It appears that the 2080 Tis cannot peer without the NVLink bridge installed. Just want to confirm that this is the intended behavior and not a bug.
Testing the peering using the simpleP2P script from the CUDA samples
Without the bridge:
[./simpleP2P] - Starting...
Checking for multiple GPUs...
CUDA-capable device count: 2
> GPU0 = "GeForce RTX 2080 Ti" IS capable of Peer-to-Peer (P2P)
> GPU1 = "GeForce RTX 2080 Ti" IS capable of Peer-to-Peer (P2P)
Checking GPU(s) for support of peer to peer memory access...
> Peer access from GeForce RTX 2080 Ti (GPU0) -> GeForce RTX 2080 Ti (GPU1) : No
> Peer access from GeForce RTX 2080 Ti (GPU1) -> GeForce RTX 2080 Ti (GPU0) : No
Two or more GPUs with SM 2.0 or higher capability are required for ./simpleP2P.
Peer to Peer access is not available amongst GPUs in the system, waiving test.
With the bridge:
[./simpleP2P] - Starting...
Checking for multiple GPUs...
CUDA-capable device count: 2
> GPU0 = "GeForce RTX 2080 Ti" IS capable of Peer-to-Peer (P2P)
> GPU1 = "GeForce RTX 2080 Ti" IS capable of Peer-to-Peer (P2P)
Checking GPU(s) for support of peer to peer memory access...
> Peer access from GeForce RTX 2080 Ti (GPU0) -> GeForce RTX 2080 Ti (GPU1) : Yes
> Peer access from GeForce RTX 2080 Ti (GPU1) -> GeForce RTX 2080 Ti (GPU0) : Yes
Enabling peer access between GPU0 and GPU1...
Checking GPU0 and GPU1 for UVA capabilities...
> GeForce RTX 2080 Ti (GPU0) supports UVA: Yes
> GeForce RTX 2080 Ti (GPU1) supports UVA: Yes
Both GPUs can support UVA, enabling...
Allocating buffers (64MB on GPU0, GPU1 and CPU Host)...
Creating event handles...
cudaMemcpyPeer / cudaMemcpy between GPU0 and GPU1: 44.81GB/s
Preparing host buffer and memcpy to GPU0...
Run kernel on GPU1, taking source data from GPU0 and writing to GPU1...
Run kernel on GPU0, taking source data from GPU1 and writing to GPU0...
Copy data back to host from GPU0 and verify results...
Disabling peer access...
Shutting down...
Test passed
This a 2 2080 Ti machine running Ubuntu 18.04.
Driver is 410.66. CUDA 10.0
Motherboard is ASUS x99-E WS/USB 3.1
CPU is Intel i7-6850k
I’ve not added/changed any IOMMU kernel parameters
Peering works as expected when the 2080 Tis are replaced with 1080 Tis, and no other changes.