[MohitAMD]

Summary

In PD-disaggregation the MoRIIO connector's READ mode treated the remote KV load as synchronous (get_num_new_matched_tokens returned load_kv_async=False). The decode request — whose request_id is the router-embedded compound id (...___prefill_addr_...___decode_addr_..._<uuid>) — was therefore sent straight to RUNNING and into scheduler_output.num_scheduled_tokens. On a step where the worker produced no output row for it, that compound id was absent from model_runner_output.req_id_to_index, so the scheduler's unconditional req_id_to_index[req_id] lookup in update_from_output raised KeyError → EngineDeadError, on the first inference request of MoRIIO 1P1D/1P2D/2P2D runs.

What changed (connector-side, per review)

Per @njhill's review ("This should be fixed in the connector, not the scheduler."), this is now fixed entirely in the MoRIIO connector — core vllm/v1/core/sched/scheduler.py is untouched. The earlier scheduler guard has been dropped.

The READ-mode load now uses vLLM's existing async KV-load lifecycle, mirroring what MoRIIO's WRITE-mode consumer leg already does:

• MoRIIOConnectorScheduler.get_num_new_matched_tokens (READ branch): returns (n, True) when n > 0, so the request is held in WAITING_FOR_REMOTE_KVS instead of being scheduled before its KV exists. Falls back to (n, False) only for the tiny-prompt / nothing-to-load edge case (the scheduler asserts > 0 for async loads).
• MoRIIOConnectorWorker.get_finished (READ branch): now surfaces done_recving = self._pop_done_transfers() (previously discarded), giving the scheduler the finished_recving signal to promote the request out of WAITING_FOR_REMOTE_KVS.

The request is therefore never scheduled-but-unindexed, so the KeyError is structurally impossible while the core scheduler invariant (every scheduled req_id has an output row) is preserved untouched.

Tests

Adds tests/v1/kv_connector/unit/test_moriio_async_read.py (CPU-only): asserts READ→async, edge-case→sync, WRITE unchanged, producer never loads, and that Scheduler.update_from_output is not wrapped/patched.

Validation

Validated on 1P1D GLM-5.3-FP8 WideEP PD-disagg (MI300X):

• READ mode — directly exercises the fix: async path triggers on the exact compound PD req_ids, WAITING_FOR_REMOTE_KVS promotion confirmed, endpoints reach startup and serve HTTP 200, zero KeyError/EngineDeadError.
• WRITE mode — no regression across a multi-hour run (128k tokens, 0 errors).

Risk

The async path relies on get_finished reporting done_recving for every READ transfer (a missed signal would leave a request waiting in WAITING_FOR_REMOTE_KVS rather than crashing); the cluster run confirms the signal fires and requests complete.

---

🤖 This change was developed with AI assistance (Cursor); all changes were reviewed and validated by the author. See the Co-authored-by trailer on the commit.

[claude]

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[mergify]

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

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

[njhill]

This should be fixed in the connector, not the scheduler.

[MohitAMD]

Thanks @njhill — addressed. I've dropped the scheduler.py guard entirely and moved the fix into the MoRIIO connector.

Instead of tolerating the broken state in the core scheduler, READ mode now uses vLLM's existing async KV-load lifecycle: get_num_new_matched_tokens returns load_kv_async=True so the (compound-ID) decode request is held in WAITING_FOR_REMOTE_KVS, and get_finished now surfaces the done_recving signal to promote it once the KV lands — mirroring what the WRITE-mode consumer leg already does. The request is therefore never scheduled-but-unindexed, so the KeyError is structurally impossible and vllm/v1/core/sched/scheduler.py is untouched (empty diff vs main).

Branch was also rebased onto latest main (merge conflicts resolved), the unit test was moved to tests/v1/kv_connector/unit/test_moriio_async_read.py, and the change was validated on 1P1D GLM-5.3-FP8 WideEP PD-disagg (MI300X) in both READ (exercises the fix) and WRITE (no regression) modes. Ready for another look.

[simondanielsson]

Hi, thanks for the work!

Could you share a reproducer and the traceback for the error you're seeing? I'm skeptical this is the right fix as allowing async loading in read mode changes the entire consumer lifecycle

[simondanielsson]

Suggestion: Don't think this one makes sense to keep

[MohitAMD]

Thanks @simondanielsson!

Repro: 1P1D PD-disagg, MoRIIO connector in READ mode (read_mode: true), block_size=1, first decode request (GLM-5.3-FP8, MI300X). Traceback:

File ".../vllm/v1/core/sched/scheduler.py", line 1666, in update_from_output
    req_index = model_runner_output.req_id_to_index[req_id]
KeyError: 'cmpl-___prefill_addr_host:...___decode_addr_host:..._<uuid>-0-<hex>'
-> vllm.v1.engine.exceptions.EngineDeadError

Root cause: in READ mode get_num_new_matched_tokens returns load_kv_async=False, so the decode request lands in num_scheduled_tokens immediately — but on the step where the worker emits no output row for it, its id is absent from req_id_to_index and the lookup kills the EngineCore.

On the lifecycle concern — agreed, that's the crux. I went async specifically to reuse the path the WRITE-mode consumer already uses (WAITING_FOR_REMOTE_KVS -> get_finished/done_recving) rather than invent new behavior, and it validated clean on 1P1D READ. That said, you know the READ consumer better than I do — and this also has to satisfy @njhill's earlier request to keep the fix out of core scheduler.py. If you'd prefer a narrower fix that keeps READ synchronous, what would you recommend? Happy to rework.

Test: I'll drop test_scheduler_update_from_output_is_not_monkeypatched (line 126) — agreed it doesn't belong.

[simondanielsson]

@MohitAMD Thanks, can you share the full commands for the engine and workload?

I'd like to understand why/when this can happens

but on the step where the worker emits no output row for it

[mergify]

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

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