Llm Inference Serving hard
Users report that their stream freezes for 1 to 2 seconds mid-response whenever another user on the same replica submits a 30K-token document. The scheduler runs each new request's entire prefill in a single iteration alongside the ongoing decodes. What is the most targeted fix?
- A Increase the maximum batch size so the long prefill shares its iteration with more decode sequences and its cost is amortized across all of them.
- B Enable speculative decoding so decode sequences emit several tokens per step and can catch up on lost progress once the prefill stall has ended.
- C Quantize the KV cache to 8-bit so the 30K-token prefill writes half as many bytes and therefore finishes in roughly half the wall-clock time.
- D Split prefill into fixed-size chunks interleaved with decode steps, or route very long prompts to a separate prefill pool of GPUs.
Answer: D. Split prefill into fixed-size chunks interleaved with decode steps, or route very long prompts to a separate prefill pool of GPUs.
Prefill is **compute-bound**, so a 30K-token prefill monopolizes one iteration and stalls every co-scheduled decode. **Chunked prefill** caps per-iteration work so ITL stays bounded, at the cost of a slightly longer TTFT for the long prompt. KV quantization saves memory but does not cut prefill FLOPs, and full prefill/decode disaggregation is worth its KV-transfer cost only at larger scale.
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