Qwen3.8 27B

User reports Qwen3.8-27B at 87.4 t/s decode and 872 t/s prefill at 16K context on one RX 7900 XTX, using a fork of llama.cpp tuned for RDNA3 Vulkan. Setup is the fork's llama.cpp with UD-Q4_K_M GGUF, q8_0 KV cache, flash attention and a DFlash2 draft model for speculative decoding; the card is 24 GB at stock 291 W on Mesa RADV. At 128K context the fork gives 69.1 t/s decode and 657 t/s prefill, against 26.3 t/s and 442 t/s upstream. The fork reaches up to 220K context where upstream fits about 192K, and DFlash2 no longer hits vk::ErrorDeviceLost past ~60K tokens. Only the 7900 XTX was tested.