Inference Metrics — TTFT, TPOT, ITL, Goodput, P99

Four metrics decide whether an inference deployment is working. TTFT is prefill plus queue plus network. TPOT (equivalently ITL) is the memory-bound decode cost per token. End-to-end latency is TTFT plus TPOT times output length. Throughput is tokens per second aggregated across the fleet. But the one that matters for product is goodput — the fraction of requests that met every SLO simultaneously. High throughput at low goodput means you are processing tokens that never reach users on time. Reference numbers for Llama-3.1-8B-Instruct on TRT-LLM in 2026: mean TTFT 162 ms, mean TPOT 7.33 ms, mean E2E 1,093 ms. Always report P50, P90, P99 — never just mean. And watch the measurement trap: GenAI-Perf excludes TTFT from ITL calculation, LLMPerf includes it; two tools disagree on TPOT for the same run.

Prerequisites: Phase 17 · 04 (vLLM Serving Internals)