[Perf] Disable DeepGEMM MoE by default when TP=8 is used (#29346)

Signed-off-by: mgoin <mgoin64@gmail.com>

vllm/model_executor/layers/quantization/fp8.py

@@ -28,6 +28,7 @@ from vllm.model_executor.layers.fused_moe import (
     FusedMoeWeightScaleSupported,
 )
 from vllm.model_executor.layers.fused_moe.config import (
+    FusedMoEParallelConfig,
     FusedMoEQuantConfig,
     RoutingMethodType,
     fp8_w8a8_moe_quant_config,
@@ -118,7 +119,9 @@ class Fp8MoeBackend(Enum):
     TRITON = 6
 
 
-def get_fp8_moe_backend(block_quant: bool) -> Fp8MoeBackend:
+def get_fp8_moe_backend(
+    block_quant: bool, moe_parallel_config: FusedMoEParallelConfig
+) -> Fp8MoeBackend:
     """
     Select the primary FP8 MoE backend
     Note: Shape-specific fallbacks may still occur at runtime.
@@ -159,8 +162,19 @@ def get_fp8_moe_backend(block_quant: bool) -> Fp8MoeBackend:
         logger.info_once("Using Marlin backend for FP8 MoE")
         return Fp8MoeBackend.MARLIN
 
-    # deepGEMM on supported platforms with block-quantized weights
-    if envs.VLLM_USE_DEEP_GEMM and envs.VLLM_MOE_USE_DEEP_GEMM and block_quant:
+    # Determine if we should use DeepGEMM with block-quantized weights:
+    # - If explicitly set by user, respect their choice
+    # - If not explicitly set (default), disable when TP size is >= 8
+    moe_use_deep_gemm = envs.VLLM_MOE_USE_DEEP_GEMM
+    if not envs.is_set("VLLM_MOE_USE_DEEP_GEMM") and moe_parallel_config.tp_size >= 8:
+        moe_use_deep_gemm = False
+        logger.info_once(
+            "DeepGEMM MoE is disabled by default when TP size is >= 8. "
+            "Set VLLM_MOE_USE_DEEP_GEMM=1 to enable it.",
+            scope="local",
+        )
+
+    if envs.VLLM_USE_DEEP_GEMM and moe_use_deep_gemm and block_quant:
         if not has_deep_gemm():
             logger.warning_once(
                 "DeepGEMM backend requested but not available.", scope="local"
@@ -641,7 +655,9 @@ class Fp8MoEMethod(FusedMoEMethodBase):
         self.quant_config = quant_config
         self.weight_block_size = self.quant_config.weight_block_size
         self.block_quant: bool = self.weight_block_size is not None
-        self.fp8_backend = get_fp8_moe_backend(self.block_quant)
+        self.fp8_backend = get_fp8_moe_backend(
+            self.block_quant, layer.moe_parallel_config
+        )
 
         self.use_marlin = self.fp8_backend == Fp8MoeBackend.MARLIN
         self.flashinfer_moe_backend: FlashinferMoeBackend | None = None