[P/D] Dynamic kv_output_aggregator collect size (#26734)

Signed-off-by: NickLucche <nlucches@redhat.com>

tests/v1/kv_connector/unit/test_nixl_connector.py

@@ -703,7 +703,7 @@ def test_kv_connector_stats_aggregation():
 
     # Create KVOutputAggregator for 3 workers (simulating TP=3), same thing
     # done in MultiprocExecutor.execute_model
-    aggregator = KVOutputAggregator(world_size=3)
+    aggregator = KVOutputAggregator(expected_finished_count=3)
 
     # Create stats for multiple workers with different transfer patterns
     worker1_stats = NixlKVConnectorStats()
@@ -768,7 +768,7 @@ def test_multi_kv_connector_stats_aggregation():
     KVOutputAggregator (used by MultiprocExecutor).
     """
 
-    aggregator = KVOutputAggregator(world_size=3)
+    aggregator = KVOutputAggregator(expected_finished_count=3)
 
     from dataclasses import dataclass
 

tests/v1/kv_connector/unit/test_output_aggregator.py

@@ -16,11 +16,13 @@ class DummyModelRunnerOutput(ModelRunnerOutput):
         finished_sending: set[str] | None = None,
         finished_recving: set[str] | None = None,
         invalid_block_ids: set[int] | None = None,
+        expected_finished_count: int = 0,
     ):
         self.kv_connector_output = KVConnectorOutput(
             finished_sending=finished_sending,
             finished_recving=finished_recving,
             invalid_block_ids=invalid_block_ids or set(),
+            expected_finished_count=expected_finished_count,
         )
 
     def __repr__(self):
@@ -33,7 +35,7 @@ class DummyModelRunnerOutput(ModelRunnerOutput):
 
 
 def test_aggregate_workers_output():
-    aggregator = KVOutputAggregator(world_size=2)
+    aggregator = KVOutputAggregator(expected_finished_count=2)
 
     output1 = DummyModelRunnerOutput()
     output2 = DummyModelRunnerOutput()
@@ -85,7 +87,7 @@ def test_aggregate_workers_output():
 
 
 def test_async_aggregate_workers_output():
-    aggregator = KVOutputAggregator(world_size=2)
+    aggregator = KVOutputAggregator(expected_finished_count=2)
 
     future1: Future[DummyModelRunnerOutput] = Future()
     future2: Future[DummyModelRunnerOutput] = Future()
@@ -158,3 +160,40 @@ def test_async_aggregate_workers_output():
     assert aggregated.finished_sending is None
     assert aggregated.finished_recving == {"req2"}
     assert aggregated.invalid_block_ids == {3, 4, 5}
+
+
+def test_aggregate_workers_output_with_expected_finished_count():
+    # We create the aggregator expecting to collect from 4 workers
+    aggregator = KVOutputAggregator(expected_finished_count=4)
+    assert aggregator._expected_finished_count == 4
+    # Some request with default expected finished requests
+    output1 = DummyModelRunnerOutput(finished_sending={"req1"})
+    aggregated = aggregator.aggregate([output1])
+    # still expecting to collect from 4 workers
+    assert aggregator._send_remaining_count["req1"] == 3
+    assert not aggregated.kv_connector_output.finished_sending
+    assert not aggregated.kv_connector_output.finished_recving
+
+    # Workers discover and find that in this setup they only need to
+    # collect from 2
+    output1 = DummyModelRunnerOutput(
+        finished_sending={"req1"}, expected_finished_count=2
+    )
+    output2 = DummyModelRunnerOutput(
+        finished_recving={"req2"}, expected_finished_count=2
+    )
+    output3 = DummyModelRunnerOutput(finished_recving={"req2"})
+    # Req2 only needs 2 acks
+    aggregated = aggregator.aggregate([output1, output2, output3])
+    assert aggregated.kv_connector_output.expected_finished_count == 2
+
+    assert not aggregated.kv_connector_output.finished_sending
+
+    # Req2 is finished
+    assert "req2" not in aggregator._recv_remaining_count
+    assert aggregated.kv_connector_output.finished_recving == {"req2"}
+
+    # Req1 is still waiting for 2 more acks (expected_finished_count has no effect)
+    # NOTE: This is to showcase dynamic update. Workers are responsible for
+    # ensuring "req1" termination in this case
+    assert aggregator._send_remaining_count["req1"] == 2

vllm/distributed/kv_transfer/kv_connector/utils.py

@@ -4,10 +4,9 @@
 KV cache helper for store.
 """
 
-from collections import defaultdict
 from collections.abc import Sequence
 from concurrent.futures import CancelledError, Future
-from typing import Literal, cast
+from typing import TYPE_CHECKING, Literal, cast
 
 import torch
 
@@ -18,6 +17,9 @@ from vllm.distributed.kv_transfer.kv_connector.factory import KVConnectorFactory
 from vllm.logger import init_logger
 from vllm.v1.outputs import KVConnectorOutput, ModelRunnerOutput
 
+if TYPE_CHECKING:
+    from vllm.distributed.kv_transfer.kv_connector.base import KVConnectorBase
+
 logger = init_logger(__name__)
 
 
@@ -124,11 +126,16 @@ class KVOutputAggregator:
     """Utility class to aggregate the output of all workers into a single
     output corresponding to Rank 0 for scheduler."""
 
-    def __init__(self, world_size: int):
+    def __init__(self, expected_finished_count: int):
         # Complete transfer tracker. Used to track finished requests
         # [req_id -> n_remaining_workers]
-        self._recv_remaining_count = defaultdict[str, int](lambda: world_size)
-        self._send_remaining_count = defaultdict[str, int](lambda: world_size)
+        self._recv_remaining_count = dict[str, int]()
+        self._send_remaining_count = dict[str, int]()
+        self._expected_finished_count = expected_finished_count
+
+    @classmethod
+    def from_connector(cls, connector: "KVConnectorBase", world_size: int):
+        return cls(connector.get_finished_count() or world_size)
 
     def aggregate(
         self, outputs: list[ModelRunnerOutput], output_rank: int = 0
@@ -141,7 +148,10 @@ class KVOutputAggregator:
             finished_set: set[str],
         ) -> None:
             for req_id in req_ids or ():
-                remaining_count_dict[req_id] -= 1
+                remaining_count = remaining_count_dict.get(
+                    req_id, self._expected_finished_count
+                )
+                remaining_count_dict[req_id] = remaining_count - 1
                 if remaining_count_dict[req_id] == 0:
                     finished_set.add(req_id)
                     del remaining_count_dict[req_id]
@@ -154,6 +164,19 @@ class KVOutputAggregator:
             kv_output = model_runner_output.kv_connector_output
             if not kv_output:
                 continue
+            # Allow the worker to dynamically update the expected number of
+            # finished sending/recving for new requests.
+            if (
+                kv_output.expected_finished_count > 0
+                and kv_output.expected_finished_count != self._expected_finished_count
+            ):
+                logger.debug(
+                    "Expected finished requests updated from %d to %d",
+                    self._expected_finished_count,
+                    kv_output.expected_finished_count,
+                )
+                self._expected_finished_count = kv_output.expected_finished_count
+
             update_finished_set(
                 kv_output.finished_sending, self._send_remaining_count, finished_sending
             )
@@ -186,6 +209,7 @@ class KVOutputAggregator:
             finished_recving=finished_recving or None,
             kv_connector_stats=aggregated_kv_connector_stats or None,
             invalid_block_ids=invalid_block_ids,
+            expected_finished_count=self._expected_finished_count,
         )
 
         return output

vllm/distributed/kv_transfer/kv_connector/v1/base.py

@@ -413,7 +413,8 @@ class KVConnectorBase_V1(ABC):
     def get_finished_count(self) -> int | None:
         """
         Get the count of requests expected to complete send/receive operations
-        via this connector.
+        via this connector. This method is used to initialize the
+        KVOutputAggregator, overwriting the default world_size.
 
         Returns:
             int: expected sending or receiving completion count.

vllm/v1/engine/core.py

@@ -160,9 +160,7 @@ class EngineCore:
         )
         self.use_spec_decode = vllm_config.speculative_config is not None
         if self.scheduler.connector is not None:  # type: ignore
-            self.model_executor.init_kv_output_aggregator(
-                self.scheduler.connector.get_finished_count()  # type: ignore
-            )
+            self.model_executor.init_kv_output_aggregator(self.scheduler.connector)  # type: ignore
 
         self.mm_registry = mm_registry = MULTIMODAL_REGISTRY
         self.mm_receiver_cache = engine_receiver_cache_from_config(

vllm/v1/executor/abstract.py

@@ -5,7 +5,7 @@ from abc import ABC, abstractmethod
 from collections.abc import Callable
 from concurrent.futures import Future
 from functools import cached_property
-from typing import Literal, TypeVar, overload
+from typing import TYPE_CHECKING, Literal, TypeVar, overload
 
 from vllm.config import VllmConfig
 from vllm.distributed.kv_transfer.kv_connector.utils import KVOutputAggregator
@@ -19,6 +19,9 @@ from vllm.v1.kv_cache_interface import KVCacheConfig, KVCacheSpec
 from vllm.v1.outputs import DraftTokenIds, ModelRunnerOutput
 from vllm.v1.worker.worker_base import WorkerBase
 
+if TYPE_CHECKING:
+    from vllm.distributed.kv_transfer.kv_connector.base import KVConnectorBase
+
 logger = init_logger(__name__)
 
 _R = TypeVar("_R")
@@ -233,10 +236,10 @@ class Executor(ABC):
         """Shutdown the executor."""
         self.collective_rpc("shutdown")
 
-    def init_kv_output_aggregator(self, finished_count: int | None) -> None:
+    def init_kv_output_aggregator(self, connector: "KVConnectorBase") -> None:
         """Init KVOutputAggregator"""
-        self.kv_output_aggregator = KVOutputAggregator(
-            finished_count or self.parallel_config.world_size
+        self.kv_output_aggregator = KVOutputAggregator.from_connector(
+            connector, self.parallel_config.world_size
         )
 
     @cached_property  # Avoid unnecessary RPC calls

vllm/v1/outputs.py

@@ -86,8 +86,14 @@ class KVConnectorOutput:
     finished_recving: set[str] | None = None
     kv_connector_stats: KVConnectorStats | None = None
     # IDs of externally computed KV blocks that failed to load.
-    # Requests referencing these blocks should be rescheduled to recompute them.
+    # Requests referencing these blocks should be rescheduled to recompute them
     invalid_block_ids: set[int] = field(default_factory=set)
+    # Configuration describing how many finished sending/receiving
+    # notifications should be expected for each request. This allows
+    # handshake-based connectors like Nixl to update the KVOutputAggregator.
+    # It captures a static setup info and should almost always remain constant
+    # for a given connector after discovery. Default value entails no change.
+    expected_finished_count: int = 0
 
     def is_empty(self):
         return (