optimize

# Optimize: throughput & memory levers Recommend levers to raise **tokens/s/GPU** (the headline metric — MFU/achieved-TFLOPS are opt-in diagnostics from `EfficiencyCallback`, *not* the goal) or cut **peak memory** for a concrete config. **[levers.md](levers.md)** holds every figure with its measured shape, each row citing the `agent-docs/optimization/` page it comes from. This page carries the flow only: read levers.md and quote the number from there, never from here. ## How to use this skill 1. Pin the **bottleneck**: throughput-bound (want more tok/s/GPU) or memory-bound (OOM / want longer seq / bigger batch)? For MoE/EP also ask: comm-bound (low top-k, e.g. gpt-oss) or permute/elementwise-bound (high top-k, e.g. qwen3.6)? When unsure, measure: `enable_torch_profiler: true` → `python scripts/profiling/trace_report.py` (TraceLens compute/comm/idle split; the `ep.dispatch`/`ep.combine` trace ranges give the all-to-all fraction directly). 2. Confirm the **shape**: dense vs MoE/EP, short vs long seq, fixed- vs variable-length data. 3. Recommend from the flow below; take the measured effect + the exact YAML/env flag from levers.md. If a lever's win is batch-1-noisy (all EP throughput), say "measure at batch ≥ 4". 4. Steer away from the **negatives** below — they are the common dead-ends. ## Already on by default (don't "enable" — they're free) These fire automatically; mention them only to confirm, not as new advice. Figures in levers.md. - **Grouped GEMM** — auto on SM90+ for MoE experts; the win is largest with many local experts per rank and narrows as batch (per-expert M) grows. - **Liger CE + RMSNorm/RoPE/SwiGLU** — `use_liger_kernel: true` default; a double-digit-% throughput win and tens of GB, on dense and MoE alike. - **FA4 on Blackwell** — auto-selected; the win over FA2 grows with sequence length and is small on MoE, where the step is expert-GEMM and all-to-all bound. FA2 is the slow outlier on B300. - **AdamWBF16 + stochastic rounding** — auto from `bf16: true`; half the per-param state of fp32 AdamW at a loss curve that tracks the fp32 master. Auto-OFF under replicated DDP. - **CDMC=1** — baked into the image env; free win on ep8, neutral dense/ep2. - **Atomic-free expert permute** — auto for high-top_k MoE (`top_k ≥ ep_size`); win grows with sequence length. gpt-oss (top-4) stays on the cheaper `index_add_` path at EP8. ## Throughput flow (raise tok/s/GPU) - **MoE/EP, any shape →** push **seq × batch as high as memory allows** first — EP at low token counts is comm-bound (fixed all-to-all), so **batch is the dominant EP lever**. Read achieved TFLOPS / tok-s, not plain MFU% (sparse MoE can't approach a dense MFU; longer seq + lower EP raise the achieved-TFLOPS ceiling). - **MoE/EP, fits without GC →** turn **gradient checkpointing off** — the largest single throughput lever, at ~2× activation memory. Keep GC on only when you need the memory. - **MoE, small model fits in FSDP →** prefer **FSDP (ep1) over EP** (no all-to-all): it keeps every param local and tops the achieved-TFLOPS table; use EP only when FSDP OOMs. - **Pure EP on 8 GPUs →** use **ep2 (throughput) or ep8 (memory)** only. **ep4 is rejected at config time**; ep4 on exactly 4 GPUs is fine. For finer expert sharding use **EP+ETP** (`ep4+etp2`); EP+TP does not help. Rule and mechanism: the `parallelism` skill (`matrix.md`, row *Multi-group >2-rank EP on one NVLink domain*). - **Dense, long context →** FA4's win over FA2 is largest here. Confirm it's selected. - **Variable-length data (avg << max_len) →** **packing** is the big win; padding-free is the smaller one — use it when cross-sequence boundaries are undesirable. Uniform/long-seq data → all collators tie (~1%), use standard. - **Convergence speed (fewer steps to target loss) →** `optim: muon` reaches a lower loss in the same step budget on matrix params, at a much costlier optimizer step and higher peak memory — far less end-to-end, measure on your model. A convergence lever, not a per-step throughput one. ## Memory flow (cut peak / fit longer seq / bigger batch) - **Long-seq OOM at the loss →** **FusedLinearCrossEntropy** (`liger_kernel_config: {cross_entropy: false, fused_linear_cross_entropy: true}` — they are sub-keys of that dict, not top-level fields): never materializes the `batch×seq×vocab` logits, for tens of GB at 32k at near-CE throughput; SFT-only, disables entropy logging, not CP-compatible. - **MoE activation OOM →** keep **GC on** (roughly half the GC-off activation footprint, and what makes 32k fit on the default elastic transport — pure ep8 GC-off OOMs there, and fits only on `ep_buffer_backend: legacy`) and/or **raise EP degree** (more ranks = less expert memory/GPU). - **Optimizer-state OOM →** AdamWBF16 (auto) is already half of fp32 AdamW's state. For more, `optim: flash_adamw` (quantized moments, convergence matches AdamW; needs `flashoptim`) — tens of GB at 70B+. - **Want exact fp32 on dense params with headroom →** `fp32_non_ep_params: true` (dense params fp32, experts stay bf16+SR); refused on an `ep_size=1` MoE whose experts FSDP shards (the default). - **Fit on a small/consumer GPU →** **QLoRA**: far less memory than full FT and faster than bf16 LoRA (the 4-bit base is bandwidth-bound). With FLCE a Qwen3-8B 32k run fits a 24 GB GPU. Under EP, LoRA targets attention **plus** native grouped expert adapters (ETP is the attention-only row); no QLoRA under EP/TP. - **Many-rank / multi-node grad precision (not memory) →** `fp32_grad_reduce: true`: a tighter grad-reduce at bf16 storage cost, ~2× cost on the reduce collective only. ## What does NOT help here (do not chase these) The honest negatives — cited in full in levers.md. At fine-grained MoE shapes (EP8, 256–512 tokens/expert, N ≤ 4096) these are dead-ends: - **Low-precision fp8/fp4 *compute* — no throughput win. Train bf16.** Per-expert GEMM is weight-bandwidth-bound at the bf16 roofline (proven roofline + Blackwell HW + measured DeepGEMM). Simulated fake-quant runs many times a bf16 step — a *convergence-validation* oracle, not a speed path. The real fp8/fp4 win is **inference memory** (`quantize_to_lowp.py`, fp8 = ½ / fp4 = ¼ expert bytes) — not training. - **Native DeepGEMM — net-slower than bf16 at every training shape**; opt-in only (`HALO_DEEPGEMM_NATIVE=1`), never auto-selected. - **torch.compile on EP MoE — composes with Liger, but adds little.** Compile and Liger target the same compilable spans between DeepEP/FA4 graph breaks, so compile alone, Liger alone and the two stacked all land within ~2% — and Liger is already on by default with no warmup cost. Reach for `torch_compile: true` when you can absorb the first-step compile latency, not as a free extra win. - **Sub-bf16 master weights — dead.** Params, checkpoint and optimizer state never go below bf16; only GEMM operands are cast. bf16 + SR is the floor. - **FA2 on Blackwell — the slow outlier** (well under half FA4's throughput at 32k). Use FA4 (auto); SDPA is the dense fallback. - **MoE throughput levers measured at batch 1** — comm-bound, run-to-run noisy. Always **batch ≥ 4**. ## Sources of truth Figures live in [levers.md](levers.md), each cited to its page in `agent-docs/optimization/`: grouped-gemm.md, liger-kernels.md, flash-attention.md, bf16-optimizer.md, padding-free-collator.md, throughput-benchmarks.md, low-precision-moe-kernels.md, muon-optimizer.md, flash-adamw.md, torch-compile.md, peft.md. Read the cited doc page before quoting an exact number — and the code is the **ultimate** authority: when a doc, this skill, or memory disagrees with the actual `src/` implementation or a fresh benchmark, or you are unsure, trust the code/measurement and read it before asserting. (`CLAUDE.md`: docs-first, the code wins.) Related skills: `data` (packing / padding-free / sharding levers), `checkpoints` (low-precision fp8/fp4 export).