add comment about hack

Signed-off-by: rshaw@neuralmagic.com <robertgshaw2@gmail.com>

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

@@ -7,13 +7,13 @@ import time
 import uuid
 from collections import defaultdict
 from collections.abc import Iterator
+from concurrent.futures import ThreadPoolExecutor
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, Optional
 
 import msgspec
 import torch
 import zmq
-from concurrent.futures import ThreadPoolExecutor, as_completed
 
 from vllm import envs
 from vllm.attention.selector import backend_name_to_enum, get_attn_backend
@@ -333,7 +333,7 @@ class NixlConnectorWorker:
         # Agent.
         import os
         num_workers = 16
-        # setting num workers on the prefiller causes the notifs to not be recved???
+        # setting num_workers on the prefiller causes no notifs to be recved???
         # this is a hack to make sure we set num workers on the prefiller to 1.
         if os.getenv("VLLM_NIXL_SIDE_CHANNEL_PORT", "") == "5557":
             num_workers = None
@@ -988,6 +988,10 @@ class NixlConnectorWorker:
         CHUNK_SIZE = 1000
         handles = []
         futures = []
+        # NOTE: this is a hack to make make_prepped_xfer into threads so that
+        # different workers are allocated for each chuck. Without this change,
+        # nixl was allocating the same worker (0) for all the chunks and the
+        # overall launch time was >300 ms.
         with ThreadPoolExecutor() as executor:
             for i in range(0, len(local_block_descs_ids), CHUNK_SIZE):
                 future = executor.submit(
@@ -1004,14 +1008,10 @@ class NixlConnectorWorker:
             for future in futures:
                 handles.append(future.result())
 
-
         # Begin async xfer.
         start = time.perf_counter()
-        # IT WORKS WITH THIS:
         # for handle in handles:
         #     self.nixl_wrapper.transfer(handle)
-
-        # IT FAILS WITH THIS:
         self.nixl_wrapper.transfer_batched(handles)
         end = time.perf_counter()
         logger.info("======== LAUNCH TIME: %s ========", end - start)