The Symptom, also reported as T71071:
A very complex scene, which one GPU can render successfully with the help of AGP memory, cannot be properly rendered by multiple GPUs. The render may fail from the start or may produce a mosaic result of normal tiles and buggy tiles.
This symptom was reproduced from the following combinations of multiple GPUs and cured by this patch.
- two NVIDIA GeForce 980 Ti GPUs
- three NVIDIA GeForce 980 Ti GPUs
- NVIDIA GeForce 980 Ti + GeForce RTX 2080 SUPER
( You can test this with the 'Cosmos Laundromat Demo' ( https://download.blender.org/demo/test/benchmark.zip ) with GPU compute device. It is a CPU render demo but can be rendered with one GPU with enough AGP memory. )
Under a multidevice, some GPUs may store a resource in a device memory while others may store and share it in a mapped host AGP memory. The code related to 'device_memory' does not handle this situation properly.