[Minor] Add some clarifying comments to recent changes (#27130)

Signed-off-by: Nick Hill <[email protected]>

Author: njhill

vllm/distributed/device_communicators/shm_broadcast.py

@@ -236,7 +236,9 @@ def __init__(
         n_reader,  # number of all readers
         n_local_reader,  # number of local readers through shared memory
         local_reader_ranks: list[int] | None = None,
-        max_chunk_bytes: int = 1024 * 1024 * 24,  # 24MiB
+        # Default of 24MiB chosen to be large enough to accommodate grammar
+        # bitmask tensors for large batches (1024 requests).
+        max_chunk_bytes: int = 1024 * 1024 * 24,
         max_chunks: int = 10,
         connect_ip: str | None = None,
     ):
@@ -538,6 +540,10 @@ def oob_callback(buf: PickleBuffer) -> bool:
                     buf[0] = 1  # overflow
                 self.local_socket.send_multipart(all_buffers, copy=False)
             else:
+                # Byte 0: 0
+                # Bytes 1-2: Count of buffers
+                # Then each buffer follows, preceded by 4 bytes containing its length:
+                # [4 byte int L][L bytes of buffer content] ...
                 with self.acquire_write(timeout) as buf:
                     buf[0] = 0  # not overflow
                     offset = 3

vllm/v1/core/sched/output.py

@@ -165,7 +165,9 @@ class SchedulerOutput:
     # freed from the encoder cache.
     free_encoder_mm_hashes: list[str]
 
-    # ids of structured outputs requests included in the bitmask, in order.
+    # ids of structured outputs requests included in the bitmask, in the
+    # same order as the corresponding stacked rows of the bitmask.
+    # There may be more than one row per request in the case of speculative decoding.
     structured_output_request_ids: list[str]
     # the bitmask for the whole batch
     grammar_bitmask: "npt.NDArray[np.int32] | None"