The concurrency cliff is a memory limit

The analysis explores the behavior of KV cache in handling concurrent sessions on an inference server, using a Qwen3-4B coding agent on an NVIDIA L4 GPU. It highlights that while traditional latency increases gradually with more users, KV cache exhibits a sharp "cliff" effect when the cache fills, dramatically increasing latency from 2.6 to 39 seconds upon exceeding 12 concurrent sessions. With a setup involving a g6.4xlarge EC2 instance and vLLM 0.20.2, the study examines the impact of KV cache precision on concurrency capacity, finding that reducing precision from fp16 to fp8 nearly doubles session capacity before hitting this cliff, and further improvements are predicted with TurboQuant 4-bit. The study emphasizes the importance of understanding KV cache memory limitations, precision adjustments, and monitoring tail latency rather than averages to ensure optimal performance and avoid latency spikes in real-world deployments.

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