p.Chaos.Spring.ParallelConstraintCount
p.Chaos.Spring.ParallelConstraintCount
#Overview
name: p.Chaos.Spring.ParallelConstraintCount
This variable is created as a Console Variable (cvar).
- type:
Var - help:
If we have more constraints than this, use parallel-for in Apply.
It is referenced in 6 C++ source files.
#Summary
#Usage in the C++ source code
The purpose of p.Chaos.Spring.ParallelConstraintCount is to control the threshold for parallel processing of spring constraints in Unreal Engine’s Chaos physics system. It determines when the system should switch from sequential to parallel execution for applying spring constraints.
This setting variable is primarily used in the Chaos physics subsystem, specifically in the spring constraint solver module. It’s part of the Experimental Chaos runtime in Unreal Engine.
The value of this variable is set through a console variable (CVar) system, allowing it to be adjusted at runtime in non-shipping builds. It’s initialized with a default value of 100.
The associated variable Chaos_Spring_ParallelConstraintCount directly interacts with p.Chaos.Spring.ParallelConstraintCount. They share the same value, with the CVar system providing a way to modify this value during runtime.
Developers should be aware that:
- This variable affects performance optimization, not physics accuracy.
- It’s only effective in non-shipping builds due to the
#if !UE_BUILD_SHIPPINGcondition. - Changing this value can impact performance, especially for simulations with many spring constraints.
Best practices when using this variable include:
- Profiling the application to determine the optimal threshold for your specific use case.
- Testing different values to find the best balance between parallel execution overhead and performance gains.
- Considering the target hardware when setting this value, as optimal thresholds may vary across different systems.
Regarding the associated variable Chaos_Spring_ParallelConstraintCount:
- It’s the actual variable used in the code to determine when to use parallel execution.
- It’s used in various constraint types beyond just springs, including altitude springs, corotated constraints, and volume constraints.
- The variable is checked in the Apply() and ApplyInParallel() functions of these constraint types to determine whether to use parallel processing.
- In some cases, the check for this variable is commented out, suggesting that parallel execution might be enforced regardless of the constraint count in certain scenarios.
Developers should note that modifying this variable can have wide-reaching effects on the physics simulation performance across different types of constraints in the Chaos system.
#References in C++ code
#Callsites
This variable is referenced in the following C++ source code:
#Loc: <Workspace>/Engine/Source/Runtime/Experimental/Chaos/Private/Chaos/PBDSpringConstraints.cpp:27
Scope (from outer to inner):
file
namespace Chaos::Softs
Source code excerpt:
static int32 Chaos_Spring_ParallelConstraintCount = 100;
#if !UE_BUILD_SHIPPING
FAutoConsoleVariableRef CVarChaosSpringParallelConstraintCount(TEXT("p.Chaos.Spring.ParallelConstraintCount"), Chaos_Spring_ParallelConstraintCount, TEXT("If we have more constraints than this, use parallel-for in Apply."));
#endif
template<typename SolverParticlesOrRange>
void FPBDSpringConstraints::InitColor(const SolverParticlesOrRange& Particles)
{
// In dev builds we always color so we can tune the system without restarting. See Apply()
#Associated Variable and Callsites
This variable is associated with another variable named Chaos_Spring_ParallelConstraintCount. They share the same value. See the following C++ source code.
#Loc: <Workspace>/Engine/Source/Runtime/Experimental/Chaos/Private/Chaos/PBDSpringConstraints.cpp:25
Scope (from outer to inner):
file
namespace Chaos::Softs
Source code excerpt:
// @todo(chaos): the parallel threshold (or decision to run parallel) should probably be owned by the solver and passed to the constraint container
static int32 Chaos_Spring_ParallelConstraintCount = 100;
#if !UE_BUILD_SHIPPING
FAutoConsoleVariableRef CVarChaosSpringParallelConstraintCount(TEXT("p.Chaos.Spring.ParallelConstraintCount"), Chaos_Spring_ParallelConstraintCount, TEXT("If we have more constraints than this, use parallel-for in Apply."));
#endif
template<typename SolverParticlesOrRange>
void FPBDSpringConstraints::InitColor(const SolverParticlesOrRange& Particles)
{
// In dev builds we always color so we can tune the system without restarting. See Apply()
#if UE_BUILD_SHIPPING || UE_BUILD_TEST
if (Constraints.Num() > Chaos_Spring_ParallelConstraintCount)
#endif
{
const TArray<TArray<int32>> ConstraintsPerColor = FGraphColoring::ComputeGraphColoringParticlesOrRange(Constraints, Particles, ParticleOffset, ParticleOffset + ParticleCount);
// Reorder constraints based on color so each array in ConstraintsPerColor contains contiguous elements.
TArray<TVec2<int32>> ReorderedConstraints;
#Loc: <Workspace>/Engine/Source/Runtime/Experimental/Chaos/Private/Chaos/PBDSpringConstraints.cpp:96
Scope (from outer to inner):
file
namespace Chaos::Softs
function void FPBDSpringConstraints::Apply
Source code excerpt:
TRACE_CPUPROFILER_EVENT_SCOPE(FPBDSpringConstraints_Apply);
SCOPE_CYCLE_COUNTER(STAT_PBD_Spring);
if ((ConstraintsPerColorStartIndex.Num() > 1) && (Constraints.Num() > Chaos_Spring_ParallelConstraintCount))
{
const int32 ConstraintColorNum = ConstraintsPerColorStartIndex.Num() - 1;
if (!Stiffness.HasWeightMap())
{
const FSolverReal ExpStiffnessValue = (FSolverReal)Stiffness;
#Loc: <Workspace>/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/PBDAltitudeSpringConstraints.h:307
Scope (from outer to inner):
file
namespace Chaos::Softs
class class FPBDAltitudeSpringConstraints
function void ApplyInParallel
Source code excerpt:
{
TRACE_CPUPROFILER_EVENT_SCOPE(STAT_ChaosPBDAltitudeSpringApply);
if ((ConstraintsPerColorStartIndex.Num() > 1))//&& (MeshConstraints.Num() > Chaos_Spring_ParallelConstraintCount))
{
const int32 ConstraintColorNum = ConstraintsPerColorStartIndex.Num() - 1;
for (int32 ConstraintColorIndex = 0; ConstraintColorIndex < ConstraintColorNum; ++ConstraintColorIndex)
{
const int32 ColorStart = ConstraintsPerColorStartIndex[ConstraintColorIndex];
#Loc: <Workspace>/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/XPBDCorotatedConstraints.h:295
Scope (from outer to inner):
file
namespace Chaos::Softs
class class FXPBDCorotatedConstraints
function void ApplyInParallel
Source code excerpt:
SCOPE_CYCLE_COUNTER(STAT_ChaosXPBDCorotated);
TRACE_CPUPROFILER_EVENT_SCOPE(STAT_ChaosXPBDCorotatedApply);
if ((ConstraintsPerColorStartIndex.Num() > 1))//&& (MeshConstraints.Num() > Chaos_Spring_ParallelConstraintCount))
{
const int32 ConstraintColorNum = ConstraintsPerColorStartIndex.Num() - 1;
for (int32 ConstraintColorIndex = 0; ConstraintColorIndex < ConstraintColorNum; ++ConstraintColorIndex)
{
const int32 ColorStart = ConstraintsPerColorStartIndex[ConstraintColorIndex];
#Loc: <Workspace>/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/XPBDVolumeConstraints.h:80
Scope (from outer to inner):
file
namespace Chaos::Softs
class class FXPBDVolumeConstraints
function void ApplyInParallel
Source code excerpt:
{
TRACE_CPUPROFILER_EVENT_SCOPE(STAT_ChaosXPBDVolumeApply);
if ((ConstraintsPerColorStartIndex.Num() > 1))//&& (MeshConstraints.Num() > Chaos_Spring_ParallelConstraintCount))
{
const int32 ConstraintColorNum = ConstraintsPerColorStartIndex.Num() - 1;
for (int32 ConstraintColorIndex = 0; ConstraintColorIndex < ConstraintColorNum; ++ConstraintColorIndex)
{
const int32 ColorStart = ConstraintsPerColorStartIndex[ConstraintColorIndex];