From 4ac870f7012964f699daa94b2f66e075ab371d8c Mon Sep 17 00:00:00 2001
From: Wojtek Figat <wojtek@figat.pl>
Date: Thu, 19 Jun 2025 13:57:50 +0200
Subject: [PATCH] Optimize physics transformation updates propagation in async
 via Job System

---
 Source/Engine/Physics/Actors/RigidBody.cpp    |  8 +++++
 .../Physics/PhysX/PhysicsBackendPhysX.cpp     | 31 +++++++++++++++++--
 2 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/Source/Engine/Physics/Actors/RigidBody.cpp b/Source/Engine/Physics/Actors/RigidBody.cpp
index a58911dfb..565bd9fbd 100644
--- a/Source/Engine/Physics/Actors/RigidBody.cpp
+++ b/Source/Engine/Physics/Actors/RigidBody.cpp
@@ -468,6 +468,14 @@ void RigidBody::OnActiveTransformChanged()
 
 void RigidBody::BeginPlay(SceneBeginData* data)
 {
+#if USE_EDITOR || !BUILD_RELEASE
+    // FlushActiveTransforms runs in async for each separate actor thus we don't support two rigidbodies that transformations depend on each other
+    if (Cast<RigidBody>(GetParent()))
+    {
+        LOG(Warning, "Rigid Body '{0}' is attached to other Rigid Body which is not unsupported and might cause physical simulation instability.", GetNamePath());
+    }
+#endif
+
     // Create rigid body
     ASSERT(_actor == nullptr);
     void* scene = GetPhysicsScene()->GetPhysicsScene();
diff --git a/Source/Engine/Physics/PhysX/PhysicsBackendPhysX.cpp b/Source/Engine/Physics/PhysX/PhysicsBackendPhysX.cpp
index 673b1e96d..99b54e8fc 100644
--- a/Source/Engine/Physics/PhysX/PhysicsBackendPhysX.cpp
+++ b/Source/Engine/Physics/PhysX/PhysicsBackendPhysX.cpp
@@ -1901,6 +1901,23 @@ void PhysicsBackend::StartSimulateScene(void* scene, float dt)
     scenePhysX->Stepper.renderDone();
 }
 
+PxActor** CachedActiveActors;
+int64 CachedActiveActorsCount;
+volatile int64 CachedActiveActorIndex;
+
+void FlushActiveTransforms(int32 i)
+{
+    PROFILE_CPU();
+    int64 index;
+    while ((index = Platform::InterlockedIncrement(&CachedActiveActorIndex)) < CachedActiveActorsCount)
+    {
+        const auto pxActor = (PxRigidActor*)CachedActiveActors[index];
+        auto actor = static_cast<IPhysicsActor*>(pxActor->userData);
+        if (actor)
+            actor->OnActiveTransformChanged();
+    }
+}
+
 void PhysicsBackend::EndSimulateScene(void* scene)
 {
     PROFILE_MEM(Physics);
@@ -1919,10 +1936,18 @@ void PhysicsBackend::EndSimulateScene(void* scene)
         // Gather change info
         PxU32 activeActorsCount;
         PxActor** activeActors = scenePhysX->Scene->getActiveActors(activeActorsCount);
-        if (activeActorsCount > 0)
+
+        // Update changed transformations
+        if (activeActorsCount > 50 && JobSystem::GetThreadsCount() > 1)
+        {
+            // Run in async via job system
+            CachedActiveActors = activeActors;
+            CachedActiveActorsCount = activeActorsCount;
+            CachedActiveActorIndex = -1;
+            JobSystem::Execute(FlushActiveTransforms, JobSystem::GetThreadsCount());
+        }
+        else
         {
-            // Update changed transformations
-            // TODO: use jobs system if amount if huge
             for (uint32 i = 0; i < activeActorsCount; i++)
             {
                 const auto pxActor = (PxRigidActor*)*activeActors++;