54 lines
1.7 KiB
Markdown
54 lines
1.7 KiB
Markdown
# Q6A llama.cpp Vulkan Patch + Benchmarks
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Everything needed to enable full GPU offload on Turnip Adreno 643.
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## The Problem
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llama.cpp's GPU backend crashes at context creation when offloading 3+ layers to Vulkan on Mesa Turnip:
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```
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pre-allocated tensor (cache_k_l3) in a buffer (Vulkan0)
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that cannot run the operation (NONE)
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```
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## The Fix
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A 12-line patch in `ggml/src/ggml-backend.cpp` that adds a fallback path: before aborting, it finds a backend matching the tensor's buffer type, ignoring op support. KV cache tensors don't need op-specific backends.
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## Quick Start
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```bash
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git clone https://github.com/ggml-org/llama.cpp
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cd llama.cpp
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git apply /path/to/ggml-backend.cpp.patch
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cmake -B build -DGGML_VULKAN=ON
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cmake --build build --target llama-cli -j$(nproc)
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./build/bin/llama-cli -m model.gguf -ngl 99
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```
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## Benchmarks
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System: Q6A / Radxa Dragon Wing / Qualcomm QCS6490 / Adreno 643 / Mesa 25.0.7 / 12GB RAM
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### Qwen3.5-0.8B Q4_K_M (521 MB)
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| Config | Prefill (pp32) | Generation (tg128) |
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|--------|---------------|-------------------|
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| CPU | 15.2 tok/s | 12.2 tok/s |
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| GPU (ngl=99) | 21.2 tok/s | 8.2 tok/s |
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| *Delta* | *+40%* | *-33%* |
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### Qwen3.5-0.8B Q8_K_XL (1.09 GB)
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| Config | Prefill (pp32) | Generation (tg64) |
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|--------|---------------|-------------------|
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| CPU | 13.0 tok/s | 8.7 tok/s |
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| GPU (ngl=99) | 21.0 tok/s | 7.5 tok/s |
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| *Delta* | *+61%* | *-14%* |
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### Summary
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- GPU prefill is **40-61% faster** than CPU
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- GPU generation is **14-33% slower** (Turnip lacks INT4 dot-product, dequantizes to fp16)
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- **Q4_K_M on CPU is the sweet spot** for interactive use: 15/12 tok/s, zero setup
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- Use GPU when you care about time-to-first-token (batch/search workloads)
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