Why does GetNiagaraParticleData stall my main thread?

GPU sim particles live in GPU memory; reading them on the game thread requires a GPU→CPU copy that flushes the render thread. Switch to CPU sim if reads are needed every frame.

Custom NiagaraDataInterface that triggers an async readback via Render Graph; results arrive several frames later but don't stall. Useful for analytics-style reads.

Single emitter is one or the other. Split the system: CPU emitter for game-thread readable state, GPU emitter for visual mass. Cross-talk via Spawn Particles From Other Emitter.

Fix: Niagara CPU Readback Stalls Game Thread

Quick answer: Use CPU sim for emitters whose data the game thread reads. For GPU sim, use async readback via custom Data Interface; never read sync.

Insight session shows a 12 ms stall every frame on the game thread waiting for GPU readback. Niagara module is GPU-simulated and BP queries particle positions every Tick.

The Fix

Niagara Emitter properties:
  Sim Target:                  CPUSim   // for game-thread readable

// Or split into two emitters:
EffectVisual (GPU, 100k particles, no readback)
EffectGameplay (CPU, 64 particles, reads on game thread)

// In BP/C++:
GetNiagaraParticlePositions(EffectGameplay, OutPositions);
// CPU sim returns immediately, no GPU stall.

GPU is for visual scale; CPU is for gameplay-readable state. Don’t mix the two purposes in one emitter.

Verifying

Insight: game-thread waits gone. Visual emitter still GPU-fast. Gameplay reads land same frame.

“Right sim target. No stalls. Gameplay reads.”

Related Issues

For Niagara user param, see user param. For collision event, see collision event.

CPU for reads. GPU for visuals.