Gahow Wang
f202351be5
Wrap each transformer block's forward in the checkpoint primitive when recompute is enabled (Phase T13 / KI-3). To make the block forward a pure segment fn (no `&self` borrow, so it can re-run in the backward closure), extract the block body + its helpers (linear / norm_gamma / attention / swiglu_mlp) into free functions parameterised by (cfg, compute_dtype) and add `Block::block_params()` (the 11 leaves in the params() per-block order). The non-recompute path calls `block_forward` directly — identical graph to before. - `TinyTransformer::with_recompute(bool)` builder (opt-in; default off keeps the unchanged tape / bit-identical numerics). - `--recompute` flag wired into bin/train and bin/train_ddp (DDP: each rank checkpoints independently). Correctness gate: tests/recompute.rs builds two identical models (recompute on/off), runs the same batched loss+backward, and asserts the forward logits, the loss, and EVERY parameter grad match within tight fp tol — parameterised over fp32 and bf16 (T12 composition). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
xtrain
A from-scratch Rust + CUDA LLM training engine — the sibling of xserv (the inference side). GPU-first.
The goal is to learn the full training-systems stack by hand: autograd / backward passes / optimizers (AdamW) / the training loop / distributed logic. Heavy lifting is borrowed where it makes sense (GEMM → cuBLAS after a hand-written version, multi-GPU comms → NCCL, tokenizer → reused from xserv), but the core is written from scratch. The target architecture is a tiny modern transformer (RoPE + RMSNorm + SwiGLU, ~1–30M params) whose forward aligns with xserv's Qwen3, so the backward passes map one-to-one onto xserv's existing forward kernels and trained weights can flow back into xserv.
Status
Bootstrapping (P0). This repo currently contains only the project skeleton and a working Rust↔CUDA build chain, verified by a trivial vector-add CUDA kernel.
Layout
xtrain/
├── Cargo.toml # workspace
├── csrc/ # CUDA sources (.cu)
│ └── test/vecadd.cu # trivial element-wise vector-add (smoke test)
└── crates/
└── xtrain-cuda/ # CUDA Runtime FFI + build.rs (nvcc → sm_120)
├── build.rs # compiles csrc/*.cu via the `cc` crate, links cudart
├── src/ # ffi / error / device / memory
└── tests/ # vecadd smoke test
The build mirrors xserv's approach: build.rs invokes nvcc (via the cc crate)
to compile csrc/*.cu targeting sm_120 (RTX 5090) and links them into the Rust
crate over hand-written extern "C" FFI.
Building & testing
CUDA compilation and execution happen on a GPU box (dash5, 8× RTX 5090, sm_120):
export PATH=/usr/local/cuda/bin:$HOME/.cargo/bin:$PATH
cargo build
cargo test -p xtrain-cuda -- --nocapture # runs the vecadd smoke test
On a machine without nvcc/GPU, build.rs detects the missing toolchain, skips
CUDA compilation, and sets a no_cuda cfg — so host-side cargo check still
works (the GPU smoke test is compiled out).