Changed scheduler to use deques instead of lists (#2290)

Co-authored-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/core/policy.py

-from typing import List
+from collections import deque
+from typing import Deque
 
 from vllm.sequence import SequenceGroup
 
@@ -15,13 +16,14 @@ class Policy:
     def sort_by_priority(
         self,
         now: float,
-        seq_groups: List[SequenceGroup],
-    ) -> List[SequenceGroup]:
-        return sorted(
+        seq_groups: Deque[SequenceGroup],
+    ) -> Deque[SequenceGroup]:
+        return deque(
+            sorted(
                 seq_groups,
                 key=lambda seq_group: self.get_priority(now, seq_group),
                 reverse=True,
-        )
+            ))
 
 
 class FCFS(Policy):

vllm/core/scheduler.py

+from collections import deque
 import enum
 import time
-from typing import Dict, Iterable, List, Optional, Tuple, Union
+from typing import Deque, Dict, Iterable, List, Optional, Tuple, Union
 
 from vllm.config import CacheConfig, SchedulerConfig
 from vllm.core.block_manager import AllocStatus, BlockSpaceManager
@@ -29,7 +30,7 @@ class SchedulerOutputs:
 
     def __init__(
         self,
-        scheduled_seq_groups: List[SequenceGroup],
+        scheduled_seq_groups: Iterable[SequenceGroup],
         prompt_run: bool,
         num_batched_tokens: int,
         blocks_to_swap_in: Dict[int, int],
@@ -75,13 +76,12 @@ class Scheduler:
             num_cpu_blocks=self.cache_config.num_cpu_blocks,
             sliding_window=self.cache_config.sliding_window)
 
-        # TODO(zhuohan): Use deque instead of list for better performance.
         # Sequence groups in the WAITING state.
-        self.waiting: List[SequenceGroup] = []
+        self.waiting: Deque[SequenceGroup] = deque()
         # Sequence groups in the RUNNING state.
-        self.running: List[SequenceGroup] = []
+        self.running: Deque[SequenceGroup] = deque()
         # Sequence groups in the SWAPPED state.
-        self.swapped: List[SequenceGroup] = []
+        self.swapped: Deque[SequenceGroup] = deque()
 
     def add_seq_group(self, seq_group: SequenceGroup) -> None:
         # Add sequence groups to the waiting queue.
@@ -152,7 +152,7 @@ class Scheduler:
                     for seq in waiting_seqs:
                         seq.status = SequenceStatus.FINISHED_IGNORED
                     ignored_seq_groups.append(seq_group)
-                    self.waiting.pop(0)
+                    self.waiting.popleft()
                     continue
 
                 # If the sequence group cannot be allocated, stop.
@@ -166,7 +166,7 @@ class Scheduler:
                     for seq in waiting_seqs:
                         seq.status = SequenceStatus.FINISHED_IGNORED
                     ignored_seq_groups.append(seq_group)
-                    self.waiting.pop(0)
+                    self.waiting.popleft()
                     continue
 
                 # If the number of batched tokens exceeds the limit, stop.
@@ -188,7 +188,7 @@ class Scheduler:
                     break
                 seq_lens = new_seq_lens
 
-                seq_group = self.waiting.pop(0)
+                seq_group = self.waiting.popleft()
                 self._allocate(seq_group)
                 self.running.append(seq_group)
                 num_curr_seqs += num_new_seqs
@@ -214,14 +214,14 @@ class Scheduler:
         self.running = self.policy.sort_by_priority(now, self.running)
 
         # Reserve new token slots for the running sequence groups.
-        running: List[SequenceGroup] = []
+        running: Deque[SequenceGroup] = deque()
         preempted: List[SequenceGroup] = []
         while self.running:
-            seq_group = self.running.pop(0)
+            seq_group = self.running.popleft()
             while not self.block_manager.can_append_slot(seq_group):
                 if self.running:
                     # Preempt the lowest-priority sequence groups.
-                    victim_seq_group = self.running.pop(-1)
+                    victim_seq_group = self.running.pop()
                     self._preempt(victim_seq_group, blocks_to_swap_out)
                     preempted.append(victim_seq_group)
                 else:
@@ -255,7 +255,7 @@ class Scheduler:
                         self.scheduler_config.max_num_seqs):
                     break
 
-                seq_group = self.swapped.pop(0)
+                seq_group = self.swapped.popleft()
                 self._swap_in(seq_group, blocks_to_swap_in)
                 self._append_slot(seq_group, blocks_to_copy)
                 num_curr_seqs += num_new_seqs
@@ -376,7 +376,7 @@ class Scheduler:
             self.block_manager.free(seq)
         # NOTE: For FCFS, we insert the preempted sequence group to the front
         # of the waiting queue.
-        self.waiting.insert(0, seq_group)
+        self.waiting.appendleft(seq_group)
 
     def _preempt_by_swap(
         self,

vllm/engine/llm_engine.py

@@ -601,8 +601,10 @@ class LLMEngine:
 
         # Create the outputs.
         request_outputs: List[RequestOutput] = []
-        for seq_group in (scheduled_seq_groups +
-                          scheduler_outputs.ignored_seq_groups):
+        for seq_group in scheduled_seq_groups:
+            request_output = RequestOutput.from_seq_group(seq_group)
+            request_outputs.append(request_output)
+        for seq_group in scheduler_outputs.ignored_seq_groups:
             request_output = RequestOutput.from_seq_group(seq_group)
             request_outputs.append(request_output)