Simplify the event loop and expose `--num-continuous-decode-steps` as an argument (#1652)

python/sglang/global_config.py

@@ -19,7 +19,6 @@ class GlobalConfig:
         self.new_token_ratio_decay = 0.001
 
         # Runtime constants: others
-        self.num_continue_decode_steps = 10
         self.retract_decode_steps = 20
         self.flashinfer_workspace_size = os.environ.get(
             "FLASHINFER_WORKSPACE_SIZE", 384 * 1024 * 1024

python/sglang/srt/managers/schedule_batch.py

@@ -831,6 +831,22 @@ class ScheduleBatch:
             sampling_info=self.sampling_info,
         )
 
+    def copy(self):
+        return ScheduleBatch(
+            reqs=self.reqs,
+            req_to_token_pool=self.req_to_token_pool,
+            token_to_kv_pool=self.token_to_kv_pool,
+            tree_cache=self.tree_cache,
+            forward_mode=self.forward_mode,
+            output_token_ids=self.output_token_ids,
+        )
+
+    def __str__(self):
+        return (
+            f"ScheduleBatch(forward_mode={self.forward_mode.name}, "
+            f"#req={(len(self.reqs))})"
+        )
+
 
 @dataclass
 class ModelWorkerBatch:

python/sglang/srt/managers/scheduler.py

@@ -20,6 +20,7 @@ import logging
 import os
 import time
 import warnings
+from types import SimpleNamespace
 from typing import List, Optional, Union
 
 import torch
@@ -106,7 +107,8 @@ class Scheduler:
             self.send_to_detokenizer = context.socket(zmq.PUSH)
             self.send_to_detokenizer.connect(f"ipc://{port_args.detokenizer_ipc_name}")
         else:
-            self.recv_from_tokenizer = self.send_to_detokenizer = None
+            self.recv_from_tokenizer = None
+            self.send_to_detokenizer = SimpleNamespace(send_pyobj=lambda x: None)
 
         # Init tokenizer
         self.model_config = ModelConfig(
@@ -190,7 +192,6 @@ class Scheduler:
         # Init running status
         self.waiting_queue: List[Req] = []
         self.running_batch: ScheduleBatch = None
-        self.out_pyobjs = []
         self.decode_forward_ct = 0
         self.stream_interval = server_args.stream_interval
         self.num_generated_tokens = 0
@@ -247,13 +248,30 @@ class Scheduler:
 
     @torch.inference_mode()
     def event_loop(self):
+        self.last_batch = None
+
         while True:
             recv_reqs = self.recv_requests()
             self.process_input_requests(recv_reqs)
 
-            self.run_step()
+            batch = self.get_next_batch_to_run()
+
+            if batch:
+                result = self.run_batch(batch)
+                self.process_batch_result(batch, result)
+
+                # Decode multiple steps to reduce the overhead
+                if batch.forward_mode.is_decode():
+                    for _ in range(self.server_args.num_continuous_decode_steps - 1):
+                        if not self.running_batch:
+                            break
+                        self.update_running_batch()
+                        if not self.running_batch:
+                            break
+                        result = self.run_batch(batch)
+                        self.process_batch_result(batch, result)
 
-            self.send_results()
+            self.last_batch = batch
 
     def recv_requests(self):
         if self.tp_rank == 0:
@@ -286,7 +304,9 @@ class Scheduler:
                 self.abort_request(recv_req)
             elif isinstance(recv_req, UpdateWeightReqInput):
                 success, message = self.update_weights(recv_req)
-                self.out_pyobjs.append(UpdateWeightReqOutput(success, message))
+                self.send_to_detokenizer.send_pyobj(
+                    UpdateWeightReqOutput(success, message)
+                )
             elif isinstance(recv_req, ProfileReq):
                 if recv_req == ProfileReq.START_PROFILE:
                     self.start_profile()
@@ -384,12 +404,6 @@ class Scheduler:
 
         self.waiting_queue.append(req)
 
-    def send_results(self):
-        if self.tp_rank == 0:
-            for obj in self.out_pyobjs:
-                self.send_to_detokenizer.send_pyobj(obj)
-            self.out_pyobjs = []
-
     def print_decode_stats(self):
         num_used = self.max_total_num_tokens - (
             self.token_to_kv_pool.available_size() + self.tree_cache.evictable_size()
@@ -427,41 +441,29 @@ class Scheduler:
             )
             exit(1) if crash_on_warning else None
 
-    def run_step(self):
-        new_batch = self.get_new_batch_prefill()
-        if new_batch is not None:
-            # Run a new prefill batch
-            # replace run_batch with the uncommented line to use pytorch profiler
-            # result = pytorch_profile(
-            #     "profile_prefill_step", self.run_batch, new_batch, data_size=len(new_batch.reqs)
-            # )
-            result = self.run_batch(new_batch)
-            self.process_batch_result(new_batch, result)
+    def get_next_batch_to_run(self):
+        # Merge prefill to the running batch
+        if (
+            self.last_batch
+            and not self.last_batch.forward_mode.is_decode()
+            and not self.last_batch.is_empty()
+        ):
+            if self.running_batch is None:
+                self.running_batch = self.last_batch
             else:
-            if self.running_batch is not None:
-                # Run a few decode batches continuously for reducing overhead
-                for _ in range(global_config.num_continue_decode_steps):
-                    batch = self.get_new_batch_decode()
+                self.running_batch.merge_batch(self.last_batch)
 
-                    if batch:
-                        # replace run_batch with the uncommented line to use pytorch profiler
-                        # result = pytorch_profile(
-                        #     "profile_decode_step",
-                        #     self.run_batch,
-                        #     batch,
-                        #     data_size=len(batch.reqs),
-                        # )
-                        result = self.run_batch(batch)
-                        self.process_batch_result(batch, result)
-
-                    if self.running_batch.is_empty():
-                        self.running_batch = None
-
-                    if self.running_batch is None:
-                        break
+        # Prefill first
+        new_batch = self.get_new_batch_prefill()
+        if new_batch is not None:
+            return new_batch
 
-                    if self.out_pyobjs and self.running_batch.has_stream:
-                        break
+        # Run decode
+        if self.running_batch is not None:
+            self.update_running_batch()
+            if not self.running_batch:
+                return None
+            return self.running_batch
         else:
             self.check_memory()
             self.new_token_ratio = global_config.init_new_token_ratio
@@ -607,7 +609,7 @@ class Scheduler:
 
         return new_batch
 
-    def get_new_batch_decode(self) -> Optional[ScheduleBatch]:
+    def update_running_batch(self):
         batch = self.running_batch
 
         # Check if decode out of memory
@@ -636,11 +638,11 @@ class Scheduler:
             if jump_forward_reqs:
                 self.batch_is_full = False
             if batch.is_empty():
-                return None
+                self.running_batch = None
+                return
 
         # Update batch tensors
         batch.prepare_for_decode()
-        return batch
 
     def run_batch(self, batch: ScheduleBatch):
         if self.is_generation:
@@ -657,16 +659,19 @@ class Scheduler:
                     )
                 else:
                     next_token_ids = torch.full((batch.batch_size(),), 0)
-            return logits_output, next_token_ids
+            ret = logits_output, next_token_ids
         else:  # embedding or reward model
             assert batch.extend_num_tokens != 0
             model_worker_batch = batch.get_model_worker_batch()
             embeddings = self.tp_worker.forward_batch_embedding(model_worker_batch)
-            return embeddings
+            ret = embeddings
+        return ret
 
     def process_batch_result(self, batch: ScheduleBatch, result):
         if batch.forward_mode.is_decode():
             self.process_batch_result_decode(batch, result)
+            if batch.is_empty():
+                self.running_batch = None
         else:
             self.process_batch_result_prefill(batch, result)
 
@@ -728,7 +733,7 @@ class Scheduler:
                     )
         else:  # embedding or reward model
             assert batch.extend_num_tokens != 0
-            embeddings = result
+            embeddings = result.tolist()
 
             # Check finish conditions
             for i, req in enumerate(batch.reqs):
@@ -750,12 +755,6 @@ class Scheduler:
 
         self.handle_finished_requests(batch)
 
-        if not batch.is_empty():
-            if self.running_batch is None:
-                self.running_batch = batch
-            else:
-                self.running_batch.merge_batch(batch)
-
     def process_batch_result_decode(self, batch: ScheduleBatch, result):
         logits_output, next_token_ids = result
         if batch.sampling_info.penalizer_orchestrator:
@@ -951,7 +950,7 @@ class Scheduler:
         # Send to detokenizer
         if output_rids:
             if self.is_generation:
-                self.out_pyobjs.append(
+                self.send_to_detokenizer.send_pyobj(
                     BatchTokenIDOut(
                         output_rids,
                         output_vids,
@@ -965,7 +964,7 @@ class Scheduler:
                     )
                 )
             else:  # embedding or reward model
-                self.out_pyobjs.append(
+                self.send_to_detokenizer.send_pyobj(
                     BatchEmbeddingOut(
                         output_rids,
                         output_embeddings,

python/sglang/srt/managers/tp_worker.py

@@ -118,7 +118,7 @@ class TpModelWorker:
     def forward_batch_embedding(self, model_worker_batch: ModelWorkerBatch):
         forward_batch = ForwardBatch.init_new(model_worker_batch, self.model_runner)
         logits_output = self.model_runner.forward(forward_batch)
-        embeddings = logits_output.embeddings.tolist()
+        embeddings = logits_output.embeddings
         return embeddings
 
     def update_weights(self, recv_req: UpdateWeightReqInput):

python/sglang/srt/server_args.py

@@ -111,6 +111,7 @@ class ServerArgs:
     torchao_config: str = ""
     enable_p2p_check: bool = False
     triton_attention_reduce_in_fp32: bool = False
+    num_continuous_decode_steps: int = 1
 
     def __post_init__(self):
         # Set missing default values
@@ -559,6 +560,14 @@ class ServerArgs:
             help="Cast the intermidiate attention results to fp32 to avoid possible crashes related to fp16."
             "This only affects Triton attention kernels.",
         )
+        parser.add_argument(
+            "--num-continuous-decode-steps",
+            type=int,
+            default=ServerArgs.num_continuous_decode_steps,
+            help="Run multiple continuous decoding steps to reduce scheduling overhead. "
+            "This can potentially increase throughput but may also increase time-to-first-token latency. "
+            "The default value is 1, meaning only run one decoding step at a time.",
+        )
 
     @classmethod
     def from_cli_args(cls, args: argparse.Namespace):