[gty111]

Summary

NIXL EC consumers can fall back to local encoding after a remote transfer fails, even when a rewritten request contains only metadata. This can trigger an assertion and terminate EngineCore instead of failing the affected request.

• Preserve local fallback when actual model input is available; otherwise report the unavailable request through take_unavailable_requests().
• Retry and log ACK timeouts, with a 60-second per-request/per-embedding wait budget that survives retries and in-flight waits. Increase the default ACK timeout from 2 to 30 seconds.
• Remove announcement leases from this PR; existing read-grant pinning is unchanged.

Validation

• .venv/bin/python -m pytest tests/v1/ec_connector/unit: 286 passed, including local-input fallback and bounded-retry coverage.
• Pre-commit hooks passed.
• MUIRBench: Qwen3.5-35B-A3B, two 4-GPU GB200 nodes, 4 E + 4 PD, TP=1, V2 runner, concurrency 96, CUDA Graph enabled through 16384 tokens on PD; 200 warmup requests + five 1,200-question rounds.

	Main 54da70c1eb	This PR 37ce970dbd
Successful formal requests	6000/6000	6000/6000
Mean accuracy	58.92%	59.08%
Median throughput	59.60 req/s	60.20 req/s

The historical crash was not reproduced on latest main in this run. Its unsafe fallback path remains in the code; this is a fault-isolation fix, not a demonstrated performance improvement. A deterministic fault-injection E2E has not yet been run for this revision.

Scope

No other open PR addressing this NIXL EC failure path was found in searches for NIXL EC request failures and 47941 in:body.

AI assistance was used for implementation and validation.

[claude]

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• tests/v1/ec_connector/unit/test_scheduler_nixl_consumer.py
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---

Walkthrough

The CPU connector now bounds remote-admission deferrals, reports unavailable requests, retries consumer ACK timeouts, and manages independent producer announcement holds with per-announcement leases.

Changes

EC transfer reliability

Layer / File(s)	Summary
Unrecoverable admission and scheduler propagation vllm/distributed/ec_transfer/ec_connector/cpu/scheduler/__init__.py, vllm/distributed/ec_transfer/ec_connector/cpu/connector.py, tests/v1/ec_connector/unit/test_scheduler_nixl_consumer.py	Remote admission failures now become unavailable requests. Deferrals use per-request budgets. Invalid announcements, size mismatches, failed transfers, and tombstoned items fail requests.
Producer announce pin lifecycle vllm/distributed/ec_transfer/ec_connector/cpu/scheduler/__init__.py, vllm/distributed/ec_transfer/ec_connector/cpu/session.py, tests/v1/ec_connector/unit/test_scheduler_nixl_producer.py	Producer announcements use independent holds and leases. Served or expired grants release individual pins. Pending holds transfer when saves become ready.
ACK timeout retry semantics vllm/distributed/ec_transfer/ec_connector/cpu/session.py, tests/v1/ec_connector/unit/test_session.py	Consumer ACK timeouts now enter the retryable result set instead of the tombstone set. Results clear after retrieval.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to 282be

The connector now defers or fails unavailable encoding requests without terminating the engine, retains announced encodings until completion or lease expiry, and retries ACK timeouts. Current coverage supports these behaviors with no remaining merge-blocking risk.

Sequence Diagram(s)

sequenceDiagram
  participant RequestScheduler
  participant ECCPUScheduler
  participant ECCPUConnector
  participant ConsumerSession
  RequestScheduler->>ECCPUScheduler: admit remote request
  ECCPUScheduler->>ECCPUScheduler: defer or fail admission
  ECCPUScheduler->>ECCPUConnector: expose unavailable request
  ECCPUConnector->>RequestScheduler: return unavailable request ID
  ConsumerSession-->>ECCPUScheduler: report ACK timeout
  ECCPUScheduler->>ConsumerSession: retry transfer

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Suggested reviewers: omerpaz95

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 43.75% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 48 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: fail the affected request instead of terminating the engine when a remote encoding cannot arrive.
Description check	✅ Passed	The description directly addresses remote transfer failures, request-level failure handling, ACK timeouts, deferral budgets, validation, and test results covered by the changeset.

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Actionable comments posted: 3

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Inline comments:
In `@vllm/distributed/ec_transfer/ec_connector/cpu/scheduler/__init__.py`:
- Line 302: Update _nixl_consumer_admit() around the size_bytes conversion to
catch TypeError and ValueError for None or non-numeric announcements, call
_fail() so the request is recorded as failed, and prevent the exception from
propagating during scheduling. Add a regression test covering invalid size_bytes
input.
- Around line 337-350: The deferred timestamp map currently collides across
requests sharing an mm_hash, causing a later request to inherit an earlier
timeout and fail prematurely. Update _deferred_since and the _defer/_fail
lifecycle to key entries by both request.request_id and mm_hash, and remove the
corresponding key when the request leaves its lifecycle.

In `@vllm/distributed/ec_transfer/ec_connector/cpu/session.py`:
- Line 265: Replace the hash-only _served set in
vllm/distributed/ec_transfer/ec_connector/cpu/session.py:265-265 with a
cardinality-preserving structure, record one event per grant in the completion
path at vllm/distributed/ec_transfer/ec_connector/cpu/session.py:371-371 and
expiry path at vllm/distributed/ec_transfer/ec_connector/cpu/session.py:394-394,
and release only the corresponding hold or after all outstanding holds are
resolved. Update _FakeProducerSession in
tests/v1/ec_connector/unit/test_scheduler_nixl_producer.py:122-130 to preserve
repeated served events, and extend
tests/v1/ec_connector/unit/test_scheduler_nixl_producer.py:222-232 to verify the
first completion keeps a duplicate announcement pinned while the final
completion or lease expiry releases it.

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• vllm/distributed/ec_transfer/ec_connector/base.py
• vllm/distributed/ec_transfer/ec_connector/cpu/connector.py
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Inline comments:
In `@vllm/distributed/ec_transfer/ec_connector/cpu/scheduler/__init__.py`:
- Line 448: Update the announcement-hold handling around the assignment to
hold.deadline so each announcement retains its own expiry rather than refreshing
existing holds for the same mm_hash. Ensure _release_announce_pins() sweeps only
expired holds, allowing older holds to expire after frequent reannouncements,
and add a regression test covering a reannouncement shortly before the first
lease expires without a served read.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @gty111.

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

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[omerpaz95]

Two main blockers:

1. Producer drops XferReqs -> consumer retries forever, never fails. _deferred_since is popped when _in_flight.add runs in _start_xfer, so every ACK_TIMEOUT retry restarts the clock and _defer never sees the retry path.
2. Regression: Pass image + ec_transfer_params to consumer, and the producer NACKs -> request FAILS (main would local-encode). _fail needs to be gated on the request actually being unable to self-encode.
   Reproduced both on 2×H100 with a fault-injected producer. source files attached. ec-repro-pr55290.zip

Some more design issues:
3. On the producer, every distinct mm_hash announced in the last 30 s is now unevictable, whether or not any consumer reads it. Before, FIFO could reclaim as soon as reads landed. So under real heavy mm workloads, the producer space utilization in the ec shared region is degraded, because the first multimodal objects are unevictable for 30 seconds, even when there's nobody consuming them. The "steady-state cost is the work in flight" comment is only true when consumers read promptly. There's no guarantee that this actually happens - depends on the orchestration.
4. In general, I don't know if this mechanism of "announce lease" is a good idea - in real life workloads, the consumer may have heavy backlog and the requests may delay in the queue for longer than 30 seconds. I think a better solution here (Future PR) is to create EC Events (Much like KV Events) that announce new EC available and EC deletions from local CPU cache.

On a more general note, I think this PR needs a split: One PR is the bugfix that fails the request, as described by the title, and a second one is the lease and announce.

Also, there are three instances of lazy imports of 'time' in vllm/distributed/ec_transfer/ec_connector/cpu/scheduler/init.py

[omerpaz95]

Overall LGTM. Some comments:

1. Docstrings need to be more informative: The docstrings got shorter as the logic got more complex. This makes the already-difficult code paths here even harder to follow. Specifically, _nixl_consumer_admit, _fail_or_fallback, take_unavailable_requests.
2. Simplify to avoid code duplication and to make the consumer admit a bit more readable. Specifically, you call:

if not self._fail_or_fallback(request, feature, "<why>"):
    return False
continue

five times in the code.
Consider something like:

for feature in request.mm_features:
    try:
        if not self._admit_item(request, feature, num_computed_tokens, now):
            pending = True
    except _RemoteUnavailable as e:
        if not self._fail_or_fallback(request, feature, str(e)):
            return False
return not pending

[gty111]

/ci run

[github-actions]

✅ Triggered Buildkite CI #87796 for commit a97bc1a2f5bf.