MoE expert backends

MoE (Mixture of Experts) models spend most of their prefill time in the expert feed-forward layers. mistralrs ships several interchangeable implementations of this computation and picks the fastest one your machine can actually run, automatically, at model load. CUTLASS selection emits a log line; the other backends are silent unless they fall back.

The backends

Backend	What it is	When it runs
cuTile	JIT-compiled grouped-GEMM kernels specialized for your exact GPU architecture and model shapes at load time. The fastest option where supported.	The `cutile` feature build, a supported CUDA/SM pair, and the `tileiras` JIT assembler available at runtime.
CUTLASS	Ahead-of-time compiled grouped GEMMs. Runs on any GPU from Ampere onward, with any CUDA toolkit, from a plain `cuda` build.	Default for unquantized BF16 MoE models with gated SiLU or tanh-approx GeLU (NewGelu / GeluPytorchTanh) when cuTile is unavailable.
Fused (WMMA)	Hand-written CUDA kernels for small batches, where grouped GEMMs are the wrong tool.	Small-batch decode under CUTLASS (below 64 tokens), and prefill when neither backend above applies (including erf-based GeLU and other activations).
Gather	Generic implementation built on the quantized-layer machinery.	Quantized experts (ISQ (in-situ quantization), UQFF (Universal Quantized File Format), pre-quantized), Metal, and CPU.

The ordering matters: cuTile outperforms CUTLASS, which substantially outperforms the fused fallback for prefill. A build without the cutile feature still gets a strong MoE path through CUTLASS - but enabling cutile on supported hardware is meaningfully faster, which is why the installer adds it automatically for supported CUDA/SM pairs.

Selection and graceful degradation

Backend selection happens once per model load and never strands you on a broken configuration:

cuTile requires a supported build CUDA and GPU pair: Ampere/Ada (sm_8x) needs CUDA >= 13.2, Hopper (sm90) needs CUDA >= 13.3, and Blackwell+ (sm_10x/sm_12x) needs CUDA >= 13.1. It also needs the tileiras JIT assembler at runtime. If either probe fails - for example, a cutile-enabled binary deployed to a machine without the toolkit - selection quietly moves to CUTLASS and logs why.
CUTLASS requires a build targeting compute capability 8.0 or newer. Below that, selection moves on.
Under CUTLASS, batches below 64 tokens delegate to the fused kernels: the grouped-GEMM setup cost exceeds the work itself at small batch sizes. cuTile runs its grouped-GEMM path at all batch sizes.

Overriding the choice

Set MISTRALRS_MOE_BACKEND to force a specific backend: cutile, cutlass, fused (also accepted: wmma, native, legacy), or fast. This is intended for debugging and A/B comparisons. Forcing a backend the build or hardware cannot support behaves in one of two ways:

Unsupported by the build (e.g. cutile on a non-cutile build): falls back to automatic selection.
Unsupported by the hardware (e.g. cutlass on a pre-Ampere build): fails at the first forward pass.

CUTILE_TILEIRAS_PATH points the cuTile JIT at a specific tileiras binary instead of resolving it from PATH.