[V1][Scheduler] Use dict for running queue

This is just a random idea, still need to benchmark

Potential advantages for large batch sizes:
- Don't need to copy entire list every iteration
- O(1) removal of aborted requests

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

vllm/v1/core/scheduler.py

@@ -66,7 +66,7 @@ class Scheduler:
         self.requests: dict[str, Request] = {}
         # Priority queues for requests.
         self.waiting: deque[Request] = deque()
-        self.running: list[Request] = []
+        self.running: dict[Request, None] = {}
         # The requests that have been scheduled and are being executed
         # by the executor.
         self.scheduled_req_ids: set[str] = set()
@@ -140,12 +140,12 @@ class Scheduler:
         scheduled_timestamp = time.monotonic()
 
         # First, schedule the RUNNING requests.
-        req_index = 0
-        while req_index < len(self.running) and token_budget > 0:
-            request = self.running[req_index]
+        running_count = len(self.running)
+        for req_index, request in enumerate(self.running):
+            if token_budget <= 0 or req_index == running_count:
+                break
             if request.request_id in self.scheduled_req_ids:
                 # This request has already been scheduled.
-                req_index += 1
                 continue
 
             num_new_tokens = (request.num_tokens_with_spec -
@@ -165,7 +165,6 @@ class Scheduler:
                 # NOTE(woosuk): Here, by doing `continue` instead of `break`,
                 # we do not strictly follow the FCFS scheduling policy and
                 # allow the lower-priority requests to be scheduled.
-                req_index += 1
                 continue
 
             while True:
@@ -174,7 +173,8 @@ class Scheduler:
                 if new_blocks is None:
                     # The request cannot be scheduled.
                     # Preempt the lowest-priority request.
-                    preempted_req = self.running.pop()
+                    preempted_req = next(reversed(self.running))
+                    running_count += 1
                     self.kv_cache_manager.free(preempted_req)
                     preempted_req.status = RequestStatus.PREEMPTED
                     preempted_req.num_computed_tokens = 0
@@ -182,7 +182,7 @@ class Scheduler:
 
                     self.waiting.appendleft(preempted_req)
                     preempted_reqs.append(preempted_req)
-                    if preempted_req == request:
+                    if req_index == running_count:
                         # No more request to preempt.
                         can_schedule = False
                         break
@@ -208,7 +208,6 @@ class Scheduler:
             ]
             num_scheduled_tokens[request.request_id] = num_new_tokens
             token_budget -= num_new_tokens
-            req_index += 1
 
             # Speculative decode related.
             if request.spec_token_ids:
@@ -230,6 +229,10 @@ class Scheduler:
                     self.encoder_cache_manager.allocate(request, i)
                 encoder_budget = new_encoder_budget
 
+        # Remove preempted requests from the running queue.
+        while len(self.running) > running_count:
+            self.running.popitem()
+
         # Record the LoRAs in scheduled_running_reqs
         requested_loras: set[int] = set()
         if self.lora_config:
@@ -255,9 +258,8 @@ class Scheduler:
                     if structured_output_req and structured_output_req.grammar:
                         request.status = RequestStatus.WAITING
                     else:
-                        waiting_structured_output_req = self.waiting.popleft()
-                        waiting_for_fsm.appendleft(
-                            waiting_structured_output_req)
+                        self.waiting.popleft()
+                        waiting_for_fsm.appendleft(request)
                         continue
 
                 # Check that adding the request still respects the max_loras
@@ -316,9 +318,8 @@ class Scheduler:
                 self.waiting.popleft()
                 if request.use_structured_output:
                     structured_output_request_ids[
-                        request.request_id] = req_index
-                req_index += 1
-                self.running.append(request)
+                        request.request_id] = running_count
+                self.running[request] = None
                 self.scheduled_req_ids.add(request.request_id)
                 self.request_scheduled(request, scheduled_timestamp)
                 if request.status == RequestStatus.WAITING:
@@ -367,7 +368,7 @@ class Scheduler:
         # This can be potentially used for cascade attention.
         num_common_prefix_blocks = 0
         if self.running:
-            any_request = self.running[0]
+            any_request = next(iter(self.running))
             num_common_prefix_blocks = (
                 self.kv_cache_manager.get_num_common_prefix_blocks(
                     any_request, len(self.running)))
@@ -531,7 +532,7 @@ class Scheduler:
         prompt_logprobs_dict = model_runner_output.prompt_logprobs_dict
         num_scheduled_tokens = scheduler_output.num_scheduled_tokens
 
-        new_running: list[Request] = []
+        stopped_requests: list[Request] = []
         outputs: list[EngineCoreOutput] = []
 
         # NOTE(woosuk): As len(self.running) can be up to 1K or more, the below
@@ -542,7 +543,6 @@ class Scheduler:
             num_tokens_scheduled = num_scheduled_tokens.get(req_id, 0)
             if num_tokens_scheduled == 0:
                 # The request was not scheduled in this step.
-                new_running.append(request)
                 continue
 
             req_index = model_runner_output.req_id_to_index[req_id]
@@ -601,6 +601,7 @@ class Scheduler:
                     stopped = self._check_stop(request)
                     if stopped:
                         self._free_request(request)
+                        stopped_requests.append(request)
                         break
 
                 # Extract sample logprobs if needed.
@@ -635,10 +636,10 @@ class Scheduler:
                         events=request.take_events()))
 
             self.scheduled_req_ids.remove(request.request_id)
-            if not stopped:
-                new_running.append(request)
 
-        self.running = new_running
+        for stopped_request in stopped_requests:
+            del self.running[stopped_request]
+
         return EngineCoreOutputs(
             outputs=outputs,
             scheduler_stats=self.make_stats(),
@@ -691,7 +692,7 @@ class Scheduler:
                 continue
 
             if request.status == RequestStatus.RUNNING:
-                self.running.remove(request)
+                del self.running[request]
                 self.scheduled_req_ids.discard(request.request_id)
             else:
                 self.waiting.remove(request)