[Core][KV-transfer] MoRIIO: heterogeneous TP<->DP prefill/decode read routing

[edwinlim0919]

Part of RFC #46107.

Purpose

MoRIIO today assumes the prefill and decode engines share the same parallelism layout. This PR enables heterogeneous disaggregated prefill/decode, where the two phases run different parallelism (TP vs DP+EP). The performance results in the RFC which encompasses this PR shows that heterogeneous parallel PD setups can achieve higher throughput and lower TTFT at high concurrency levels than their homogeneous parallel PD counterparts on MI300X (gfx942). This PR contains the support for both mixed-parallel configurations below:

• Forward: P_DP8EP → D_TP8 (DP/EP prefill, TP decode)
• Mirror: P_TP8 → D_DP8EP (TP prefill, DP/EP decode)

Three focused changes, one per commit:

1. Read from the request's prefill DP rank, not hardcoded DP0

Previously the READ path resolved the remote session and read-completion notify port from a hardcoded prefill DP rank 0, regardless of which prefill DP rank actually computed the KV. Under heterogeneous DP prefill the proxy forwards the owning rank in kv_transfer_params["remote_dp_rank"]; the read now targets it. remote_dp_rank == 0 (the symmetric single-DP case) is byte-identical to prior behaviour.

2. Eager pre-forward handshake (deadlock fix)

A decode TP worker reads from whichever prefill rank owns each request, so across requests every worker must reach several remote ranks. The previous lazy per-rank handshake on the read path lets the TP workers diverge. One rank can race ahead into a forward collective (TP all-gather, or the DP+EP all-to-all on the decode side) while a peer is still blocked in a handshake recv() → 600s NCCL timeout / hang. We handshake every referenced remote rank up front in start_load_kv, once per engine, behind a CPU all_reduce(MIN) barrier so all workers finish together and fail fast + uniformly on error.

3. Flexible prefill-TP read (mirror)

For a TP-replicated prefill + MLA feeding a DP decode, the latent KV is replicated across every prefill TP rank, so any is a valid source. Instead of every decode DP rank funnelling onto prefill tp0 (a single-GPU NIC bottleneck), each decode rank round-robins its reads across tp0..N-1 to spread the RDMA/prefill-NIC load. The choice is gated (all four must hold, so non-mirror configs are untouched):

• world_size == 1: decode is TP1, no local TP collective to keep in lockstep;
• use_mla: latent KV replicated across prefill TP ranks → any rank valid;
• remote_dp_size == 1: pure-TP prefill (no DP partition constraining source);
• remote_tp_size > 1: more than one prefill TP rank to spread across.

Outside the gate the source rank is the fixed local-rank mapping (_remote_tp_rank), byte-identical to prior behaviour. The chosen rank is threaded identically through the (dp, tp) session key, the handshake dial, and the notify port, so a read and its completion notify always address the same prefill rank. A drift would read one rank but notify another, leaving that rank's prefill buffer unfreed (KV leak → MR overflow).

The round-robin is deterministic (staggered by the decode rank's dp_rank), not random: it gives exactly-uniform NIC spread and is trivially testable.

Affected files

• vllm/distributed/kv_transfer/kv_connector/v1/moriio/moriio_connector.py
• vllm/distributed/kv_transfer/kv_connector/v1/moriio/moriio_common.py
• tests/v1/kv_connector/unit/test_moriio_routing_fairness.py

Test Plan

tests/v1/kv_connector/unit/test_moriio_routing_fairness.py: dependency-light (no GPU/ROCm/mori), drives the real routing functions (_resolve_read_source, _next_flex_tp_rank) and asserts hardware-fair prefill-read distribution across
every homogeneous and mixed P/D config in the RFC, plus the flexible gate, the symmetric-TP bijection, owner-DP coverage, round-robin determinism/stagger, and the read/notify same-rank threading.

Config                 coverage  balanced  flexible-gate
1P_TP8:1D_TP8          PASS      PASS      False
2P_TP8:1D_DP8EP        PASS      PASS      True
2P_TP8:2D_TP8          PASS      PASS      False
2P_DP8EP:3D_DP8EP      PASS      PASS      False
2P_DP8EP:4D_TP8        PASS      PASS      False

symmetric TP is a bijection                    PASS
owner-DP read covers every rank                PASS
round-robin deterministic/uniform/staggered    PASS
_read_blocks_for_req threads the chosen tp     PASS
21/21 assertions PASS

AI assistance

This PR was prepared with AI assistance (Claude). The submitter has reviewed every
changed line and is responsible for the change end-to-end.

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[mergify]

Documentation preview: https://vllm--46116.org.readthedocs.build/en/46116/

[edwinlim0919]

Tagging co-author chaeminlim-mb (Chaemin Lim, MangoBoost), who co-authored the fixes tracked in this issue, so he is looped in and can follow progress here.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @edwinlim0919.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @edwinlim0919.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[edwinlim0919]

hi @tjtanaa this is one of the last PRs we require to get through. Do you giving it a review?

[tanpinsiang]

What was tested

Item	Configuration
Hardware	Two AMD Instinct MI355X nodes, 8 GPUs per node
Model	MiniMaxAI/MiniMax-M3-MXFP8; identical resolved model snapshot and verified weights on both arms
P/D topology	TP8 prefill/router -> TP8 decode; 16 GPUs total
KV transfer	MoRIIO READ mode over RDMA

Functional and focused-test results

Check	Baseline	PR head	Result
Focused MoRIIO unit suite	59 passed	73 passed	Pass; head includes the new routing-fairness tests
Router, prefill, and decode health	Healthy	Healthy	Pass
End-to-end P/D request	HTTP 200	HTTP 200	Pass
Router path	Both endpoints used	Both endpoints used	Pass
Decode transfer handshakes	All 8 TP ranks	All 8 TP ranks	Pass; head eager handshake reported ok=True for ranks 0-7
Exception, OOM, or HTTP 5xx during valid workloads	None	None	Pass

Accuracy: GSM8K 25-shot

Strict exact match over all 1,319 prompts:

Arm	Run 1	Run 2	Two-run mean
Baseline	94.8446% (1,251/1,319)	94.2381% (1,243/1,319)	94.5413%
PR head	93.4799% (1,233/1,319)	94.0864% (1,241/1,319)	93.7832%
Head - baseline	-1.3647 pp	-0.1516 pp	-0.7582 pp

Performance: fixed-shape P/D benchmark

Metric	Baseline	PR head	Head vs. baseline
Successful / failed requests	1,280 / 0	1,280 / 0	Same
Output throughput	3,568.583 tok/s	3,575.756 tok/s	+0.201%
Request throughput	3.48494 req/s	3.49195 req/s	+0.201%
Total-token throughput	32,117.245 tok/s	32,181.803 tok/s	+0.201%
Duration	367.294 s	366.557 s	-0.201%
Mean TTFT	1,850.30 ms	1,852.04 ms	+0.094%
Median TTFT	387.37 ms	387.00 ms	-0.096%
p99 TTFT	26,407.13 ms	26,384.78 ms	-0.085%
Mean TPOT	32.994 ms	32.911 ms	-0.253%
Median TPOT	33.440 ms	33.382 ms	-0.172%
p99 TPOT	33.610 ms	33.536 ms	-0.222%
p99 ITL	59.826 ms	59.463 ms	-0.606%
Mean E2E latency	35,603.29 ms	35,519.65 ms	-0.235%
Median E2E latency	34,613.28 ms	34,565.19 ms	-0.139%
p99 E2E latency	60,729.76 ms	60,494.16 ms	-0.388%

Commands used

# Router
vllm-router \
  --host 0.0.0.0 \
  --port 30000 \
  --vllm-pd-disaggregation \
  --kv-connector moriio \
  --vllm-discovery-address 0.0.0.0:36367 \
  --policy consistent_hash \
  --prefill-policy consistent_hash \
  --decode-policy consistent_hash \
  --log-level info

# Run once for the TP8 producer and once for the TP8 consumer with the
# corresponding role-specific MoRIIO configuration.
VLLM_MORIIO_CONNECTOR_READ_MODE=1 \
VLLM_USE_V1=1 \
VLLM_ROCM_USE_AITER=1 \
VLLM_USE_BREAKABLE_CUDAGRAPH=0 \
vllm serve "$MODEL_SNAPSHOT" \
  --served-model-name MiniMaxAI/MiniMax-M3-MXFP8 \
  --host 0.0.0.0 \
  --port "$ROLE_HTTP_PORT" \
  --trust-remote-code \
  --tensor-parallel-size 8 \
  --block-size 128 \
  --language-model-only \
  --kv-cache-dtype fp8 \
  --attention-backend TRITON_ATTN \
  --no-enable-prefix-caching \
  --gpu-memory-utilization 0.90 \
  --tool-call-parser minimax_m3 \
  --reasoning-parser minimax_m3 \
  --enable-auto-tool-choice \
  --kv-transfer-config "$ROLE_SPECIFIC_MORIIO_CONFIG"

ROLE_SPECIFIC_MORIIO_CONFIG selected kv_producer or kv_consumer and set read_mode=true, backend=rdma, qp_per_transfer=4, and num_workers=4.

# Run twice per arm; lm_eval==0.4.12
lm_eval run \
  --model local-completions \
  --model_args model=MiniMaxAI/MiniMax-M3-MXFP8,base_url=http://127.0.0.1:30000/v1/completions,num_concurrent=200,max_retries=10,max_gen_toks=2048,max_length=1048576,timeout=60000 \
  --tasks gsm8k \
  --num_fewshot 25 \
  --batch_size auto \
  --device cpu \
  --seed 0,1234,1234,1234 \
  --output_path "$GSM8K_OUTPUT" \
  --log_samples

# Warmup: NUM_PROMPTS=128 and SAVE_ARGS=().
# Measured: NUM_PROMPTS=1280 and
# SAVE_ARGS=(--save-result --save-detailed --result-dir "$PERF_OUTPUT" --result-filename result.json).
vllm bench serve \
  --backend openai \
  --host 127.0.0.1 \
  --port 30000 \
  --endpoint /v1/completions \
  --model "$MODEL_SNAPSHOT" \
  --served-model-name MiniMaxAI/MiniMax-M3-MXFP8 \
  --tokenizer "$MODEL_SNAPSHOT" \
  --dataset-name random \
  --random-input-len 8192 \
  --random-output-len 1024 \
  --random-range-ratio 0.0 \
  --num-prompts "$NUM_PROMPTS" \
  --num-warmups 0 \
  --max-concurrency 128 \
  --request-rate inf \
  --seed 0 \
  --ignore-eos \
  --trust-remote-code \
  --percentile-metrics ttft,tpot,itl,e2el \
  "${SAVE_ARGS[@]}"

[edwinlim0919]

@tjtanaa @tanpinsiang Thank you so much for the review, testing, and approval!!!

Currently I see that the CI fails, but the failing lanes are unrelated to this PR. The diff here is three MoRIIO files, and is unrelated to the failures.

• docker-build-cpu-image: infra failure, not a code failure (runner lost connectivity to during an apt-get build, CI node network flakiness)
• pytorch-compilation-unit-tests (exit −1): known lane timeout that is addressed by [CI] Increase timeout of pytorch-compilation-unit-tests #49450
• multi-modal-processor, pipeline-plus-context-parallelism-4-gpus, v1-sample-plus-logits: inherited from main --> the six most recent commits on main (as of Jul 22) all report failing CI, and this PR has no import path into any of these subsystems

Will re-merge main and retrigger once it stabilizes (picking up #49450 in the process).

[mergify]

Hi @edwinlim0919, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.