[Bugfix][NIXL] Stop clearing finished_sending for replicated-PCP producer ranks by sohom-cs · Pull Request #59325 · vllm-project/vllm

sohom-cs · 2026-09-30T00:18:55Z

Purpose

When a prefill instance runs with prefill context parallelism (PCP) and the KV cache is replicated rather than sharded across PCP ranks, only rank 0 actually sends KV to the decode instance. The scheduler frees a finished request's blocks on the prefill side only after every worker has reported the send as done. #53903 made that work by having ranks > 0 report a synthetic "done sending" for each request they were asked to send. A later refactor brought back the line that throws those reports away, so today the prefill side waits for reports that never arrive and never frees the blocks of any request it served. This PR removes that line again.

Concretely:

[NIXL][PCP] Report replicated-PCP ranks > 0 as done sending instead of hiding them #53903 (09-10) added _replicated_pcp_done_sending (nixl/base_worker.py:838), merged it into done_sending in the worker's get_transfer_results (base_worker.py:2907), and deleted the code in NixlBaseConnector.get_finished that cleared finished_sending for replicated PCP ranks > 0.
[3/N] HiSparse: host-resident sparse-MLA decode hot-buffering #53781 (09-12) added NixlBaseConnector.get_transfer_results (nixl/connector.py:243), which the model runner now calls instead of get_finished (kv_connector_model_runner_mixin.py:94). It carries over the old clear (connector.py:253) for kv_producer with pcp_rank > 0 and not pcp_dcp_sharded.
KVOutputAggregator (kv_connector/utils.py:64) expects world_size reports per request, since NIXL's get_finished_count() returns None. With ranks > 0 cleared it gets one of pcp_size and never adds the request to finished_sending, so the scheduler never frees its blocks on P. Rank 0's abort-timeout expiry doesn't help, because it is still only one report.

Fix: return the worker's results unchanged, as get_finished already does. The replicated ranks' synthetic completions are produced in exactly one place (the worker), so there is nothing left for the connector to filter. An alternative is to keep the clear and have get_finished_count() return world_size // pcp_size. That also changes how receive completions are counted, so I went with the smaller change, but I'm happy to switch if you prefer it.

Not a duplicate: no open PR touches NixlBaseConnector.get_transfer_results. #55398 edits nixl/connector.py elsewhere, and #55471 changes tests of the worker's get_transfer_results, not the connector override; neither touches this clear. This restores the behaviour #53903 introduced; the clear appears to have come back when #53781 was rebased.

Test Plan

python -m pytest tests/v1/kv_connector/unit/test_nixl_connector.py -q -k "pcp_producer_exposes or replicated_pcp_producer_send"
python -m pytest tests/v1/kv_connector/unit/test_nixl_connector.py tests/v1/kv_connector/unit/test_nixl_push_connector.py tests/v1/kv_connector/unit/test_nixl_connector_hma.py tests/v1/kv_connector/unit/test_nixl_heartbeat.py -q
pre-commit run --from-ref origin/main --to-ref HEAD
pre-commit run mypy-3.12 --hook-stage manual --from-ref origin/main --to-ref HEAD

test_pcp_producer_exposes_dcp_shards_or_canonical_replica: the final assertion checked get_transfer_results against get_finished's old behaviour (empty for replicated ranks > 0). It now expects the completion to pass through, matching get_finished.
New test_replicated_pcp_producer_send_aggregation_completes: builds two worker connectors for a PCP=2 replicated producer. D notifies only rank 0; rank 1 has the synthetic completion. It feeds both ranks' get_transfer_results into a KVOutputAggregator(world_size=2) and checks that the request comes out in finished_sending.

Test Result

With the fix (CPU, macOS arm64, on main 4e0a414c4):

-k "pcp_producer_exposes or replicated_pcp_producer_send" ... 9 passed
test_nixl_connector.py + test_nixl_push_connector.py + test_nixl_connector_hma.py + test_nixl_heartbeat.py ... 354 passed, 3 failed

The 3 failures are environment-only and fail the same way on main: test_abort_timeout_on_prefiller starts a real engine, and test_fewer_blocks_with_hma[google/gemma-3-1b-it-512] needs a gated model.

On main (fix reverted, tests kept):

test_pcp_producer_exposes_dcp_shards_or_canonical_replica[1-2-1-False]
E   AssertionError: assert set() == {'sent'}
test_replicated_pcp_producer_send_aggregation_completes
E   AssertionError: assert None == {'req'}      (aggregated finished_sending)
2 failed, 7 passed

pre-commit (--from-ref origin/main --to-ref HEAD) and mypy-3.12 (manual stage): clean.

This changes when the prefill side frees blocks after a transfer; model outputs are unaffected, so no evals are needed.

Related: one of a few independent fixes from an audit of the KV transfer paths (CPU offload, NIXL, P2P): #59096, #59099, #59102, #59329. None depends on another; they can be reviewed and merged in any order.

AI assistance

I used an AI coding assistant (Claude) to audit this code path, write the fix and write the tests. I reviewed every changed line and ran the tests above myself. The commit carries a Co-authored-by trailer, as AGENTS.md asks.

…ucer ranks vllm-project#53903 made replicated-PCP producer ranks > 0 report every request they were asked to send as done, so that the world_size aggregation in KVOutputAggregator completes, and removed the code that hid those completions from get_finished(). vllm-project#53781 then added NixlBaseConnector.get_transfer_results(), which the model runner now calls instead of get_finished(), with the old clear() for those ranks carried over. The synthetic completions were dropped again, the aggregator waited for pcp_size reports and received one, and the prefill side never freed a finished request's blocks. Return the worker's results unchanged, as get_finished() does. Update the handshake test that asserted the empty set, and add an aggregator test for a PCP=2 replicated producer. Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Sohom Chakraborty <16609933+sohom-cs@users.noreply.github.com>

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github-actions · 2026-09-30T01:03:18Z

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YannikHinteregger

Thanks for the contribution, the fix/restore seems to be correct.

Do you have a way to run the engine with the fix and PCP on so we can verify end to end?

YannikHinteregger · 2026-10-04T16:09:46Z

+        # completions too, or the world_size aggregation never finishes.
        results = connector.get_transfer_results(set())
-        assert results.finished_sending == ({"sent"} if expected_tracked else set())
+        assert results.finished_sending == {"sent"}


With get_transfer_results mocked this only checks a passthrough, so it passes regardless of the fix. The new test below covers the real path, so I think this block can go.

YannikHinteregger · 2026-10-04T16:09:46Z

+        req_id = "req"
+        connectors = []
+        for pcp_rank in (0, 1):
+            with (


This repeats the connector setup from the parametrized test above. Could it be pulled into a small helper both tests share?

YannikHinteregger · 2026-10-04T16:09:46Z

+
+        aggregator = KVOutputAggregator.from_connector(connectors[0], world_size=2)
+        outputs = [
+            ModelRunnerOutput(


create_model_runner_output from tests/v1/kv_connector/unit/utils.py could replace the hand-built ModelRunnerOutput here.

YannikHinteregger · 2026-10-04T16:09:46Z

        worker.get_transfer_results = MagicMock(
            return_value=KVConnectorTransferResults(finished_sending={"sent"})
        )
+        # The runner reads completions through get_transfer_results, so it must


This comment describes the old bug. I think it fits better in the PR description.

snadampal · 2026-10-05T13:48:10Z

+        "vllm.distributed.kv_transfer.kv_connector.v1.nixl.base_worker.NixlWrapper",
+        FakeNixlWrapper,
+    )
+    def test_replicated_pcp_producer_send_aggregation_completes(


please add a similar test for test_nixl_push_connector.py as well.

snadampal · 2026-10-05T13:57:22Z

-        ):
-            results.finished_sending.clear()
-        return results
+        return self.connector_worker.get_transfer_results()


it's good to add a note in both producer and consumer logic about the contract for replicated-KV PCP scenario.

ibrahimnd2000 · 2026-10-07T09:14:20Z

End-to-end verification of this change on 8× B300 (SM103) with vLLM 0.31.0: GLM-5.3 FP8 (MLA + DSA indexer), prefill TP1 + PCP8 (DP1) + EP with NixlConnector as kv_producer over LIBFABRIC/EFA; decode on a separate node (TP1 DP8 + EP, kv_both).

Without the change: under a burst of ~188k-token prompts, the prefill engine's KV usage reached 91–98% within a minute and never came down. The scheduler sat at 0 running / 40–60 waiting with no progress. Even at idle, KV usage crept up after each P/D request.

With exactly this change applied on top of 0.31.0: 48 unique ~188k-token prompts at concurrency 24 through P/D completed 48/48. Prefill KV usage returned to 0.0% on all four prefill instances right after. Needle-in-a-haystack at ~40k and ~200k tokens was answered correctly through P/D, matching the non-PCP path.

I also checked the aggregation with the real KVOutputAggregator(expected_finished_count=8). Ranks 1–7 report first and the request stays pending with 1 report outstanding. Rank 0 reports and it's added to finished_sending.

sohom-cs requested review from ApostaC, NickLucche, ivanium, orozery and xuechendi as code owners September 30, 2026 00:18

claude Bot reviewed Sep 30, 2026

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mergify Bot added bug Something isn't working kv-connector labels Sep 30, 2026

sohom-cs mentioned this pull request Oct 3, 2026

[Bugfix][Core] Unblock scheduling after a failed async KV load #59901

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YannikHinteregger reviewed Oct 4, 2026

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[Bugfix][NIXL] Stop clearing finished_sending for replicated-PCP producer ranks - #59325

[Bugfix][NIXL] Stop clearing finished_sending for replicated-PCP producer ranks#59325
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