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What Is Continuous Batching in LLM Serving, and When Does It Help?

Continuous batching can keep an LLM serving batch occupied as requests finish and new ones arrive. Here is how it works, when it helps, and what to measure.
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Continuous batching is a way to schedule LLM requests during text generation: as soon as one request finishes, the serving system can add a waiting request to the active batch instead of waiting for every request in a fixed batch to finish. This can keep hardware busier and increase aggregate throughput when requests overlap and finish at different times. It is not a guarantee of lower latency: prompt processing, decode scheduling, KV-cache capacity, and admission limits all matter.

How continuous batching works

An LLM serving request typically moves through a queue, prompt processing (called prefill), token generation (called decode), and completion. During decode, the model generates output autoregressively, one token at a time. In a fixed request-level batch, requests are grouped together and the batch may have to wait for its slowest member to finish. With continuous batching, the scheduler can remove a completed request at a generation step and admit another queued request while the remaining requests continue decoding.

Hugging Face describes this approach as keeping the GPU occupied, with higher throughput and lower average latency as potential benefits. Those are general effects, not a promise for every workload or configuration. The scheduler still has to decide which work fits in each forward pass.

What limits the active batch

Continuous batching does not mean that an unlimited number of requests can run simultaneously. The Transformers scheduler documented by Hugging Face uses a query-token budget for a forward pass, a KV-cache/page budget, and a request cap. The KV cache stores information needed to continue generating each active sequence, so available cache capacity limits how many and how long requests can remain in flight.

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If a new prompt is too large to fit within the available token budget, the documented scheduler can process part of it, defer the remainder, and continue that prompt in later steps interleaved with ongoing decode. This lets the system fit work into constrained iterations, but the balance between prompt work and active generation remains a scheduling decision.

When it is most likely to help

The clearest fit is concurrent traffic with requests arriving over time and output lengths that vary. When short requests finish while longer ones are still generating, a continuous scheduler can use the freed capacity for queued work. A fixed batch can leave capacity unused while waiting for its slowest member; continuous replacement can improve utilization and aggregate throughput in that situation.

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The outcome depends on the actual arrival pattern, prompt and output lengths, model, hardware, cache headroom, and latency target. If traffic is sparse, requests rarely overlap, or another resource is the bottleneck, continuous batching may have little practical effect. Measure it on representative traffic rather than treating it as an automatic speedup.

Why prefill and decode create a latency trade-off

Prefill processes the input prompt; decode generates the response. They have different scheduling demands. A long prompt can consume an iteration and delay token generation for requests already in progress. Giving prompt work priority can improve the rate at which new prompts are processed but worsen time between tokens for active users. Prioritizing decode can protect ongoing generation while making new requests wait longer to start.

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The Sarathi-Serve authors frame this as a throughput–latency trade-off: “We introduce an efficient LLM inference scheduler, Sarathi-Serve, to address this throughput-latency tradeoff.” Their paper proposes chunked prefill, which splits prompt processing across iterations so it can be interleaved with decode; its stall-free schedule is designed to add prefill chunks without pausing ongoing decode. Chunking can improve the balance, but it does not remove the need to choose scheduling priorities or set resource limits.

What benchmark results show—and do not show

In its 2024 evaluation, the Sarathi-Serve paper reported the following serving-capacity comparisons with vLLM under the paper’s models, hardware, workloads, and latency constraints:

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Evaluation Reported result
Mistral-7B on one A100 GPU 2.6× higher serving capacity than vLLM, as reported by the Sarathi-Serve authors in 2024
Yi-34B on two A100 GPUs Up to 3.7× higher serving capacity than vLLM, as reported by the Sarathi-Serve authors in 2024
Falcon-180B using pipeline parallelism Up to 5.6× gain in end-to-end serving capacity, as reported by the Sarathi-Serve authors in 2024

These are results for that study’s scheduler, systems, workloads, and latency constraints—not general multipliers for continuous batching. The paper’s Yi-34B evaluation, for example, considers throughput against p99 time-between-token latency. A system that serves more requests may still feel worse to users if token delivery becomes less responsive.

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How to compare serving configurations fairly

Compare systems under the workload and service objective you actually care about. Keep the model, hardware, prompt and output length distributions, request arrival pattern or concurrency, and latency target aligned. Report throughput or serving capacity alongside interactive latency, rather than using a single headline number.

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  • Measure time to first token, which captures how long users wait before generation starts.
  • Measure time between tokens, including tail latency such as p99 where available, to capture stalls during generation.
  • Report aggregate throughput or serving capacity so that a latency improvement is not achieved merely by leaving hardware underused.
  • Record scheduler settings and token/KV-cache budgets, since these affect admission and the work that can be scheduled together.

vLLM’s serving documentation exposes controls for scheduled or batched tokens, sequence counts, chunked prefill, KV-cache admission safeguards, asynchronous scheduling, and streaming intervals. The exact options and defaults can change, so consult the documentation for the version you deploy rather than copying configuration values from an unrelated release. Its documentation says asynchronous scheduling can avoid GPU utilization gaps and may improve latency and throughput; treat that as an implementation-specific claim to verify under your workload.

Operational limits and engine context

Continuous batching alone does not solve queueing, tail latency, fairness, or memory pressure. Admission policy determines when a request is allowed to run, and KV-cache capacity can constrain concurrent sequences. When capacity is tight, the system may need to defer work or apply cache-related safeguards; settings that raise concurrency can also increase resource pressure.

Model size and deployment layout matter too. vLLM documents tensor-parallel serving across GPUs and multi-node deployment for cases where one node lacks enough GPUs to hold a model, with Ray and multiprocessing execution options. These are scaling choices for particular deployment needs, not prerequisites for continuous batching itself.

Engine status is also worth checking when choosing an implementation. Hugging Face currently describes Text Generation Inference (TGI) as being in maintenance mode and recommends downstream inference engines including vLLM and SGLang; its documentation also lists continuous batching and tensor parallelism among TGI’s features. Project status and live framework documentation can change, so confirm current guidance for the specific version and deployment you intend to use.

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