Metal

Metal is Apple's low-level graphics and compute API that provides direct access to the GPU on Apple devices, including iPhones, iPads, Macs, and Apple Silicon based machines. For AI builders, Metal is the underlying technology that enables GPU acceleration for machine learning frameworks like PyTorch (via MPS backend), TensorFlow, and specialized LLM inference engines like llama.cpp and MLX. Metal offers a unified memory model on Apple Silicon, meaning the CPU and GPU share the same memory pool, eliminating the expensive data copies required in traditional discrete GPU architectures. This is particularly beneficial for LLM inference where large model weights and KV caches must be frequently accessed. Metal also supports GPU-accelerated operations for mixed-precision computation (FP16, BF16) and matrix multiplication through the Metal Performance Shaders (MPS) framework, which includes optimized convolutional and transformer primitives. For running LLMs, llama.cpp's Metal backend provides near-native speeds for models up to what fits in unified memory, often outperforming CPU-only inference by 10-50x. Metal's ecosystem includes the Metal Shading Language (MSL) for writing custom GPU kernels, and Xcode's debugging tools. The primary limitation is vendor lock-in: Metal code cannot run on NVIDIA or AMD GPUs. AI builders targeting Apple ecosystem users should consider Metal compatibility, especially for apps that run LLMs locally on-device, like Mac-native GUIs or iOS apps. Metal's support for quantization kernels (e.g., Q4_K_M via llama.cpp) is actively improved, but it generally lags behind CUDA in ultra-low latency fine-tuning support.