OptimizeBroadPhase usage

[turanszkij]

Hi, I'm currently running OptimizeBroadPhase every frame on a background thread - just in case - and I wonder if there could be a problem with my approach in theory (I don't notice a problem in practice currently):

1. The OptimizeBroadPhase always starts on a background thread after PhysicsSystem::Update was called for the last time in the current frame.
2. Meanwhile, I update my game objects from the Jolt bodies, which are accessed through GetBodyInterfaceNoLock()
3. During the rest of the frame, it's also possible that constraints will be created infrequently or forces, etc. applied on bodies. I also use GetBodyInterfaceNoLock() for these, the PhysicsSystem::Update() is guaranteed to not be updating here, but the OptimizeBroadPhase() could still be running
4. Before the next frame PhysicsSystem::Update(), the thread with OptimizeBroadPhase() is guaranteed to be finished.

Point 3) is the most worrying since that could be modifying the physics world while OptimizeBroadPhase() hasn't finished. Does this sound like could lead to threading issues?

[jrouwe]

Hello,

There's no point in calling OptimizeBroadPhase every frame. PhysicsSystem::Update does the same thing but only does part of the work every frame. OptimizeBroadPhase is only there to be called the first frame after loading so that you don't start the game with a really inefficient broadphase (which will then take 10 PhysicsSystem::Update's or so to incrementally fix).

If you call OptimizeBroadphase and use GetBodyInterfaceNoLock from another thread, you could have a race condition where you're modifying the bounding box of a body while it is being processed in the broadphase.

[Mahmoud1205]

If I call OptimizeBroadphase the first frame, then after a few seconds add a lot of new physics bodies in one frame, should I call OptimizeBroadphase again?

[jrouwe]

It doesn't hurt (other than that it could cause a frame rate hiccup). As long as you've batch added all those new bodies using BodyInterface::AddBodiesPrepare / AddBodiesFinalize and they're somewhat separated in space from the existing bodies (e.g. it's a new level section) then you shouldn't really need to call OptimizeBroadPhase. The best way to find out is to profile it, if the physics update is significantly faster after OptimizeBroadPhase then it could be worth it.

[Mahmoud1205]

Okay! Thanks for your continuous help

[turanszkij]

Thank you, I will rewrite this part.