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Last technical review · 17.08.2026 · vLLM API

vLLM API Server: Turn a Local Model Into an OpenAI-Compatible Service

vLLM is not merely a model loader; continuous batching, KV-cache management and OpenAI-compatible endpoints are designed for efficient multi-request serving. The useful benchmark is not single-user token/s but throughput and p95 latency under concurrency.

Production note

Do not blindly copy a `docker run` command without validating model license, tokenizer/chat template and quantization format. “Server started” does not prove correct answer formatting.

vllm setupvllm openai api servervllm gpu server
TECHNICAL IMPLEMENTATION PROFILE
EKA CORE
vLLM API

The official vLLM server exposes OpenAI-compatible endpoints such as Chat/Completions and can run from the `vllm/vllm-openai` Docker image. Even if weights fit in VRAM, reserve headroom for KV cache and concurrency.

OpenAICompatible API
Checked
BatchContinuous batching
Checked
KVContext memory
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TPMulti-GPU
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Technical guide · production-focused · official sources
Quick answer

The official vLLM server exposes OpenAI-compatible endpoints such as Chat/Completions and can run from the `vllm/vllm-openai` Docker image. Even if weights fit in VRAM, reserve headroom for KV cache and concurrency.

01

Technical scope at a glance

Deploy vLLM’s OpenAI-compatible server on a GPU host with Docker, model caching, tensor parallelism, endpoint tests, concurrency, prefix caching and real benchmark methodology.

OpenAICompatible API

Lets many OpenAI-SDK applications connect by changing the base URL.

BatchContinuous batching

Designed to schedule concurrent requests efficiently on the GPU.

KVContext memory

Long context and concurrency consume VRAM beyond model weights.

TPMulti-GPU

Tensor parallelism can shard models across GPUs; interconnect bottlenecks must be measured.

On this page

  1. 1. Budget serving VRAM, not just model file size
  2. 2. Validate GPU and driver before the container
  3. 3. Keep model cache outside ephemeral container storage
  4. 4. Test `/v1/models` before Chat Completions
  5. 5. Account for NVLink/PCIe topology in tensor parallelism
  6. 6. Measure prefix caching against real prompt repetition
  7. 7. Find saturation with a concurrency ladder
  8. Frequently asked questions
02

1. Budget serving VRAM, not just model file size

Weights, quantization overhead, CUDA/runtime, KV cache and fragmentation share VRAM. Targeting 100% occupancy increases OOM risk.

BudgetDriver
WeightsParameters × bits
KV cacheContext × concurrency × model arch
RuntimeCUDA/kernels/graphs
HeadroomPeak/fragmentation
03

2. Validate GPU and driver before the container

If host `nvidia-smi` is unhealthy, do not debug it inside the container first. Validate driver/CUDA compatibility at host level.

Command
nvidia-smi
Command
nvidia-smi --query-gpu=name,memory.total,driver_version,pstate --format=csv
Command
docker info | grep -i runtime
Command
docker run --rm --gpus all nvidia/cuda:12.8.0-base-ubuntu24.04 nvidia-smi
04

3. Keep model cache outside ephemeral container storage

Redownloading a large model on each deployment increases downtime, bandwidth and startup time. Persist model cache outside the container.

Command
mkdir -p /srv/vllm-cache
Command
docker run --rm --gpus all -p 8000:8000 -v /srv/vllm-cache:/root/.cache/huggingface vllm/vllm-openai:latest --model MODEL_ID --host 0.0.0.0 --port 8000
05

4. Test `/v1/models` before Chat Completions

A server can appear ready while tokenizer/chat-template errors emerge on the first chat request. Put a simple smoke test into the deployment gate.

Command
curl -s http://127.0.0.1:8000/v1/models | jq .
Command
curl -s http://127.0.0.1:8000/v1/chat/completions -H 'Content-Type: application/json' -d '{"model":"MODEL_ID","messages":[{"role":"user","content":"Say only READY"}],"max_tokens":8}' | jq .
06

5. Account for NVLink/PCIe topology in tensor parallelism

Two GPUs increase aggregate VRAM but do not guarantee 2× performance. Collective communication and PCIe/NVLink bandwidth shape scaling.

Command
nvidia-smi topo -m
Command
nvidia-smi nvlink --status 2>/dev/null || true
07

6. Measure prefix caching against real prompt repetition

Workloads repeating long system/RAG prefixes may benefit more from caching; random-prompt benchmarks can miss that advantage.

Measure cold-request TTFT.
Measure warm TTFT with the same prefix.
Record cache hit rate and VRAM impact.
08

7. Find saturation with a concurrency ladder

At 1, 2, 4, 8 and 16 concurrent requests, measure TTFT, output tokens/s, p95 latency and error rate. Maximum concurrency within SLA matters more than peak tokens/s.

ConcurrencyTTFT p95Output tok/sError %
1
2
4
8
16
EKA SUNUCU · TECHNICAL

Choose the GPU for concurrency SLA, not model size alone

Plan single- or multi-GPU vLLM benchmark environments on Eka Sunucu around model, context and concurrent-request targets.

Production principleMeasure → Test → DeployNo fabricated benchmark data.
SRC

Official sources

Primary documentation and technical references used by this guide.

EKA

Related technical guides

Continue with related infrastructure and implementation guides.

FAQ

Frequently asked questions

vLLM API

Can vLLM work with the OpenAI SDK?

Its OpenAI-compatible endpoints allow many SDKs/apps to connect by changing base URL and model name; verify the specific endpoint features you use.

Is fitting model weights into VRAM enough for vLLM?

No. KV cache, runtime and concurrency require additional VRAM. Include context length and concurrent users in capacity planning.

Are two GPUs twice as fast?

No guarantee. Scaling depends on tensor-parallel communication, model size, batch and GPU interconnect.

What is the official vLLM Docker image?

Official documentation shows `vllm/vllm-openai` for OpenAI-compatible serving.

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