r.RHICmdWidth

r.RHICmdWidth

#Overview

name: r.RHICmdWidth

This variable is created as a Console Variable (cvar).

• type: Var
• help: Controls the task granularity of a great number of things in the parallel renderer.

It is referenced in 4 C++ source files.

#Summary

#Usage in the C++ source code

The purpose of r.RHICmdWidth is to control the task granularity of various operations in the parallel renderer of Unreal Engine 5. This setting variable plays a crucial role in managing the parallelization of rendering tasks.

• The RHI (Rendering Hardware Interface) subsystem of Unreal Engine 5 relies on this setting variable. It’s primarily used in the rendering pipeline to optimize parallel execution of rendering tasks.

• The value of this variable is set through a console variable (CVar) system. It’s initialized with a default value of 8 in the RHICommandList.cpp file.

• This variable interacts closely with its associated variable CVarRHICmdWidth. They share the same value and are used interchangeably in the codebase.

• Developers must be aware that changing this value can significantly impact the performance and behavior of the parallel renderer. A higher value may increase parallelism but could also lead to increased overhead, while a lower value might reduce parallelism but potentially improve cache coherency.

• Best practices when using this variable include:

  1. Profiling the application with different values to find the optimal setting for specific hardware and scene complexities.
  2. Considering the target hardware capabilities when adjusting this value.
  3. Testing thoroughly after changes, as it can affect rendering performance and potentially introduce artifacts if set inappropriately.

Regarding the associated variable CVarRHICmdWidth:

• It serves the same purpose as r.RHICmdWidth, controlling the task granularity in the parallel renderer.
• It’s defined as a TAutoConsoleVariable, allowing runtime modification of the value.
• This variable is used directly in the FParallelCommandListSet constructor to set the Width member, which likely determines the number of parallel command lists to be used.
• It’s exposed as an external variable in RHICommandList.h, allowing other parts of the engine to access and potentially modify its value.
• When working with CVarRHICmdWidth, developers should be cautious about thread safety, as it’s accessed on the render thread (GetValueOnRenderThread()).

In summary, both r.RHICmdWidth and CVarRHICmdWidth are critical for fine-tuning the parallel rendering performance in Unreal Engine 5, and their proper use requires careful consideration of the specific application requirements and target hardware capabilities.

#References in C++ code

#Callsites

This variable is referenced in the following C++ source code:

#Loc: <Workspace>/Engine/Source/Runtime/RHI/Private/RHICommandList.cpp:43

Scope: file

Source code excerpt:

TAutoConsoleVariable<int32> CVarRHICmdWidth(
	TEXT("r.RHICmdWidth"), 
	8,
	TEXT("Controls the task granularity of a great number of things in the parallel renderer."));

TAutoConsoleVariable<int32> CVarRHICmdFlushRenderThreadTasks(
	TEXT("r.RHICmdFlushRenderThreadTasks"),
	0,

#Associated Variable and Callsites

This variable is associated with another variable named CVarRHICmdWidth. They share the same value. See the following C++ source code.

#Loc: <Workspace>/Engine/Source/Runtime/RHI/Private/RHICommandList.cpp:42

Scope: file

Source code excerpt:

	TEXT("1: Enable (convenient for debugging low level graphics API calls, can suppress artifacts from multithreaded renderer code)"));

TAutoConsoleVariable<int32> CVarRHICmdWidth(
	TEXT("r.RHICmdWidth"), 
	8,
	TEXT("Controls the task granularity of a great number of things in the parallel renderer."));

TAutoConsoleVariable<int32> CVarRHICmdFlushRenderThreadTasks(
	TEXT("r.RHICmdFlushRenderThreadTasks"),

#Loc: <Workspace>/Engine/Source/Runtime/RHI/Public/RHICommandList.h:169

Scope: file

Source code excerpt:

extern RHI_API bool GEnableAsyncCompute;
extern RHI_API TAutoConsoleVariable<int32> CVarRHICmdWidth;
extern RHI_API TAutoConsoleVariable<int32> CVarRHICmdFlushRenderThreadTasks;


struct FRHICopyTextureInfo
{
	FIntRect GetSourceRect() const

#Loc: <Workspace>/Engine/Source/Runtime/Renderer/Private/SceneRendering.cpp:755

Scope (from outer to inner):

file
function     FParallelCommandListSet::FParallelCommandListSet

Source code excerpt:

	, bHasRenderPasses(bInHasRenderPasses)
{
	Width = CVarRHICmdWidth.GetValueOnRenderThread();
	MinDrawsPerCommandList = CVarRHICmdMinDrawsPerParallelCmdList.GetValueOnRenderThread();
	QueuedCommandLists.Reserve(Width * 8);
	check(!GOutstandingParallelCommandListSet);
	GOutstandingParallelCommandListSet = this;
}