Support layerwise rebalancing experts (#6851)

python/sglang/srt/managers/eplb_manager.py

 import logging
 import time
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, List
 
 import torch.cuda
 
@@ -20,6 +20,10 @@ class EPLBManager:
         super().__init__()
         self._model_runner = model_runner
         self._server_args = model_runner.server_args
+        self._rebalance_layers_per_chunk = (
+            self._server_args.eplb_rebalance_layers_per_chunk
+        )
+        self._rebalance_num_iterations = self._server_args.eplb_rebalance_num_iterations
 
         # Otherwise, the circular buffer will contain stale data. If the case is needed, it can be implemented.
         assert (
@@ -31,15 +35,28 @@ class EPLBManager:
             get_global_expert_distribution_recorder().start_record()
 
         logger.info(
-            f"[EPLBManager] system started, will rebalance per {self._server_args.eplb_rebalance_num_iterations} iterations."
+            f"[EPLBManager] system started, will rebalance per {self._rebalance_num_iterations} iterations."
         )
 
-    def on_forward_pass_end(self, forward_pass_id: int):
-        if forward_pass_id % self._server_args.eplb_rebalance_num_iterations == 0:
-            self.rebalance()
+        self._main_generator = self._entrypoint()
+
+    def on_forward_pass_end(self):
+        next(self._main_generator)
+
+    # can be more complex if needed
+    def _entrypoint(self):
+        while True:
+            for _ in range(self._rebalance_num_iterations):
+                yield
+
+            yield from self.rebalance()
 
     def rebalance(self):
         logger.info("[EPLBManager] rebalance start")
+
+        enable_timing = self._rebalance_layers_per_chunk is None
+
+        if enable_timing:
             torch.cuda.synchronize()
             time_start = time.time()
 
@@ -49,8 +66,31 @@ class EPLBManager:
         expert_location_metadata = ExpertLocationMetadata.init_by_eplb(
             self._server_args, self._model_runner.model_config, logical_count
         )
-        self._model_runner.update_expert_location(expert_location_metadata)
 
+        update_layer_ids_chunks = self._compute_update_layer_ids_chunks()
+        for chunk_index, update_layer_ids in enumerate(update_layer_ids_chunks):
+            if len(update_layer_ids_chunks) > 1:
+                yield
+            self._model_runner.update_expert_location(
+                expert_location_metadata,
+                update_layer_ids=update_layer_ids,
+            )
+
+        msg = f"[EPLBManager] rebalance end"
+        if enable_timing:
             torch.cuda.synchronize()
             time_end = time.time()
-        logger.info(f"[EPLBManager] rebalance end time={time_end - time_start:.3f}s")
+            msg += f" time={time_end - time_start:.3f}s"
+        logger.info(msg)
+
+    def _compute_update_layer_ids_chunks(self) -> List[List[int]]:
+        all_layer_ids = sorted(
+            list(self._model_runner.model.routed_experts_weights_of_layer.keys())
+        )
+        chunk_size = self._rebalance_layers_per_chunk or 1000000
+        return list(_chunk_list(all_layer_ids, chunk_size=chunk_size))
+
+
+def _chunk_list(items: List, chunk_size):
+    for start_index in range(0, len(items), chunk_size):
+        yield items[start_index : start_index + chunk_size]

python/sglang/srt/managers/expert_location.py

@@ -33,6 +33,7 @@ logger = logging.getLogger(__name__)
 @dataclass
 class ExpertLocationMetadata:
     physical_to_logical_map: torch.Tensor  # (layers, num_physical_experts)
+    physical_to_logical_map_cpu: torch.Tensor
     logical_to_all_physical_map: torch.Tensor  # (layers, num_logical_experts, X)
     logical_to_all_physical_map_num_valid: torch.Tensor  # (layers, num_logical_experts)
     # (layers, num_logical_experts)
@@ -203,6 +204,7 @@ class ExpertLocationMetadata:
 
         return ExpertLocationMetadata(
             physical_to_logical_map=physical_to_logical_map,
+            physical_to_logical_map_cpu=physical_to_logical_map.cpu(),
             logical_to_all_physical_map=logical_to_all_physical_map_padded,
             logical_to_all_physical_map_num_valid=logical_to_all_physical_map_num_valid,
             logical_to_rank_dispatch_physical_map=(
@@ -223,6 +225,7 @@ class ExpertLocationMetadata:
     def update(
         self,
         other: "ExpertLocationMetadata",
+        update_layer_ids: List[int],
     ):
         for field in [
             "ep_size",
@@ -231,15 +234,21 @@ class ExpertLocationMetadata:
 
         for field in [
             "physical_to_logical_map",
+            "physical_to_logical_map_cpu",
             "logical_to_all_physical_map",
             "logical_to_all_physical_map_num_valid",
             "logical_to_rank_dispatch_physical_map",
         ]:
-            src = getattr(other, field)
-            dst = getattr(self, field)
-            assert (src is not None) == (dst is not None)
-            if dst is not None:
-                dst[...] = src
+            other_field = getattr(other, field)
+            self_field = getattr(self, field)
+            assert (other_field is not None) == (self_field is not None)
+            if self_field is not None:
+                mask_update = torch.tensor(
+                    [i in update_layer_ids for i in range(self.num_layers)]
+                )
+                mask_update = mask_update.view(*([-1] + [1] * (self_field.dim() - 1)))
+                mask_update = mask_update.to(self_field.device, non_blocking=True)
+                self_field[...] = torch.where(mask_update, other_field, self_field)
 
     # -------------------------------- usage ------------------------------------
 

python/sglang/srt/model_executor/expert_location_updater.py

@@ -24,6 +24,7 @@ from sglang.srt.managers.expert_location import (
     ExpertLocationMetadata,
     get_global_expert_location_metadata,
 )
+from sglang.srt.managers.schedule_batch import global_server_args_dict
 from sglang.srt.utils import get_bool_env_var
 
 logger = logging.getLogger(__name__)
@@ -37,6 +38,7 @@ class ExpertLocationUpdater:
         self,
         routed_experts_weights_of_layer: Dict[int, List[torch.Tensor]],
         new_expert_location_metadata: ExpertLocationMetadata,
+        update_layer_ids: List[int],
         nnodes: int,
         rank: int,
     ):
@@ -46,45 +48,47 @@ class ExpertLocationUpdater:
 
         old_expert_location_metadata = get_global_expert_location_metadata()
         _update_expert_weights(
-            routed_experts_weights_of_layer,
-            old_expert_location_metadata,
+            routed_experts_weights_of_layer=routed_experts_weights_of_layer,
+            old_expert_location_metadata=old_expert_location_metadata,
+            new_expert_location_metadata=new_expert_location_metadata,
+            update_layer_ids=update_layer_ids,
+            nnodes=nnodes,
+            rank=rank,
+        )
+        old_expert_location_metadata.update(
             new_expert_location_metadata,
-            nnodes,
-            rank,
+            update_layer_ids=update_layer_ids,
         )
-        old_expert_location_metadata.update(new_expert_location_metadata)
 
 
 def _update_expert_weights(
     routed_experts_weights_of_layer: Dict[int, List[torch.Tensor]],
     old_expert_location_metadata: ExpertLocationMetadata,
     new_expert_location_metadata: ExpertLocationMetadata,
+    update_layer_ids: List[int],
     nnodes: int,
     rank: int,
 ):
     log_metrics = get_bool_env_var("SGLANG_EXPERT_LOCATION_UPDATER_LOG_METRICS")
 
     temp_buffers = create_temp_buffers(
-        next(iter(routed_experts_weights_of_layer.values()))
+        routed_experts_weights_of_layer[update_layer_ids[0]]
     )
 
     world_size = torch.distributed.get_world_size()
     num_local_physical_experts = old_expert_location_metadata.num_local_physical_experts
     num_gpu_per_node = world_size // nnodes
 
-    old_physical_to_logical_map = (
-        old_expert_location_metadata.physical_to_logical_map.tolist()
-    )
-    new_physical_to_logical_map = (
-        new_expert_location_metadata.physical_to_logical_map.tolist()
-    )
-
-    for layer_id in sorted(routed_experts_weights_of_layer.keys()):
+    for layer_id in update_layer_ids:
         update_expert_weights_single_layer(
             routed_experts_weights=routed_experts_weights_of_layer[layer_id],
             temp_buffers=temp_buffers,
-            old_physical_to_logical_map=old_physical_to_logical_map[layer_id],
-            new_physical_to_logical_map=new_physical_to_logical_map[layer_id],
+            old_physical_to_logical_map=old_expert_location_metadata.physical_to_logical_map_cpu[
+                layer_id
+            ].tolist(),
+            new_physical_to_logical_map=new_expert_location_metadata.physical_to_logical_map_cpu[
+                layer_id
+            ].tolist(),
             num_local_physical_experts=num_local_physical_experts,
             num_gpu_per_node=num_gpu_per_node,
             rank=rank,

python/sglang/srt/model_executor/model_runner.py

@@ -611,11 +611,14 @@ class ModelRunner:
             ) from None
 
     def update_expert_location(
-        self, new_expert_location_metadata: ExpertLocationMetadata
+        self,
+        new_expert_location_metadata: ExpertLocationMetadata,
+        update_layer_ids: List[int],
     ):
         self.expert_location_updater.update(
             self.model.routed_experts_weights_of_layer,
             new_expert_location_metadata,
+            update_layer_ids=update_layer_ids,
             nnodes=self.server_args.nnodes,
             rank=self.tp_rank,
         )
@@ -1203,7 +1206,7 @@ class ModelRunner:
             )
 
         if self.eplb_manager is not None:
-            self.eplb_manager.on_forward_pass_end(self.forward_pass_id)
+            self.eplb_manager.on_forward_pass_end()
 
         return output
 

python/sglang/srt/server_args.py

@@ -180,6 +180,7 @@ class ServerArgs:
     enable_eplb: bool = False
     eplb_algorithm: str = "auto"
     eplb_rebalance_num_iterations: int = 1000
+    eplb_rebalance_layers_per_chunk: Optional[int] = None
     expert_distribution_recorder_mode: Optional[
         Literal["stat", "per_pass", "per_token"]
     ] = None
@@ -1367,6 +1368,12 @@ class ServerArgs:
             default=ServerArgs.eplb_rebalance_num_iterations,
             help="Number of iterations to automatically trigger a EPLB re-balance.",
         )
+        parser.add_argument(
+            "--eplb-rebalance-layers-per-chunk",
+            type=int,
+            default=ServerArgs.eplb_rebalance_layers_per_chunk,
+            help="Number of layers to rebalance per forward pass.",
+        )
         parser.add_argument(
             "--expert-distribution-recorder-mode",
             type=str,

test/srt/test_eplb.py

@@ -5,7 +5,6 @@ from pathlib import Path
 from types import SimpleNamespace
 
 import sglang as sgl
-from sglang.srt.managers.expert_distribution_storage import ExpertDistributionStorage
 from sglang.srt.utils import kill_process_tree
 from sglang.test.run_eval import run_eval
 from sglang.test.test_utils import (
@@ -17,7 +16,9 @@ from sglang.test.test_utils import (
 )
 
 
-class TestDynamicEPLB(CustomTestCase):
+class _BaseTestDynamicEPLB(CustomTestCase):
+    extra_args = []
+
     @classmethod
     def setUpClass(cls):
         cls.model = DEFAULT_MLA_MODEL_NAME_FOR_TEST
@@ -51,8 +52,13 @@ class TestDynamicEPLB(CustomTestCase):
                 "stat",
                 "--ep-dispatch-algorithm",
                 "static",
+                *cls.extra_args,
             ],
-            env={"SGL_ENABLE_JIT_DEEPGEMM": "0", **os.environ},
+            env={
+                "SGL_ENABLE_JIT_DEEPGEMM": "0",
+                "SGLANG_EXPERT_LOCATION_UPDATER_CANARY": "1",
+                **os.environ,
+            },
         )
 
     @classmethod
@@ -72,6 +78,14 @@ class TestDynamicEPLB(CustomTestCase):
         self.assertGreater(metrics["score"], 0.5)
 
 
+class TestDynamicEPLBSimple(_BaseTestDynamicEPLB):
+    pass
+
+
+class TestDynamicEPLBMultiChunk(_BaseTestDynamicEPLB):
+    extra_args = ["--eplb-rebalance-layers-per-chunk", "1"]
+
+
 class TestStaticEPLB(CustomTestCase):
     def test_save_expert_distribution_and_init_expert_location(self):
         os.environ["SGL_ENABLE_JIT_DEEPGEMM"] = "0"