110 lines
6.3 KiB
Markdown
110 lines
6.3 KiB
Markdown
# Hexagon backend developer details
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## Backend libraries
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The Hexagon backend consist of two parts:
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- `libggml-hexagon`
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This is the regular CPU-side GGML backend library, either shared or statically linked
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- `libggml-htp-vNN`
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This is the NPU-side (HTP stands for Hexagon Tensor Processor) shared library that contains the Op dispatcher and kernels.
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The correct library is selected automatically at runtime based on the HW version.
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Here is an example of the build artifacts
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```
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~/src/llama.cpp$ ls -l pkg-adb/llama.cpp/lib/libggml*
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pkg-adb/llama.cpp/lib/libggml-base.so
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pkg-adb/llama.cpp/lib/libggml-cpu.so
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pkg-adb/llama.cpp/lib/libggml-hexagon.so <<< CPU library
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pkg-adb/llama.cpp/lib/libggml-htp-v73.so <<< HTP op/kernels for Hexagon v73
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pkg-adb/llama.cpp/lib/libggml-htp-v75.so
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pkg-adb/llama.cpp/lib/libggml-htp-v79.so
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pkg-adb/llama.cpp/lib/libggml-htp-v81.so
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```
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## Memory buffers
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Hexagon NPU backend takes advantage of the Snapdragon's unified memory model where all buffers are fully accessible by the CPU and GPU.
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The NPU does have a dedicated tightly-coupled memory called VTCM but that memory is used only for intermediate data (e.g. dynamically
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quantized tensors) or temporary data (chunks of the weight tensors fetched via DMA).
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Please note that currently the Hexagon backend does not implement SET/GET_ROWS Ops because there is no advantage in offloading those
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to the NPU at this point.
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The backend does allocates non-host buffers for the tensors with datatypes that require repacking: Q4_0, Q8_0, MXFP4.
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From the MMU perspective these buffers are still regular buffers (normal access by the CPU) they are marked as non-host simply to force
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the repacking.
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## Large model handling
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Hexagon NPU session (aka Process Domain (PD) in the Hexagon docs) is limited to a memory mapping of around 3.5GB.
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In llama.cpp/GGML the Hexagon session is mapped to a single GGML backend device (HTP0, HTP1, etc).
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In order to map models larger than 3.5GB we need to allocate multiple devices and split the model.
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For this we're taking advantage of the llama.cpp/GGML multi-GPU layer-splitting support.
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Each Hexagon device behaves like a GPU from the offload and model splitting perspective.
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Here is an example of running GPT-OSS-20B model on a newer Snapdragon device with 16GB of DDR.
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```
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M=gpt-oss-20b-Q4_0.gguf NDEV=4 D=HTP0,HTP1,HTP2,HTP3 P=surfing.txt scripts/snapdragon/adb/run-cli.sh -no-cnv -f surfing.txt -n 32
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...
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LD_LIBRARY_PATH=/data/local/tmp/llama.cpp/lib
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ADSP_LIBRARY_PATH=/data/local/tmp/llama.cpp/lib
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GGML_HEXAGON_NDEV=4 ./bin/llama-cli --no-mmap -m /data/local/tmp/llama.cpp/../gguf/gpt-oss-20b-Q4_0.gguf
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-t 4 --ctx-size 8192 --batch-size 128 -ctk q8_0 -ctv q8_0 -fa on -ngl 99 --device HTP0,HTP1,HTP2,HTP3 -no-cnv -f surfing.txt
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...
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llama_model_loader: - type f32: 289 tensors
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llama_model_loader: - type q4_0: 96 tensors
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llama_model_loader: - type q8_0: 2 tensors
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llama_model_loader: - type mxfp4: 72 tensors
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...
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load_tensors: offloaded 25/25 layers to GPU
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load_tensors: CPU model buffer size = 1182.09 MiB
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load_tensors: HTP1 model buffer size = 6.64 MiB
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load_tensors: HTP1-REPACK model buffer size = 2505.94 MiB
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load_tensors: HTP3 model buffer size = 5.55 MiB
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load_tensors: HTP3-REPACK model buffer size = 2088.28 MiB
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load_tensors: HTP0 model buffer size = 7.75 MiB
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load_tensors: HTP0-REPACK model buffer size = 2923.59 MiB
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load_tensors: HTP2 model buffer size = 6.64 MiB
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load_tensors: HTP2-REPACK model buffer size = 2505.94 MiB
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...
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llama_context: n_ctx_per_seq (8192) < n_ctx_train (131072) -- the full capacity of the model will not be utilized
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llama_context: CPU output buffer size = 0.77 MiB
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llama_kv_cache_iswa: creating non-SWA KV cache, size = 8192 cells
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llama_kv_cache: HTP1 KV buffer size = 25.50 MiB
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llama_kv_cache: HTP3 KV buffer size = 25.50 MiB
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llama_kv_cache: HTP0 KV buffer size = 25.50 MiB
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llama_kv_cache: HTP2 KV buffer size = 25.50 MiB
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llama_kv_cache: size = 102.00 MiB ( 8192 cells, 12 layers, 1/1 seqs), K (q8_0): 51.00 MiB, V (q8_0): 51.00 MiB
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llama_kv_cache_iswa: creating SWA KV cache, size = 256 cells
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llama_kv_cache: HTP1 KV buffer size = 0.80 MiB
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llama_kv_cache: HTP3 KV buffer size = 0.53 MiB
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llama_kv_cache: HTP0 KV buffer size = 1.06 MiB
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llama_kv_cache: HTP2 KV buffer size = 0.80 MiB
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llama_kv_cache: size = 3.19 MiB ( 256 cells, 12 layers, 1/1 seqs), K (q8_0): 1.59 MiB, V (q8_0): 1.59 MiB
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llama_context: HTP0 compute buffer size = 16.06 MiB
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llama_context: HTP1 compute buffer size = 16.06 MiB
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llama_context: HTP2 compute buffer size = 16.06 MiB
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llama_context: HTP3 compute buffer size = 16.06 MiB
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llama_context: CPU compute buffer size = 98.19 MiB
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...
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llama_perf_context_print: prompt eval time = 3843.67 ms / 197 tokens ( 19.51 ms per token, 51.25 tokens per second)
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llama_perf_context_print: eval time = 1686.13 ms / 31 runs ( 54.39 ms per token, 18.39 tokens per second)
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llama_perf_context_print: total time = 6266.30 ms / 228 tokens
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llama_perf_context_print: graphs reused = 30
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llama_memory_breakdown_print: | memory breakdown [MiB] | total free self model context compute unaccounted |
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llama_memory_breakdown_print: | - HTP0 (Hexagon) | 2048 = 2048 + ( 0 = 0 + 0 + 0) + 0 |
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llama_memory_breakdown_print: | - HTP1 (Hexagon) | 2048 = 2048 + ( 0 = 0 + 0 + 0) + 0 |
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llama_memory_breakdown_print: | - HTP2 (Hexagon) | 2048 = 2048 + ( 0 = 0 + 0 + 0) + 0 |
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llama_memory_breakdown_print: | - HTP3 (Hexagon) | 2048 = 2048 + ( 0 = 0 + 0 + 0) + 0 |
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llama_memory_breakdown_print: | - Host | 1476 = 1208 + 105 + 162 |
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llama_memory_breakdown_print: | - HTP1-REPACK | 2505 = 2505 + 0 + 0 |
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llama_memory_breakdown_print: | - HTP3-REPACK | 2088 = 2088 + 0 + 0 |
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llama_memory_breakdown_print: | - HTP0-REPACK | 2923 = 2923 + 0 + 0 |
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llama_memory_breakdown_print: | - HTP2-REPACK | 2505 = 2505 + 0 + 0 |
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```
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