llm-c-transformer: A High-Performance CPU Inference Engine Implemented in Pure C

This article introduces llm-c-transformer, a Transformer inference engine entirely written in C. Through INT8 quantization and AVX2 SIMD optimization, it achieves 8.6x faster performance and 4x less memory usage than PyTorch on CPUs, providing an ideal solution for edge deployment and cost-sensitive scenarios.

With the popularization of large models, inference cost has become a key consideration. Although GPUs are powerful, CPU inference is irreplaceable in scenarios such as edge deployment (no GPU support), cost sensitivity (cloud GPUs are expensive), cold start latency (serverless is not suitable), and power consumption constraints (data center power and cooling). Traditional frameworks like PyTorch lack sufficient optimization for CPUs, wasting resources, hence the llm-c-transformer project was born.

llm-c-transformer adopts two key technologies:

1. Post-training INT8 quantization: Including weight and activation dynamic range calibration, quantization-aware forward propagation, and dequantization precision recovery, reducing memory by 4x and improving computation speed.
2. AVX2 SIMD matrix multiplication: Using the x86 SIMD instruction set to process 256-bit data simultaneously, achieving a 3.1x speedup and 4x memory reduction. It uses a cache-friendly blocking strategy to avoid data copying.

Performance test results:

Metric	C INT8-AVX2	PyTorch CPU (FP32)	GPU (T4)
Latency (seq=16)	0.275 ms	2.355 ms	~0.05 ms
Throughput	3,636 tok/s	425 tok/s	~20,000 tok/s
Memory (model weights)	0.50 MB	2.01 MB	2.01 MB
Cost per million tokens	$0.014	$0.120	$0.050
Compared to PyTorch CPU, latency is reduced by 8.6x, throughput is increased by 8.6x, and memory usage is reduced by 4x.

TCO considers costs such as hardware, cloud computing, power, cooling, storage, and operation and maintenance:

Cost Item	C INT8-AVX2	PyTorch CPU	GPU (T4)
Hardware (amortized)	$100/year	$100/year	$1,000/year
Cloud computing (1 billion tokens/month)	$168/year	$1,440/year	$600/year
Power (24/7 operation)	$78/year	$341/year	$73/year
Cooling (data center)	$16/year	$68/year	$15/year
Memory/Storage	$10/year	$40/year	$50/year
Development & Operation	$500/year	$200/year	$800/year
Total TCO	$872/year	$2,189/year	$2,538/year
In the scenario of 1 billion tokens per month, the C solution is 2.5x cheaper than PyTorch CPU and 2.9x cheaper than GPU.

Based on TCO analysis, deployment recommendations are as follows:

• Low traffic (<100 million tokens/month) : C INT8-AVX2 (CPU) — lowest TCO, fast cold start
• Medium traffic (100 million - 10 billion tokens/month) :
  • C solution wins: Edge/serverless deployment, acceptable <1ms latency
  • GPU solution wins: Batch processing, requires <100μs latency
• High traffic (>10 billion tokens/month) : GPU (A100/H100) — large-scale amortization cost
• Edge/mobile/IoT : C INT8-AVX2 — only feasible option (no GPU support)

llm-c-transformer includes a complete Transformer technology stack:

• Causal language model (lm_train.c)
• NER fine-tuning (main.c)
• Inference benchmark (bench.c)
• TCO calculator (tco_analysis.py) Core components: Custom tensor library, post-training INT8 quantization, AVX2 SIMD matrix multiplication, Adam optimizer, gradient clipping, complete backpropagation.

llm-c-transformer provides an ideal solution for scenarios such as edge AI, Serverless architecture, cost-sensitive applications, and educational research. It demonstrates the amazing effects of low-level optimization, offers new possibilities for the popularization of large model deployment, and will play an important role in the growing demand for edge AI.