transformers/docs/source/en/experts_interface.md

Experts backends

All Mixture-of-Experts (MoE) implementations perform the same high-level computation. For each token, a router selects k experts. The token hidden state is then projected through the selected experts' parameters and aggregated with routing weights. The difference between experts backends is how those expert matrix multiplications execute.

The [ExpertsInterface] provides optimized experts backends. It decouples the experts implementation from the model code to simplify experimentation with different functions. Add new backends through the same interface.

experts backend	description	GPU	CPU
"eager"	Reference implementation that loops over selected experts and applies projections on their tokens.	Reasonable baseline performance without requiring compilation.	Slower than grouped_mm but faster than batched_mm.
"batched_mm"	Duplicates selected expert parameters for each token and projects all tokens in a single batched GEMM using torch.bmm.	Fastest for small inputs, especially with compilation. Uses more memory due to parameter duplication.	Not recommended (significantly slower than other backends).
"grouped_mm"	Orders tokens by selected experts and uses torch.nn.functional.grouped_mm to project all tokens in a single grouped GEMM (requires PyTorch 2.9+).	Best for larger inputs and more memory efficient as it avoids duplicating expert parameters. Fast with compilation.	Most efficient backend for all input sizes.

Note

When using experts_implementation="grouped_mm" on GPU, the model automatically switches to "batched_mm" during the decode stage of generation (after prefill). This is because batched_mm is significantly faster on lower token count during autoregressive decoding on GPU. On CPU, grouped_mm remains active throughout generation as it is more efficient for all input sizes.

Set an experts backend

Use the experts_implementation argument in [~PreTrainedModel.from_pretrained] to instantiate a model with a specific experts backend.

from transformers import AutoModelForCausalLM

model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen1.5-MoE-A2.7B",
    dtype="bfloat16",
    experts_implementation="batched_mm",
)

Switch between experts backends at runtime without reloading the model using [~PreTrainedModel.set_experts_implementation].

model.set_experts_implementation("eager")

Backbone-specific experts backend

Multimodal models can have multiple sub-configs (for example, different backbones). You can set a different experts backend per sub-config by passing a dict to experts_implementation at load time.

Keys in the mapping must match sub-config names.

from transformers import AutoModelForImageTextToText

experts_implementation_per_backbone = {
    "text_config": "grouped_mm",
    "vision_config": "eager",
}

model = AutoModelForImageTextToText.from_pretrained(
    "Qwen/Qwen3-VL-Moe",
    experts_implementation=experts_implementation_per_backbone,
)

Set the experts backend globally with an empty key.

model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen1.5-MoE-A2.7B",
    experts_implementation={"": "batched_mm"},
)

torch.compile

All three backends ("eager", "batched_mm", "grouped_mm") are compatible with torch.compile to certain extents. The following table summarizes compatibility:

Implementation	compilation modes	dtypes	fullgraph=True
grouped_mm	None, max-autotune-no-cudagraphs	bfloat16	Yes
grouped_mm (fallback)	None, max-autotune-no-cudagraphs	bfloat16, float16, float32	Yes
batched_mm	all	bfloat16, float16, float32	Yes
eager	all	bfloat16, float16, float32	No

Notes:

• The grouped_mm experts backend currently only supports bfloat16 when compiled with torch.compile. Additionally, it is not compatible with CUDA graphs, so you must use mode=None or mode="max-autotune-no-cudagraphs" when compiling.
• The eager experts backend uses a data-dependent operation to find which experts are used in a forward pass. This operation is not compatible with full graph compilation (fullgraph=True).

import torch
from transformers import AutoModelForCausalLM

model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen1.5-MoE-A2.7B",
    dtype="bfloat16",
    experts_implementation="grouped_mm",
).eval().cuda()

# Works for grouped_mm (no CUDA graphs)
model.forward = torch.compile(model.forward, mode="max-autotune-no-cudagraphs")

Benchmarks

This benchmark compares different input sizes and experts implementations with and without torch.compile.