Improve EPLB logical to physical dispatch map (#6727)

python/sglang/srt/managers/expert_location.py

@@ -13,6 +13,7 @@
 # ==============================================================================
 import json
 import logging
+import random
 from dataclasses import dataclass
 from pathlib import Path
 from typing import List, Optional
@@ -205,10 +206,10 @@ class ExpertLocationMetadata:
             logical_to_all_physical_map_num_valid=logical_to_all_physical_map_num_valid,
             logical_to_rank_dispatch_physical_map=compute_logical_to_rank_dispatch_physical_map(
                 logical_to_all_physical_map=logical_to_all_physical_map,
-                logical_to_all_physical_map_num_valid=logical_to_all_physical_map_num_valid,
                 num_gpus=ep_size,
                 num_physical_experts=num_physical_experts,
-                ep_rank=torch.distributed.get_rank(),
+                # TODO improve when we have real EP rank
+                ep_rank=torch.distributed.get_rank() % ep_size,
             ),
         )
 
@@ -296,49 +297,82 @@ def _pad_nested_array(arr, pad_value):
     return padded
 
 
-# TODO use more sophisticated approaches
+# TODO optimize performance (rewrite and/or run in separate process with overlap)
 def compute_logical_to_rank_dispatch_physical_map(
     logical_to_all_physical_map: torch.Tensor,
-    logical_to_all_physical_map_num_valid: torch.Tensor,
     num_gpus: int,
     num_physical_experts: int,
     ep_rank: int,
-    base_seed: int = 42,
+    seed: int = 42,
 ):
-    device = logical_to_all_physical_map.device
+    r = random.Random(seed)
 
     num_local_physical_experts = num_physical_experts // num_gpus
     num_layers, num_logical_experts, _ = logical_to_all_physical_map.shape
+    dtype = logical_to_all_physical_map.dtype
 
-    g = torch.Generator(device=device)
-    g.manual_seed(base_seed + ep_rank)
-
-    output_shape = (num_layers, num_logical_experts)
-    chosen_index = (
-        torch.randint(
-            0, 65536, output_shape, dtype=torch.int32, device=device, generator=g
-        )
-        % logical_to_all_physical_map_num_valid
+    logical_to_rank_dispatch_physical_map = torch.full(
+        size=(num_gpus, num_layers, num_logical_experts),
+        fill_value=-1,
+        dtype=dtype,
     )
-    logical_to_rank_dispatch_physical_map = torch.gather(
-        logical_to_all_physical_map, dim=2, index=chosen_index.unsqueeze(-1)
-    ).squeeze(-1)
-    assert logical_to_rank_dispatch_physical_map.shape == output_shape
-
-    for index in range(logical_to_all_physical_map_num_valid.max().item()):
-        partial_logical_to_all_physical_map = logical_to_all_physical_map[:, :, index]
-        is_valid = partial_logical_to_all_physical_map != -1
-        is_same_gpu = (
-            partial_logical_to_all_physical_map // num_local_physical_experts
-        ) == ep_rank
-        logical_to_rank_dispatch_physical_map = torch.where(
-            is_valid & is_same_gpu,
-            partial_logical_to_all_physical_map,
-            logical_to_rank_dispatch_physical_map,
-        )
+
+    for layer_id in range(num_layers):
+        for logical_expert_id in range(num_logical_experts):
+            candidate_physical_expert_ids = _logical_to_all_physical_raw(
+                logical_to_all_physical_map, layer_id, logical_expert_id
+            )
+            output_partial = logical_to_rank_dispatch_physical_map[
+                :, layer_id, logical_expert_id
+            ]
+
+            for gpu_id in range(num_gpus):
+                same_gpu_physical_expert_ids = [
+                    physical_expert_id
+                    for physical_expert_id in candidate_physical_expert_ids
+                    if _compute_gpu_id_of_physical_expert(
+                        physical_expert_id, num_local_physical_experts
+                    )
+                    == gpu_id
+                ]
+                if len(same_gpu_physical_expert_ids) > 0:
+                    output_partial[gpu_id] = same_gpu_physical_expert_ids[0]
+
+            num_remain = torch.sum(output_partial == -1).item()
+            output_partial[output_partial == -1] = torch.tensor(
+                _fair_choices(candidate_physical_expert_ids, k=num_remain, r=r),
+                dtype=dtype,
+            )
 
     assert torch.all(logical_to_rank_dispatch_physical_map != -1)
-    return logical_to_rank_dispatch_physical_map
+
+    device = logical_to_all_physical_map.device
+    return logical_to_rank_dispatch_physical_map[ep_rank, :, :].to(device)
+
+
+def _logical_to_all_physical_raw(
+    logical_to_all_physical_map, layer_id: int, logical_expert_id: int
+) -> List[int]:
+    return [
+        physical_expert_id
+        for physical_expert_id in logical_to_all_physical_map[
+            layer_id, logical_expert_id
+        ].tolist()
+        if physical_expert_id != -1
+    ]
+
+
+def _compute_gpu_id_of_physical_expert(
+    physical_expert_id: int, num_local_physical_experts: int
+) -> int:
+    return physical_expert_id // num_local_physical_experts
+
+
+def _fair_choices(arr: List, k: int, r: random.Random) -> List:
+    quotient, remainder = divmod(k, len(arr))
+    ans = arr * quotient + r.sample(arr, k=remainder)
+    r.shuffle(ans)
+    return ans
 
 
 @dataclass