LLM Inference

• KV-Cache is a cache used to store result of computations from forward pass to avoid unnecessarily rerunning parts of the forward pass for each subsequent token

  • KV-Cache can be huge

Key Metrics

• Time to first token (TTFT): How quickly user starts seeing the first token after entering their query
• Time per output token (TPOT): Time to generate an output token for each user that is querying the model. Corresponds with how each user will percieve the “speed” of the model

Bandwidth vs. Compute Bound

• At lower batch size LLM inference is bandwidth bound

  • LImited by how quickly can page weights in/out of memory into cache in order to do computation

• At higher batch size LLM inference is compute bound

Model Flop Utililization(MFU) = (achieved FLOPS) / (peak FLOPs)

Model Bandwidth Utilization(MBU) = (achieved bandwidth) / (peak bandwidth)

• Achieved bandiwdth is ((total model parameter size + KV cache size)/ TPOT)

Reference

• LLM Inference Performance Engineering: Best Practices
  • Blog from MOSAIC with some good information about LLM inference
• Serving Quantized LLMS on NVIDIA H100 Tensor Core GPUs
• Speeding up LLM Inference with TensorRT-LLM - video from GTC
  • Lots of good information about serving
  • Has benchmarking results for TRT-LLM
  • Results illustrate various tradeoffs; e.g. as you have longer context size you have less memory for KV cache so max batch size decreases
    • As batch size decreases your utilization of compute decreases
• Blog post illustrating batching techningues