FRDGAsyncTask

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An RDG pass execution lambda MAY be executed in a parallel task IF the lambda references a non-immediate command list AND the builder flags are set to execute in parallel. By default, if a pass executes in parallel, the task will be awaited at the end of FRDGBuilder::Execute(). This behavior may be overridden by tagging the lambda with FRDGAsyncTask as the first argument. A tagged lambda, when executed in parallel, is NOT awaited at the end of FRDGBuilder::Execute(). Instead, the task is recorded as an outstanding RHI command list task (which share semantics with mesh passes or other parallel command list tasks) and can be manually awaited by calling FRDGBuilder::WaitForAsyncExecuteTasks() or formed into a task graph with FRDGBuilder::GetAsyncExecuteTask() (both static methods). The lifetime of RDG allocations is tied to these tasks and RDG will not release any memory or allocated objects until the last task completes, even though the FRDGBuilder instance itself may go out of scope and destruct.

Consider the following examples: // Builder is marked as supporting parallel execute. FRDGBuilder GraphBuilder(RDG_EVENT_NAME("MyBuilder"), ERDGBuilderFlags::Parallel)

GraphBuilder.AddPass(RDG_EVENT_NAME("..."), PassParameters, PassFlags, [...] (FRHICommandList& RHICmdList) { // This will execute in parallel and is awaited by RDG on the render thread at the end of FRDGBuilder::Execute(). });

GraphBuilder.AddPass(RDG_EVENT_NAME("..."), PassParameters, PassFlags, [...] (FRHICommandListImmediate& RHICmdList) { // This will execute inline on the render thread, because the immediate command list is referenced. });

FMyObject* Object = GraphBuilder.AllocObject();

GraphBuilder.AddPass(RDG_EVENT_NAME("..."), PassParameters, PassFlags, [Object] (FRDGAsyncTask, FRHICommandList& RHICmdList) { // This will execute in parallel and is NOT awaited at the end of FRDGBuilder::Execute(). Accessing 'Object' is safe. });

GraphBuilder.Execute();

Tasks can be synced in a few different ways. RDG async execute tasks are chained, so syncing the last batch will sync ALL prior batches. // This will sync all RDG async execute tasks. RHICmdList.ImmediateFlush(EImmediateFlushType::WaitForOutstandingTasksOnly);

// This will also sync all RDG async execute tasks. FRDGBuilder::WaitForAsyncExecuteTasks();

// Launch a task that will do something when RDG async execute tasks complete. UE::Tasks::Launch(UE_SOURCE_LOCATION, [...] { ... }, FRDGBuilder::GetAsyncExecuteTask());

Syntax

struct FRDGAsyncTask  
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struct FRDGAsyncTask