Operating system: all OS
Graphics card: CUDA cards
Worked: 2.80 partly before animation denoising was committed
Short description of error
Tiles need to be transmitted between GPUs for denoising to have all neighbor tiles. The transfer can only happen when the GPU with the requested tile refreshes the tiles it was rendering. The requesting GPU in the mean time idles.
Exact steps for others to reproduce the error
Any production files with denoising will show the behavior. Depending on the scene and the GPU combination (the more GPU and the more heterogenous they are, the more it will be slowed down.
When the GPU is not connected to display, it increases step samples and make the idling even longer. As multiGPU setups often have only one card connected, it increase the chance.
With 2 GPUs on some evermotion scenes, the slowdown goes up to 80%. (so 2 cards being just 20% faster than a single card)