[PerfFix] Avoid separate thread for MP executor shm spin (#28012)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/distributed/kv_transfer/kv_connector/utils.py

@@ -221,39 +221,24 @@ class KVOutputAggregator:
 
     def async_aggregate(
         self,
-        output_futures: Sequence[Future[ModelRunnerOutput | None]],
+        output_future: Future[Sequence[ModelRunnerOutput | None]],
         output_rank: int = 0,
     ) -> Future[ModelRunnerOutput | None]:
-        """Takes a list of futures and returns a single future which resolves
-        to the respective list of outputs."""
+        """Takes a future that resolves to a list of outputs and returns a future
+        which resolves to a single aggregated output."""
         result_future: Future[ModelRunnerOutput | None] = Future()
 
-        outputs: list[ModelRunnerOutput | None] = [None] * len(output_futures)
-        remaining = len(output_futures)
-
-        def make_callback(idx):
-            def callback(fut):
-                if result_future.done():
-                    return
-
-                try:
-                    outputs[idx] = fut.result()
-                except CancelledError:
-                    result_future.cancel()
-                except Exception as e:
-                    result_future.set_exception(e)
-
-                # this check assumes io_thread_pool uses a single thread
-                nonlocal remaining
-                remaining -= 1
-                if not remaining:
-                    result_future.set_result(self.aggregate(outputs, output_rank))
-
-            return callback
-
-        for i, output_future in enumerate(output_futures):
-            output_future.add_done_callback(make_callback(i))
+        def callback(fut):
+            if result_future.done():
+                return
+            try:
+                result_future.set_result(self.aggregate(fut.result(), output_rank))
+            except CancelledError:
+                result_future.cancel()
+            except Exception as e:
+                result_future.set_exception(e)
 
+        output_future.add_done_callback(callback)
         return result_future