pd-disaggregation.md

title	Model inference with Prefill-Decode disaggregation
date	2026-02-19
description	TBA
slug	pd-disaggregation
image	https://dstack.ai/static-assets/static-assets/images/dstack-pd-disaggregation.png
categories	Changelog
links	SGLang router integration https://dstack.ai/blog/sglang-router/

Model inference with Prefill-Decode disaggregation

While dstack started as a GPU-native orchestrator for development and training, over the last year it has increasingly brought inference to the forefront — making serving a first-class citizen.

At the end of last year, we introduced SGLang router integration — bringing cache-aware routing to services. Today, building on that integration, we’re adding native Prefill–Decode (PD) disaggregation.

Unlike many PD disaggregation setups tied to Kubernetes as the control plane, dstack does not depend on Kubernetes. It’s an open-source, GPU-native orchestrator that can provision GPUs directly in your cloud accounts or on bare-metal infrastructure — while also running on top of existing Kubernetes clusters if needed.

For inference, dstack provides a services abstraction. While remaining framework-agnostic, we integrate more deeply with leading open-source frameworks — SGLang being one of them for model inference.

If you’re new to Prefill–Decode disaggregation, see the official SGLang docs.

!!! note "Deprecation notice" Configuring the SGLang router in a gateway is deprecated and will be disallowed in a future release. To run router and workers as separate replica groups, see SGLang PD disaggregation (router as replica group).

Services

With dstack 0.20.10, you can define a service with separate replica groups for Prefill and Decode workers and enable PD disaggregation directly in the router configuration.

type: service
name: glm45air

env:
  - HF_TOKEN
  - MODEL_ID=zai-org/GLM-4.5-Air-FP8

image: lmsysorg/sglang:latest

replicas:
  - count: 1..4
    scaling:
      metric: rps
      target: 3
    commands:
      - |
          python -m sglang.launch_server \
            --model-path $MODEL_ID \
            --disaggregation-mode prefill \
            --disaggregation-transfer-backend mooncake \
            --host 0.0.0.0 \
            --port 8000 \
            --disaggregation-bootstrap-port 8998
    resources:
      gpu: H200

  - count: 1..8
    scaling:
      metric: rps
      target: 2
    commands:
      - |
          python -m sglang.launch_server \
            --model-path $MODEL_ID \
            --disaggregation-mode decode \
            --disaggregation-transfer-backend mooncake \
            --host 0.0.0.0 \
            --port 8000
    resources:
      gpu: H200

port: 8000
model: zai-org/GLM-4.5-Air-FP8

probes:
  - type: http
    url: /health_generate
    interval: 15s

router:
  type: sglang
  pd_disaggregation: true

Deploy it as usual:

$ HF_TOKEN=...
$ dstack apply -f glm45air.dstack.yml

Gateway

Just like dstack relies on the SGLang router for cache-aware routing, Prefill–Decode disaggregation also requires a gateway configured with the SGLang router.

type: gateway
name: inference-gateway

backends: [kubernetes]
region: any

domain: example.com

router:
  type: sglang
  policy: cache_aware

Limitations

• Because the SGLang router requires all workers to be on the same network, and dstack currently runs the router inside the gateway, the gateway and the service must be running in the same cluster.
• Autoscaling supports RPS as the metric for now; TTFT and ITL metrics are planned next.
• Prefill–Decode disaggregation is currently available with the SGLang backend (vLLM support is coming).

With native support for inference and now Prefill–Decode disaggregation, dstack makes it easier to run high-throughput, low-latency model serving across GPU clouds, and Kubernetes or bare-metal clusters.

What's next?

We’re working on PD disaggregation benchmarks and tuning guidance — coming soon.

In the meantime:

1. Read about services, gateways, and fleets
2. Check out Quickstart
3. Join Discord