Software

The software stack for inference has four layers of abstraction:

• CUDA: Direct communication to the GPU for explicit control over computations and memory
• Deep learning frameworks: Abstractions over CUDA for training, exporting, and running neural networks in Python
• Inference engines: Highly configurable PyTorch-backed inference for common architectures
• NVIDIA Dynamo: Sits on top of inference engines to power large-scale deployments

Most inference engineering today happens at the higher levels, configuring and deploying inference engines. No matter what level you work at, it's essential to have a strong mental model for the adjacent levels.

CUDA#

CUDA has four key components:

• CUDA kernel: A function that executes parallelized code on the GPU
• CUDA graph: A DAG of kernels and GPU operations for optimizing repeated workflows
• CUDA driver: Low-level interface between the application and GPU hardware
• CUDA runtime: Developer-facing API for launching kernels and managing memory

CUDA is not a programming language — programs are written in C++, then compiled into separate CPU and GPU code by a compiler like nvcc.

Writing CUDA kernels shifts inference engineering from thinking about algorithms to thinking about implementations. The traditional attention algorithm can be expressed in a few dozen lines, but FlashAttention — the same mathematical operation — takes tens of thousands of lines to implement efficiently for a specific GPU.

CUDA Kernels for Inference

The prior art predates CUDA by decades. BLAS (Basic Linear Algebra Subprograms) is a specification for common linear algebra operations. cuBLAS brings this to GPUs, and cuDNN provides neural network primitives.

The most frequently used operation is GEMM (General Matrix-Matrix Multiplication). Key libraries:

CUDA Kernel Selection

Kernel implementations are highly specialized with hard-coded values based on specific GPU hardware. A kernel written for an H100 will likely not take advantage of B200 architecture. Most kernel selection is automatic — deep learning frameworks have pre-configured kernels for various architectures.

Reducing Memory Accesses with Kernel Fusion

Running two kernels back-to-back on the same data creates unnecessary round-trips to memory. Kernel fusion combines multiple kernels into a single kernel that handles both operations, eliminating intermediate reads and writes.

Deep Learning Frameworks#

PyTorch is the industry standard technology underlying both training and inference. Originally created at Meta, now part of the Linux Foundation.

PyTorch balances built-in functions and automatic optimizations with manual control. The step that transforms a model from training to inference is compilation (torch.compile), which performs automatic kernel selection and fusion for a specific GPU.

Model File Formats

• safetensors: The dominant format for serializing model weights. Uses memory mapping for fast, safe loading. Only holds tensor data, not executable code.
• ONNX: Stores weights along with an execution graph. Highly portable across hardware.

ONNX Runtime and TensorRT

Transformers and Diffusers

The transformers and diffusers libraries by Hugging Face offer reference implementations — great for learning and tinkering, but not designed for large-scale production inference. Use them for local inference and notebooks, then switch to production inference engines.

Inference Engines#

Three competitive engines: vLLM, SGLang, and TensorRT-LLM.

All three support continuous batching, post-training quantization, speculative decoding, prefix caching, parallelism, and disaggregation out of the box.

vLLM

Largest market share. First released summer 2023. Best selling point: broad support — most hardware options, most model architectures, plus multimodal inference via vLLM Omni. Use when you want solid out-of-the-box performance for almost any model.

SGLang

Pairs a fast backend runtime with a flexible frontend language. Strong support for Chinese open models (DeepSeek, Qwen). Heavy investment in large-scale MoE deployments on systems like GB200 NVL72.

TensorRT-LLM

Steeper learning curve but usually best performance. Deep NVIDIA hardware integration with graph-level optimizations. At Baseten, it's the most-used engine.

NVIDIA Dynamo#

NVIDIA Dynamo is an open-source distributed serving platform that sits on top of inference engines. It manages KV cache reuse, disaggregation, and multi-GPU/multi-node orchestration — the coordination layer for large-scale deployments.

Performance Benchmarking#

Key benchmarking tools: genai-perf (NVIDIA), vllm-benchmark, sglang-bench. Profiling tools: PyTorch Profiler, NVIDIA Nsight Systems and Nsight Compute.

Tips for useful benchmarking:

1. Define realistic workloads: Match your actual ISL/OSL distributions
2. Warm up first: Let the engine stabilize before measuring
3. Test multiple configurations: Vary batch size, parallelism, quantization
4. Measure what matters: TTFT and TPS for user-facing, total throughput for batch