PD Disaggregation¶

Prefill-Decode (PD) disaggregation separates the two phases of LLM inference — prompt processing (prefill) and token generation (decode) — onto specialized workers. This optimizes Time to First Token (TTFT) and throughput independently.

Why Disaggregate?¶

Running both on the same worker creates contention — prefill batches wait for decode slots, and decode batches stay small due to memory pressure. Dedicating workers to each phase removes this conflict.

SGLang PD¶

SMG sends the request to both prefill and decode workers simultaneously, and they coordinate KV cache transfer through a bootstrap mechanism.

Start SGLang Workers¶

# Prefill worker
python -m sglang.launch_server \
  --model-path meta-llama/Llama-3.1-70B-Instruct \
  --port 8000 \
  --prefill-only

# Decode worker
python -m sglang.launch_server \
  --model-path meta-llama/Llama-3.1-70B-Instruct \
  --port 8001 \
  --decode-only

Start SMG¶

Each prefill worker needs a bootstrap port for coordination:

smg \
  --pd-disaggregation \
  --prefill http://prefill:8000 9001 \
  --decode http://decode:8001 \
  --host 0.0.0.0 \
  --port 30000

Multiple Workers¶

smg \
  --pd-disaggregation \
  --prefill http://prefill1:8000 9001 \
  --prefill http://prefill2:8000 9002 \
  --decode http://decode1:8001 \
  --decode http://decode2:8001 \
  --prefill-policy cache_aware \
  --decode-policy power_of_two

vLLM PD¶

SMG sends to prefill first with max_tokens=1, then sends the original request to decode, relaying KV-transfer metadata between the two legs:

• NIXL: SMG tags the prefill request with do_remote_decode=true, harvests the kv_transfer_params the prefill engine returns (engine id, request id, block ids, side-channel address, TP size), and forwards them verbatim with the decode request so decode pulls the KV cache over NIXL.
• Mooncake: the connector is push-based and returns nothing, so SMG mints a shared transfer_id, tags the prefill request with it, and synthesizes the decode params (remote_engine_id discovered from the worker at registration, remote_bootstrap_addr from the worker's bootstrap host/port). With an older servicer that doesn't report kv_engine_id, SMG falls back to legacy host/port injection.

Start vLLM Workers with NIXL¶

# Prefill worker
VLLM_NIXL_SIDE_CHANNEL_PORT=5600 \
python -m vllm.entrypoints.grpc_server \
  --model /path/to/model \
  --port 50051 \
  --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_producer"}'

# Decode worker
VLLM_NIXL_SIDE_CHANNEL_PORT=5601 \
python -m vllm.entrypoints.grpc_server \
  --model /path/to/model \
  --port 50052 \
  --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_consumer"}'

VLLM_NIXL_SIDE_CHANNEL_PORT must be unique per worker on the same host (with data parallelism each rank uses port + dp_rank). When prefill and decode run on different machines, also set VLLM_NIXL_SIDE_CHANNEL_HOST to an address reachable from the decode worker — prefill embeds this host/port in the handoff params that decode uses to fetch the KV cache.

To verify KV transfer is active, send a request and look for Transfer plan: in the decode worker log (vLLM >= 0.20). If the router logs prefill returned no kv_transfer_params, upgrade the servicer (smg-grpc-servicer >= 0.5.4, smg-grpc-proto >= 0.4.9) or check the --kv-transfer-config on the workers.

Start SMG¶

vLLM workers use grpc:// URLs and require --model-path for tokenizer loading:

smg \
  --pd-disaggregation \
  --prefill grpc://prefill:50051 \
  --decode grpc://decode:50052 \
  --model-path /path/to/model \
  --host 0.0.0.0 \
  --port 30000

Alternative: Mooncake Backend¶

Mooncake supports TCP transport (no RDMA required). Each prefill worker needs a unique bootstrap port:

# Prefill worker
VLLM_MOONCAKE_BOOTSTRAP_PORT=8998 \
python -m vllm.entrypoints.grpc_server \
  --model /path/to/model \
  --port 50051 \
  --kv-transfer-config '{"kv_connector":"MooncakeConnector","kv_role":"kv_producer","engine_id":"prefill-0"}'

# Decode worker
python -m vllm.entrypoints.grpc_server \
  --model /path/to/model \
  --port 50052 \
  --kv-transfer-config '{"kv_connector":"MooncakeConnector","kv_role":"kv_consumer"}'

Set an explicit engine_id on each prefill worker in production. SMG discovers the id once at worker registration; without a pinned id, vLLM generates a new one per process, so a restarted prefill container would invalidate the registered id until the worker is re-registered.

With vLLM data parallelism (data_parallel_size > 1), run SMG with --dp-aware: SMG pins each request to a DP rank and mints the decode params with the matching {engine_id}_dp{rank} engine-core id. Without --dp-aware, DP>1 prefill workers are not minted for and SMG falls back to legacy host/port injection (decode recomputes the prompt locally). External-LB DP (--data-parallel-external-lb, one pod per rank) is unsupported for Mooncake minting: every pod's engine core is {engine_id}_dp0, so register pods as plain workers and pin a distinct engine_id per pod.

smg \
  --pd-disaggregation \
  --prefill grpc://prefill:50051 8998 \
  --decode grpc://decode:50052 \
  --model-path /path/to/model

Helper Script¶

Use the provided script to launch workers with either backend:

# NIXL (default)
./scripts/launch-pd-workers.sh vllm /path/to/model

# Mooncake
KV_BACKEND=mooncake ./scripts/launch-pd-workers.sh vllm /path/to/model

Verify¶

# Check workers and their roles
curl http://localhost:30000/workers | jq

# Send a request
curl http://localhost:30000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "meta-llama/Llama-3.1-70B-Instruct",
    "messages": [{"role": "user", "content": "Hello!"}]
  }'

vLLM vs SGLang PD at a Glance¶

Next Steps¶

For sizing guidelines, per-phase routing policies, Kubernetes service discovery, and monitoring, see the full PD Disaggregation Concepts page.