnalyze>::CreateTask(&CalTasks, ENamedThreads::AnyThread).
		ConstructAndDispatchWhenReady(&TaskContext);

}

延伸阅读

CalMin = TGraphTask::CreateTask(&RootTasks, ENamedThreads::AnyThread).
ConstructAndDispatchWhenReady(&TaskContext);

FGraphEventRef CalAverage = TGraphTask::CreateTask(&RootTasks, ENamedThreads::AnyThread).
	ConstructAndDispatchWhenReady(&TaskContext);

// compeletion
FGraphEventArray CalTasks = { CalMax, CalMin, CalAverage };
TGraphTask::CreateTask(&CalTasks, ENamedThreads::AnyThread).
	ConstructAndDispatchWhenReady(&TaskContext);

}



## 延伸阅读

- [Wiki: Fork-Join](http://en.wikipedia.org/wiki/Fork%E2%80%93join_model)

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