[Core][KV-transfer] MoRIIO multi-node TP prefill→decode dispatch

[chaeminlim-mb]

Purpose

Fix multi-node TP=16 prefill→decode dispatch in the MoRI-IO KV connector.
When prefill spans two hosts (rank 0-7 on P_NODE, rank 8-15 on P2_NODE),
each decode rank must dial the producer rank that holds its KV blocks.
Today, every decode rank dials a single remote_host parsed from
request_id, which always resolves to the prefill head. Decode ranks
8-15 then handshake the wrong producer (always the head, never the
worker), the subsequent MoRI-IO RDMA read never completes, and the
engine wedges at shm_broadcast.py:698 (repeated No available shared memory broadcast block found in 60 seconds).

This PR makes the proxy forward the prefill TP-group host list and adds
worker-side rank→host dispatch.

Changes

Connector (moriio_common.py, moriio_connector.py):

• MoRIIOConnectorScheduler reads VLLM_MORIIO_NODE_HOSTS at init and
  emits the ordered TP-group host list in request_finished's
  kv_transfer_params.remote_hosts.
• ReqMeta gains a remote_hosts field; MoRIIOConnectorMetadata.add_new_req
  populates it from kv_transfer_params.
• MoRIIOConnectorWorker._pick_remote_host(meta) picks
  remote_hosts[tp_rank // (tp_size // len(remote_hosts))] when
  len(remote_hosts) > 1, falling back to meta.remote_host for
  single-host setups. Used by _background_moriio_handshake and
  _read_blocks_for_req so both the handshake dial and the
  post-transfer notify callback route to the correct prefill node per
  rank.
• ReqMeta.tp_size reads tp_size first, then remote_tp_size,
  defaulting to 1 — defense-in-depth so a caller forwarding only
  remote_tp_size still resolves correctly.

Reference proxy (examples/disaggregated/disaggregated_serving/moriio_toy_proxy_server.py):

• READ-mode handle_request: forward remote_hosts from prefill_response
  into the decode-side kv_transfer_params.
• Forward bare tp_size alongside remote_tp_size (always, not just READ).

Backward compatibility

TP=8 1P1D and any single-host setup short-circuits at
len(remote_hosts) <= 1, so behavior is unchanged. The proxy fix only
changes behavior when the prefill side actually produces a multi-entry
remote_hosts list (gated by VLLM_MORIIO_NODE_HOSTS).

Not a duplicate of

• Fix engine_id collision + MoRIIO robustness for multi-node disagg DP #39276 (raviguptaamd) — DP engine_id collision in headless
  multi-node disagg + kv_transfer_params caching across scheduler
  steps. Different parallelism dimension (DP, not TP) and different
  failure (engine routing vs. per-rank handshake target).
• [ROCm] Add XGMI backend for MoRI Connector #41753 (simondanielsson) — XGMI transport backend; no overlap
  with the routing layer this PR touches.
• [Bugfix][KVConnector][NIXL] Sync engine_id across nodes for headless multi-node deployments #42928 (esmeetu) — NIXL connector's engine_id sync; not MoRIIO.

Searched open PRs/issues on 2026-05-19 with MoRIIO multi-node,
MoRIIO TP=16, remote_hosts, kv_connector multi-node TP,
kv_transfer multi-node TP=16 — nothing else targets the per-rank
prefill dispatch path.

Verification

Software stack on every node

	version
OS	Ubuntu 24.04.4 LTS (Noble Numbat)
Kernel	Linux 6.8.0-110-generic, x86_64
ROCm	7.2.0
Python	3.12 (container)
vLLM	base = upstream main @ 4a4fdabe2, plus this PR
NCCL	2.26.6 (vendored in vLLM)
Container	internal build of upstream vLLM main + MoRI-IO connector

Hardware

	per node
GPU	8× AMD Instinct MI300X (gfx942), 192 GB HBM, SCLK ~2050 MHz
NIC	10× Broadcom Thor (PCI vendor 0x14e4)
Fabric	RDMA over 100 GbE, MoRI-IO transport (UCX + NCCL HCAs above)

4 nodes total for the TP=16 sym 1P1D experiment (2 prefill + 2 decode).
Proxy runs on the prefill head.

Cluster topology used for the experiment

role	IP	TP ranks
prefill head + proxy	$P_NODE	rank 0-7
prefill worker	$P2_NODE	rank 8-15
decode head	$DH_NODE	rank 0-7
decode worker	$DW_NODE	rank 8-15

Launch invocations

Reference proxy (on prefill head, $P_NODE):

python3 examples/disaggregated/disaggregated_serving/moriio_toy_proxy_server.py \
  --port 10001

Prefill head ($P_NODE, --node-rank 0):

VLLM_HOST_IP=$P_NODE \
VLLM_NIXL_SIDE_CHANNEL_HOST=$P_NODE \
VLLM_NIXL_SIDE_CHANNEL_PORT=6300 \
VLLM_MORIIO_NODE_HOSTS=$P_NODE,$P2_NODE \
VLLM_ENABLE_V1_MULTIPROCESSING=1 VLLM_WORKER_MULTIPROC_METHOD=spawn \
VLLM_MORI_PD_ROLE=prefill \
vllm serve deepseek-ai/DeepSeek-R1-0528 \
  --served-model-name DeepSeek-R1-0528 \
  --tensor-parallel-size 16 \
  --max-model-len 16384 --max-num-batched-tokens 8192 \
  --gpu-memory-utilization 0.80 \
  --block-size 1 --no-enable-prefix-caching \
  --kv-cache-dtype fp8_e4m3 --dtype auto --trust-remote-code \
  --distributed-executor-backend mp \
  --speculative-config '{"method":"deepseek_mtp","num_speculative_tokens":1}' \
  --nnodes 2 --node-rank 0 --master-addr $P2_NODE --master-port 29500 \
  --kv-transfer-config '{"kv_connector":"MoRIIOConnector","kv_role":"kv_producer","kv_connector_extra_config":{"proxy_ip":"$P_NODE","proxy_port":36367,"proxy_ping_port":36367,"http_port":8100,"handshake_port":6301,"notify_port":61005}}' \
  --host 0.0.0.0 --port 8100

Prefill worker ($P2_NODE, --node-rank 1 --headless): same
shape, with --headless, --node-rank 1, VLLM_HOST_IP=$P2_NODE,
and the same VLLM_MORIIO_NODE_HOSTS=$P_NODE,$P2_NODE.

Decode head ($DH_NODE, --node-rank 0): same shape with
kv_role=kv_consumer, VLLM_MORI_PD_ROLE=decode,
VLLM_MORIIO_NODE_HOSTS=$DH_NODE,$DW_NODE, --http_port 8200,
--master-addr $DW_NODE. Decode worker ($DW_NODE): the
--headless counterpart on --node-rank 1.

Resolved vLLM config (from the prefill head's startup log)

INFO 05-15 10:58:24 [utils.py:240] non-default args:
  {'host': '0.0.0.0', 'port': 8100,
   'model': 'deepseek-ai/DeepSeek-R1-0528',
   'trust_remote_code': True,
   'max_model_len': 16384,
   'served_model_name': ['DeepSeek-R1-0528'],
   'distributed_executor_backend': 'mp',
   'master_addr': '$P2_NODE', 'master_port': 29500,
   'nnodes': 2,
   'tensor_parallel_size': 16,
   'block_size': 1,
   'gpu_memory_utilization': 0.8,
   'kv_cache_dtype': 'fp8_e4m3',
   'enable_prefix_caching': False,
   'max_num_batched_tokens': 8192,
   'speculative_config': {'method': 'deepseek_mtp',
                          'num_speculative_tokens': 1},
   'kv_transfer_config': KVTransferConfig(
       kv_connector='MoRIIOConnector', kv_role='kv_producer', ...
       kv_connector_extra_config={
         'proxy_ip': '$P2_NODE', 'proxy_port': 36367,
         'http_port': 8100, 'handshake_port': 6301, 'notify_port': 61005})}

INFO 05-15 10:59:07 [core.py:109] Initializing a V1 LLM engine (v0.1.dev0)
  with config:
   dtype=torch.bfloat16, max_seq_len=16384,
   tensor_parallel_size=16, pipeline_parallel_size=1, data_parallel_size=1,
   quantization=fp8, kv_cache_dtype=fp8_e4m3,
   enable_chunked_prefill=True (auto), enable_prefix_caching=False,
   speculative_config=SpeculativeConfig(method='mtp',
                                        num_spec_tokens=1),
   compilation: VLLM_COMPILE, CUDAGraphMode.FULL_AND_PIECEWISE,
   moe_backend=AITER Fp8 MoE,
   attention_backend(decode)=ROCM_AITER_MLA,
   attention_backend(prefill)=FLASH_ATTN MLA,
   sampler=aiter (ROCm)

Workload driver

lm_eval on the proxy port, gsm8k test split (1319 prompts):

python3 -m lm_eval --model local-chat-completions --apply_chat_template \
  --tasks gsm8k \
  --model_args 'model=DeepSeek-R1-0528,base_url=http://$P_NODE:10001/v1/chat/completions,api_key=EMPTY,max_retries=5,num_concurrent=30,timeout=1800,tokenized_requests=False,max_length=16384' \
  --gen_kwargs max_tokens=12288,temperature=0,top_p=1 \
  --log_samples --output_path ./gsm8k-output

Pre-fix observation

results/exp34-tp16-sym-1p1d-cycle12/, 2026-05-15 08:53 UTC, same
hardware and launch as above except this PR is not applied. gsm8k
progresses to 141 / 1319 prompts then the prefill engine wedges
and floods the log with a 60-second-interval message:

(EngineCore pid=1118) INFO 05-15 08:53:11 [shm_broadcast.py:698] No available shared memory broadcast block found in 60 seconds.
(EngineCore pid=1118) INFO 05-15 08:54:11 [shm_broadcast.py:698] No available shared memory broadcast block found in 60 seconds.
(EngineCore pid=1118) INFO 05-15 08:55:11 [shm_broadcast.py:698] No available shared memory broadcast block found in 60 seconds.
...

Root cause confirmed by inspecting decode-worker handshake logs: decode
ranks 8-15 dial $P_NODE (the prefill head) for KV blocks that
actually live on $P2_NODE, the RDMA read never completes, and
shm_broadcast waits on the unrendered tensor.

Post-fix observation

results/exp34-tp16-sym-1p1d-cycle3-v2/, 2026-05-15 11:50 UTC, with
this PR applied. gsm8k completes 1319 / 1319 in one run:

"results": {
  "gsm8k": {
    "exact_match,strict-match": 0.9492039423805914,
    "exact_match_stderr,strict-match": 0.006048352096878092,
    "exact_match,flexible-extract": 0.9476876421531463,
    "exact_match_stderr,flexible-extract": 0.006133057708959227
  }
}
"n-samples": {"gsm8k": {"original": 1319, "effective": 1319}}

Decode ranks 8-15 now handshake $P2_NODE (correct producer); no
shm_broadcast.py:698 spam on either prefill engine.

In-tree unit tests

The existing five tests in
tests/v1/kv_connector/unit/test_moriio_connector.py still pass (no
regression — they cover the single-host remote_host path that this
PR keeps as a fallback):

pytest tests/v1/kv_connector/unit/test_moriio_connector.py -v
# ✓ test_write_mode_saves_local_block_ids
# ✓ test_write_mode_with_chunked_prefill_saves_local_block_ids
# ✓ test_read_mode_loads_remote_block_ids
# ✓ test_register_kv_caches
# ✓ test_moriio_handshake_returns_metadata

These tests do not exercise the new _pick_remote_host multi-host
branch or the remote_hosts plumbing; the new branch is covered only
by the end-to-end cluster run above. Happy to add a targeted unit test
if reviewers prefer.

Lint

pre-commit run --files \
  vllm/distributed/kv_transfer/kv_connector/v1/moriio/moriio_common.py \
  vllm/distributed/kv_transfer/kv_connector/v1/moriio/moriio_connector.py \
  examples/disaggregated/disaggregated_serving/moriio_toy_proxy_server.py
pre-commit run mypy-3.10 \
  --files vllm/distributed/kv_transfer/kv_connector/v1/moriio/moriio_common.py \
          vllm/distributed/kv_transfer/kv_connector/v1/moriio/moriio_connector.py \
  --hook-stage manual
# All hooks: Passed.

AI assistance disclosure

This change was drafted with AI assistance (Claude Code). The diff is
~70 lines across 3 files. I (chaemin.lim@mangoboost.io) have reviewed
and defend each changed line; the exact run artifacts (eval JSON +
server logs) are under
results/exp34-tp16-sym-1p1d-cycle3-v2/accuracy/gsm8k/c30/results_2026-05-15T11-50-18.020558.json
and results/exp34-tp16-sym-1p1d-cycle3-v2/server_logs/20260515_105747/,
available on request. The intent and fallback semantics in
_pick_remote_host and the remote_tp_size read-order were authored
to match the existing single-host code paths so that TP=8 / monolithic
deployments cannot regress.

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[mergify]

Documentation preview: https://vllm--43063.org.readthedocs.build/en/43063/

[gemini-code-assist]

Code Review

This pull request introduces support for multi-node Tensor Parallelism (TP) within the MoRIIO KV transfer connector. It enables decode workers to resolve and connect to the specific prefill node containing their KV cache slice by introducing a remote_hosts list and mapping logic based on tp_rank. Key changes include the addition of the VLLM_MORIIO_NODE_HOSTS environment variable for configuration and updates to the handshake and block reading processes to utilize the correct peer host. I have no feedback to provide as there were no review comments.