One AMD driver flag boosts dual-GPU Vulkan LLM inference up to 4x
tabletuser_blogspot · reddit · 2026-09-28
A Reddit user running llama.cpp with the Vulkan backend on dual Radeon GPUs (MI50 16GB + RX 7900 GRE) diagnosed why benchmarks were low and found a major fix:
- Hardware layout: moving the MI50 to the primary PCIe 16x slot and demoting the RX 7900 GRE to a 4x slot improved overall inference performance.
- Key flag: RADVPERFTEST=nogttspill — an AMD Linux driver flag that forces Mesa's RADV Vulkan driver to keep model allocations in VRAM instead of spilling into system memory (GTT).
- Results: pp512 gains up to +402% (Laguna-XS-2.1), +398% for gemma-4-31B, +387% for Muse-Glimmer-30B (which also gained +48.6% tg128); a few MoE models like Qwen3-Coder-30B-A3B saw marginal losses.
- The author used Google Gemini to help tune llama-bench settings and shared full commands plus a 13-model benchmark table.
Anyone running local LLMs on AMD GPUs should try this flag immediately.
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