Glossary
Short definitions for the acronyms and names that recur in this book and the Metrale Engine codebase.
| Term | Definition |
|---|---|
| axum | Rust async web framework (built on tokio + tower). Metrale Engine's HTTP layer. |
| BF16 | Brain Floating-Point 16. 1 sign + 8 exponent + 7 mantissa. Standard precision for Metrale Engine activations and residual streams. |
| Circuit | An architecture written as a string diagram: ops wired by typed edges, in blocks laid out per layer kind (kernels/circuits/<arch>.toml). See The Circuit Compiler. |
| CommBackend | Metrale Engine's trait for collective ops (all-reduce, broadcast, send/recv). NCCL-backed in production; no-op in single-GPU. |
| ComputeTarget | Metrale Engine's build-time trait for vendor-specific compilers (nvcc, xcrun metal, hipcc, icpx). |
| conv1d | 1D convolution, typically causal with small kernel width (3–4). Used in Mamba-style SSMs. |
| CUTLASS | NVIDIA's open-source CUDA template library for GEMM and related ops. Metrale Engine uses it for certain NVFP4 paths. |
| cp.async | CUDA instruction for asynchronous global-to-shared-memory copies. Key to pipelining on SM80+ architectures. |
| DGX Spark | NVIDIA's GB10-based workstation. Metrale Engine's initial hardware target. |
| DType | Data type enum in metrale-core — E2M1, FP8E4M3, FP8E5M2, BF16, FP16, FP32. |
| E2M1 | 4-bit float format: 1 sign + 2 exponent + 1 mantissa. Values: {0, ±0.5, ±1, ±1.5, ±2, ±3, ±4, ±6}. The storage format of NVFP4 weights. |
| E4M3 | 8-bit float format: 1 sign + 4 exponent + 3 mantissa. The standard FP8 format. |
| EP=2 | Expert Parallelism across 2 nodes. Metrale Engine's multi-GPU shape — experts split across ranks, other layers replicated. |
| Evidence envelope | The (class, point, row count) combinations at which a kernel family has a microbench record. "Optimized" is claimed only inside it. See the LKB. |
| Flash Attention | Tiled online-softmax attention kernel family. Metrale Engine's prefill kernel builds on FA-2 + FA-4. |
| FP8 | 8-bit floating-point. In Metrale Engine context, usually E4M3. |
| GB10 | NVIDIA Grace-Blackwell GB10 Superchip. SM121. 119.7 GB unified memory. |
| GDN | Gated Delta Rule. Qwen3.5's variant of the delta-net SSM. |
| GDR | Gated Delta Rule (see above) / also NCCL's GPUDirect RDMA level. Context-dependent. |
| GEMM | General matrix-matrix multiplication. The workhorse tensor-core op. |
| GeGLU | Gated GELU. Gemma-4's activation. |
| GpuBackend | Metrale Engine's runtime trait for GPU ops (memory, launch, streams, graphs). CUDA-backed in production; mockable for tests. |
| Grace | The ARM CPU half of GB10. Used for CPU-side NEON SIMD precomputation (e.g. RoPE tables). |
| HARDWARE.toml | Per-hardware metadata in kernels/<hw>/. Vendor, arch, memory specs. |
| HF | HuggingFace. Metrale Engine loads HF-format checkpoints via safetensors. |
| HyperCompiling | "AI Kernel HyperCompiling" — Metrale Engine's philosophy. Specialize per (H, M_q) target; abstractions stay above the kernel layer. |
| IORouter | The SBIO pattern name for an I/O-side trait (GpuBackend, CommBackend, WeightStore). |
| Kernel family | One kernel algorithm for one op class, with its parameters and instantiated points (KERNEL_FAMILIES.toml). A generator of the LKB. |
| KernelTarget | The (arch, model, quant) dispatch key. metrale-core::target::KernelTarget. |
| KV cache | Cached key and value tensors from attention. Paged in Metrale Engine. |
| LAB | Latent Architecture Blueprint: architectures as parameterized block families, so a new model is a parameter point plus a named residual. See the LAB. |
| Laxity | The gain of a fusion: the cost of its parts minus the cost of the fused kernel. Time and energy laxity are measured separately. See the LKB in Mathematics. |
| LKB | Latent Kernel Blueprint: kernel families (generators) and bit-identical rewrites (relations), shared by every hardware class and realized on each. See the LKB. |
| LKB residual | The kernels of a hardware class that are not points of any LKB family: shadows, class-only sources, per-point copies. Always named, with evidence. Not to be confused with the residual stream. |
| Lowering rule | A [[rule]] in FUSIONS.toml: an op pattern mapped to kernels under given modes, rows and settings. Its numerics tag is reference, bit_identical or differs. |
| LPDDR5X | The memory technology GB10 uses. Unified with CPU; 273 GB/s peak bandwidth. |
| Mamba / Mamba-2 | Selective state-space models. Mamba-2 is the variant used by Nemotron-H. |
| Marconi | Metrale Engine's SSM snapshot cache. Extension of RadixAttention to hybrid models. |
| MMA | Matrix Multiply-Accumulate. NVIDIA's tensor-core operation (mma.sync.aligned.m16n8k16.*). |
| MoE | Mixture of Experts. FFN split into N experts with per-token top-k routing. |
| MODEL.toml | Per-model metadata in kernels/<hw>/<model>/. Sampling presets, thinking budget, behavior defaults. |
| MRoPE | Multi-RoPE. Variant of RoPE that splits head dim into spatial (H, W) and temporal (T) segments. Used by vision models. |
| MTP | Multi-Token Prediction. Metrale Engine's speculative-decoding mechanism using a model-native draft head. |
| NCCL | NVIDIA's collective-ops library for multi-GPU / multi-node. |
| Numerics point | A choice of reduction order, rounding points, FMA contraction and MMA atom for a kernel. Two kernels at the same numerics point give identical bytes. |
| NVFP4 | 4-bit E2M1 weights + FP8 E4M3 per-block scales (block=16). Metrale Engine's flagship quant format on GB10. |
| O_DIRECT | Linux open flag that bypasses the page cache. Used by Metrale Engine's fast safetensors loader. |
| PCND | "Prefer Config / No Defaults" — a user-instruction principle: no implicit defaults in production paths. |
| Point | One instantiation of a kernel family: a value for each compile-time, policy and numerics parameter. |
| PTX | Parallel Thread Execution. NVIDIA's virtual ISA; Metrale Engine's compiled kernels ship as PTX. |
| RadixAttention | Prefix-caching mechanism built on a radix tree over token sequences. |
| Realization | How a hardware class turns LKB points into kernels it can launch, along its HARDWARE.toml inheritance chain. In the math, a functor F_H. |
| Relation | A lowering rule proven byte-identical to the chain it replaces (numerics = "bit_identical"). It may become the default on bytes plus a same-box speed and energy A/B. |
| RMSNorm | Root-mean-square normalization. The norm used in every modern transformer Metrale Engine supports. |
| RoCE | RDMA over Converged Ethernet. Metrale Engine's multi-node transport. |
| RoPE | Rotary Position Embedding. Position encoding used by every transformer in the support matrix. |
| Rust | The language Metrale Engine is written in (stable, edition 2024). |
| SafeTensors | Format for HF model checkpoints. Metrale Engine's loader reads it directly. |
| SBIO | Separation of Business logic and I/O. The architectural pattern that keeps Metrale Engine ~80% unit-testable without a GPU. |
| SDD | Structure-Driven Development. User-instruction principle for abstraction design. |
| SLAI | SLO-Aware Inference. Metrale Engine's TBT-deadline-aware scheduling policy. |
| SM100 / SM101 / SM120 / SM121 | NVIDIA Streaming Multiprocessor architecture identifiers. GB10 is SM121. |
| SSM | State-Space Model. Mamba / delta-net style layer. |
| SSOT | Single Source of Truth. User-instruction principle. |
| TBT | Time Between Tokens. The decode-step latency SLAI optimises. |
| TTBP / TTPV | Time to bit parity; time to a performance (speed and energy) win over the baseline, per hardware and model combination. |
| TTFT | Time To First Token. The prefill-stage latency. |
| Tensor core | Dedicated MMA hardware on NVIDIA GPUs; Metrale Engine targets the BF16 and E4M3 tensor cores on SM121. |
| TurboQuant | Metrale Engine's WHT + Lloyd-Max 4/3/8-bit KV-cache quant format. Lower MSE than NVFP4 at the same bit rate. |
| Uncovered op | An op no kernel family lowers on a class; reported as novel by met circuit venn and plan. A gap in the LKB, the opposite of the LKB residual. |
| vLLM | Popular open-source LLM inference framework. Metrale Engine's primary throughput baseline. |
| WHT | Walsh-Hadamard Transform. Used in TurboQuant to flatten outliers before quantization. |
| XGrammar | Token-bitmap automaton for constrained decoding. Metrale Engine's tool-call + structured-output enforcement substrate. |