[Docs] Improve V1 KVConnector interface documentation (#19172)

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

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

@@ -8,9 +8,15 @@ The class provides the following primitives:
     Scheduler-side: runs in the scheduler, binds metadata, which
     is used by the worker-side to load/save KV cache.
         get_num_new_matched_tokens() - get number of new tokens 
-            that exist in the remote KV cache
+            that exist in the remote KV cache. Might be called multiple
+            times for a given request and should be side-effect free.
         update_state_after_alloc() - update KVConnector state after
             temporary buffer alloc by the CacheManager.
+        request_finished() - called when a request is finished, with
+            the computed kv cache blocks for the request.
+            Returns whether KV cache should be freed now or will be
+            freed asynchronously and optionally returns KV transfer
+            params.
 
     Worker-side: runs in each worker, loads/saves KV cache to/from
     the Connector based on the metadata.
@@ -19,6 +25,9 @@ The class provides the following primitives:
 
         save_kv_layer() - starts saving KV for layer i (maybe async)
         wait_for_save() - blocks until all saves are done
+
+        get_finished() - called with ids of finished requests, returns
+            ids of requests that have completed async sending/recving.
 """
 
 import enum
@@ -184,7 +193,8 @@ class KVConnectorBase_V1(ABC):
         finished generating tokens.
 
         Returns:
-            ids of requests that have finished asynchronous transfer,
+            ids of requests that have finished asynchronous transfer
+            (requests that previously returned True from request_finished()),
             tuple of (sending/saving ids, recving/loading ids).
             The finished saves/sends req ids must belong to a set provided in a
             call to this method (this call or a prior one).
@@ -215,7 +225,8 @@ class KVConnectorBase_V1(ABC):
                 - The number of tokens that can be loaded from the 
                   external KV cache beyond what is already computed.
                 - `True` if external KV cache tokens will be loaded
-                  asynchronously (between scheduler steps).
+                  asynchronously (between scheduler steps). Must be
+                  'False' if the first element is 0.
         """
         pass
 
@@ -225,6 +236,18 @@ class KVConnectorBase_V1(ABC):
                                  num_external_tokens: int):
         """
         Update KVConnector state after block allocation.
+
+        If get_num_new_matched_tokens previously returned True for a
+        request, this function may be called twice for that same request -
+        first when blocks are allocated for the connector tokens to be
+        asynchronously loaded into, and second when any additional blocks
+        are allocated, after the load/transfer is complete.
+
+        Args:
+            request (Request): the request object.
+            blocks (KVCacheBlocks): the blocks allocated for the request.
+            num_external_tokens (int): the number of tokens that will be
+                loaded from the external KV cache.
         """
         pass
 

vllm/v1/core/sched/scheduler.py

@@ -101,7 +101,7 @@ class Scheduler(SchedulerInterface):
         # This is flushed at the end of each scheduling step.
         self.finished_req_ids: set[str] = set()
 
-        # P/D: requests in process of recving KV transfers
+        # KV Connector: requests in process of async KV loading or recving
         self.finished_recving_kv_req_ids: set[str] = set()
 
         # OPTIMIZATION: Cache the CachedRequestData objects to avoid creating
@@ -822,7 +822,7 @@ class Scheduler(SchedulerInterface):
             if not stopped:
                 new_running.append(request)
 
-        # P/D: update state for finished KV Transfers.
+        # KV Connector: update state for finished KV Transfers.
         self._update_from_kv_xfer_finished(model_runner_output)
 
         # Return the cached request data to the queue so they can be reused.
@@ -969,7 +969,7 @@ class Scheduler(SchedulerInterface):
             self.kv_event_publisher.shutdown()
 
     ########################################################################
-    # P/D Related Methods
+    # KV Connector Related Methods
     ########################################################################
 
     def get_kv_connector(self) -> Optional[KVConnectorBase_V1]:
@@ -992,7 +992,7 @@ class Scheduler(SchedulerInterface):
 
     def _update_waiting_for_remote_kv(self, request: Request) -> bool:
         """
-        P/D: check if the request_id is finished_recving.
+        KV Connector: check if the request_id is finished_recving.
 
         The finished_recving_kv_req_ids list is populated
         on the previous steps()'s update_from_output based
@@ -1029,7 +1029,7 @@ class Scheduler(SchedulerInterface):
     def _update_from_kv_xfer_finished(self,
                                       model_runner_output: ModelRunnerOutput):
         """
-        P/D: update the scheduler state based on the output.
+        KV Connector: update the scheduler state based on the output.
 
         The Worker side connectors add finished_recving and
         finished_sending reqs to the output.
@@ -1037,7 +1037,7 @@ class Scheduler(SchedulerInterface):
         # if finished_recving: add to state so we can
             scheduler the request during the next step.
         """
-        # P/D: update recv and send status from last step.
+        # KV Connector:: update recv and send status from last step.
         for req_id in (model_runner_output.finished_recving or ()):
             logger.debug("Finished recving KV transfer for request %s", req_id)
             self.finished_recving_kv_req_ids.add(req_id)