[ModelOpt] Introduce VLLM_MAX_TOKENS_PER_EXPERT_FP4_MOE env var to control blockscale tensor allocation (#18160)

Signed-off-by: Pavani Majety <pmajety@nvidia.com>

vllm/_custom_ops.py

@@ -1085,7 +1085,6 @@ def scaled_fp4_experts_quant(
     blockscale_offsets: torch.Tensor,
     topk: int,
     expert_map: Optional[torch.Tensor] = None,
-    MAX_TOKENS_PER_EXPERT: int = 163840,
 ) -> tuple[torch.Tensor, torch.Tensor]:
     """
     Quantize input tensor to FP4 and return quantized tensor and scale, for
@@ -1107,9 +1106,16 @@ def scaled_fp4_experts_quant(
     input_tensor = input_tensor[
         expert_map] if expert_map is not None else input_tensor
     m_numtopk, k = input_tensor.shape
+    # Control the maximum number of tokens per expert supported by the
+    # NVFP4 MoE Expert Quantization. This is used to prevent the kernel
+    # from running out of memory. This value can also be increased to support
+    # larger models.
+    MAX_TOKENS_PER_EXPERT = envs.VLLM_MAX_TOKENS_PER_EXPERT_FP4_MOE
     assert (m_numtopk <= MAX_TOKENS_PER_EXPERT * topk), (
-        f"m_numtopk must be less than MAX_TOKENS_PER_EXPERT * topk for"
-        f" scaled_fp4_experts_quant kernel, observed m_numtopk = {m_numtopk}")
+        f"m_numtopk must be less than MAX_TOKENS_PER_EXPERT("
+        f"{MAX_TOKENS_PER_EXPERT})"
+        f" for cutlass_moe_fp4, observed m_numtopk = {m_numtopk}. Use"
+        f" VLLM_MAX_TOKENS_PER_EXPERT_FP4_MOE to set this value.")
     scales_k = k // 16
     padded_k = (scales_k + (4 - 1)) // 4
 

vllm/envs.py

@@ -117,6 +117,7 @@ if TYPE_CHECKING:
     VLLM_NIXL_SIDE_CHANNEL_HOST: str = "localhost"
     VLLM_NIXL_SIDE_CHANNEL_PORT: int = 5557
     VLLM_ALL2ALL_BACKEND: str = "naive"
+    VLLM_MAX_TOKENS_PER_EXPERT_FP4_MOE: int = 163840
 
 
 def get_default_cache_root():
@@ -814,6 +815,13 @@ environment_variables: dict[str, Callable[[], Any]] = {
     # - "pplx": use pplx kernels
     "VLLM_ALL2ALL_BACKEND":
     lambda: os.getenv("VLLM_ALL2ALL_BACKEND", "naive"),
+
+    # Control the maximum number of tokens per expert supported by the
+    # NVFP4 MoE CUTLASS Kernel. This value is used to create a buffer for
+    # the blockscale tensor of activations NVFP4 Quantization.
+    # This is used to prevent the kernel from running out of memory.
+    "VLLM_MAX_TOKENS_PER_EXPERT_FP4_MOE":
+    lambda: int(os.getenv("VLLM_MAX_TOKENS_PER_EXPERT_FP4_MOE", "163840")),
 }
 
 # --8<-- [end:env-vars-definition]

vllm/model_executor/layers/fused_moe/cutlass_moe.py

@@ -1,6 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 """ CUTLASS based Fused MoE kernels."""
-import os
 from typing import Optional
 
 import torch
@@ -271,8 +270,6 @@ def cutlass_moe_fp8(
 
 FLOAT4_E2M1_MAX = scalar_types.float4_e2m1f.max()
 FLOAT8_E4M3_MAX = torch.finfo(torch.float8_e4m3fn).max
-MAX_TOKENS_PER_EXPERT = int(
-    os.environ.get('VLLM_MODELOPT_MAX_TOKENS_PER_EXPERT', '65536'))
 
 
 def cutlass_moe_fp4(a: torch.Tensor, a1_gscale: torch.Tensor,
@@ -330,10 +327,7 @@ def cutlass_moe_fp4(a: torch.Tensor, a1_gscale: torch.Tensor,
     assert a.dtype in [torch.half, torch.bfloat16], "Invalid input dtype"
     assert (topk_weights.shape[0] == m and topk_ids.shape[0]
             == m), ("topk must be provided for each row of a")
-    assert (m <= MAX_TOKENS_PER_EXPERT), (
-        f"m must be less than MAX_TOKENS_PER_EXPERT({MAX_TOKENS_PER_EXPERT})"
-        f" for cutlass_moe_fp4, observed m = {m}. Use"
-        f" VLLM_MODELOPT_MAX_TOKENS_PER_EXPERT to set this value.")
+
     out_dtype = a.dtype
     num_topk = topk_ids.shape[1]
 
@@ -362,8 +356,7 @@ def cutlass_moe_fp4(a: torch.Tensor, a1_gscale: torch.Tensor,
         expert_offsets,
         blockscale_offsets,
         num_topk,
-        expert_map=a_map,
-        MAX_TOKENS_PER_EXPERT=MAX_TOKENS_PER_EXPERT)
+        expert_map=a_map)
 
     c1 = ops.cutlass_fp4_moe_mm(rep_a_fp4, w1_fp4, rep_a_blockscale,
                                 w1_blockscale, w1_alphas, problem_sizes1,
@@ -378,12 +371,7 @@ def cutlass_moe_fp4(a: torch.Tensor, a1_gscale: torch.Tensor,
     torch.ops._C.silu_and_mul(intermediate, c1)
 
     int_fp4, int_blockscale = ops.scaled_fp4_experts_quant(
-        intermediate,
-        a2_gscale,
-        expert_offsets,
-        blockscale_offsets,
-        num_topk,
-        MAX_TOKENS_PER_EXPERT=MAX_TOKENS_PER_EXPERT)
+        intermediate, a2_gscale, expert_offsets, blockscale_offsets, num_topk)
 
     c2 = ops.cutlass_fp4_moe_mm(int_fp4, w2_fp4, int_blockscale, w2_blockscale,
                                 w2_alphas, problem_sizes2, expert_offsets[:-1],