llama.cpp for IBM POWER8

AltiVec/VSX Optimized LLM Inference for POWER8

This provides POWER8-specific optimizations for llama.cpp, enabling efficient LLM inference on IBM POWER8 servers.

What's Included

• power8-compat.h - POWER9 intrinsics compatibility layer for POWER8
• ggml-dcbt-resident.h - Full L2/L3 cache-resident prefetch hints
• altivec_benchmark.c - AltiVec/VSX performance benchmark

Performance

Tested on IBM Power System S824 (dual 8-core POWER8, 576GB RAM):

Model	pp128 (tokens/s)	tg32 (tokens/s)
TinyLlama 1.1B Q4	~85	~15
Llama-7B Q4	~20	~5
DeepSeek-33B Q4	~5	~1

Building llama.cpp for POWER8

Prerequisites

• Ubuntu 20.04 LTS (last POWER8-supported release)
• GCC with POWER8 support
• CMake 3.14+

Build Commands

# Clone llama.cpp
git clone https://github.com/ggerganov/llama.cpp
cd llama.cpp

# Copy POWER8 headers
cp /path/to/powerpc/* ggml/src/ggml-cpu/arch/powerpc/

# Configure for POWER8
mkdir build-power8 && cd build-power8
cmake .. \
    -DCMAKE_BUILD_TYPE=Release \
    -DGGML_OPENMP=ON \
    -DCMAKE_C_FLAGS="-mcpu=power8 -mvsx -maltivec -O3 -mtune=power8 -funroll-loops" \
    -DCMAKE_CXX_FLAGS="-mcpu=power8 -mvsx -maltivec -O3 -mtune=power8 -funroll-loops"

# Build
make -j$(nproc)

With IBM MASS Library (Optional)

IBM Mathematical Acceleration Subsystem (MASS) provides optimized math functions:

cmake .. \
    -DCMAKE_BUILD_TYPE=Release \
    -DGGML_OPENMP=ON \
    -DCMAKE_C_FLAGS="-mcpu=power8 -mvsx -maltivec -O3 -mtune=power8 -funroll-loops -DGGML_USE_MASS=1 -I/opt/ibm/mass/include" \
    -DCMAKE_CXX_FLAGS="-mcpu=power8 -mvsx -maltivec -O3 -mtune=power8 -funroll-loops -DGGML_USE_MASS=1 -I/opt/ibm/mass/include" \
    -DCMAKE_EXE_LINKER_FLAGS="-L/opt/ibm/mass/lib -lmassvp8 -lmass"

Running Inference

# Basic inference
./bin/llama-cli -m ~/models/llama-7b-q4.gguf -p "Hello world" -n 64

# With optimal thread count (64 threads is usually best on POWER8)
OMP_NUM_THREADS=64 ./bin/llama-cli -m ~/models/llama-7b-q4.gguf -p "Hello" -n 64

# NUMA-aware (for dual-socket systems)
numactl --interleave=all ./bin/llama-cli -m ~/models/large-model.gguf -p "Test" -n 32

# Benchmark
./bin/llama-bench -m ~/models/tinyllama-1.1b-q4.gguf -t 64 -p 128 -n 32

POWER8 Optimization Notes

Thread Scaling

64 threads is typically optimal on POWER8 (NOT 128):

• 16 threads: ~40 t/s
• 32 threads: ~65 t/s
• 64 threads: ~85 t/s (optimal)
• 96 threads: ~75 t/s
• 128 threads: ~65 t/s

Cache Prefetch

The ggml-dcbt-resident.h header provides cache-resident prefetch hints:

• DCBT_RESIDENT_FULL() - Keeps data in L2/L3 until explicit eviction
• Critical for weight reuse in attention/matmul

Memory Alignment

POWER8 prefers 128-byte aligned data for optimal VSX performance. The power8-compat.h handles alignment requirements.

Files

powerpc/
├── power8-compat.h       # POWER9 → POWER8 intrinsic compatibility
└── ggml-dcbt-resident.h  # Cache-resident prefetch hints

altivec_benchmark.c       # VSX/AltiVec performance test

Hardware Tested

• System: IBM Power System S824 (8286-42A)
• CPUs: Dual 8-core POWER8, 128 threads (SMT8)
• RAM: 576 GB DDR3
• OS: Ubuntu 20.04 LTS

Video Demos

DeepSeek Running on POWER8 - LLM inference on IBM POWER8 S824

Claude Code on POWER8 - Proving Claude Code works on POWER8!

Community

Join the RustChain Discord for PowerPC/POWER8 AI discussion:

Related Projects

• llama.cpp - Main project
• Claude Code POWER8 - Claude Code for POWER8

Attribution

Months of research, tuning, and testing on real POWER8 hardware went into this.

If you use this project, please give credit:

llama.cpp POWER8 Optimizations by Scott (Scottcjn)
https://github.com/Scottcjn/llama-cpp-power8

If this helped you, please:

• ⭐ Star this repo - It helps others find it
• 📝 Credit in your project - Keep the attribution
• 🔗 Link back - Share the love

Credits

• ggerganov/llama.cpp - Original project
• Scott (Scottcjn) - Creator, architect, hardware lab, POWER8 optimizations
• Claude (Opus 4.1/4.5) - Implementation assistance

Designed by Scott, coded with Claude

License

MIT License - Free to use, but please keep the copyright notice and attribution.

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"576GB RAM. 128 threads. Your POWER8 was built for AI - it just didn't know it yet."

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