ai-distributed-training

Distributed Training - Systems Performance Skill

Operational focus: picking and implementing the right parallelism strategy, not the theory. Covers data parallelism through FSDP/ZeRO/tensor+pipeline parallelism, memory-efficient attention, mixed precision at scale, activation checkpointing, rented-GPU cost discipline, and reproducing GPT-2 124M as the canonical sanity check.

Profile before you scale. Debug on the smallest GPU that fits. Stop the instance when done.

ASCII Flow

single GPU (debug/prototype)
  └─ DDP: replicate model, all-reduce gradients — linear scale up to ~8 GPUs
      └─ FSDP2 / ZeRO: shard optimizer state, gradients, params across GPUs
          └─ tensor parallelism: split weight matrices across GPUs (intra-node)
              └─ pipeline parallelism: split layers across nodes (inter-node)
                  └─ context parallelism: shard the sequence dim (long context)
                      └─ expert parallelism: route MoE experts across GPUs (all-to-all)
                          └─ N-D parallelism: DP + TP + PP + CP + EP (frontier MoE)

profile-before-scale
  └─ nsys / torch.profiler → find bottleneck (compute? memory? dataloader?)
      └─ fix bottleneck at small scale, then scale

When to Use This Skill

Activate when the user asks about:

• Choosing between DDP, FSDP2, DeepSpeed ZeRO stages 1/2/3, or Megatron-LM
• Training Mixture-of-Experts (MoE) models: expert parallelism, all-to-all, load balancing
• OOM errors on multi-GPU training runs
• Memory-efficient attention (FlashAttention-2/3, xformers)
• Mixed precision (bf16, fp8, nvfp4) trade-offs at pre-training scale
• Optimizer choice at scale (AdamW vs Muon/MuonClip)
• Targeting current-gen hardware (H100, Blackwell B200/GB200 NVL72, early Rubin NVL72 access)
• Gradient checkpointing vs activation checkpointing cost
• Pre-training frameworks: litgpt, torchtitan, nanotron, levanter
• Reproducing GPT-2 (llm.c or modded-nanoGPT as the reference)
• Rented GPU cost management (RunPod, Lambda, Vast.ai, Modal)
• Spot / interruptible instance checkpoint strategies
• Profiling a training run before deciding to scale

Scope Boundaries (Use These Skills for Depth)

• Single-GPU pre-training build, data pipelines, tokenization -> ai-pretraining
• Token budget, compute-optimal scaling, Chinchilla -> ai-scaling-laws
• Serving optimization, batching, quantization, inference -> ai-llm-inference
• General cloud/infra cost optimization -> ops-cost-optimization
• Production MLOps, model registry, monitoring, deployment -> ai-mlops

Default Workflow

1. Confirm scale and budget: how many GPUs, which provider, on-demand or spot, target hours.
2. Profile at small scale: run nsys or torch.profiler on 1-2 GPUs before adding more.
3. Pick parallelism strategy: data parallel (DDP) -> FSDP/ZeRO -> tensor+pipeline only as needed.
4. Enable memory optimizations: FlashAttention, gradient checkpointing, bf16, activation offload.
5. Wire checkpointing: use Distributed Checkpoint (DCP) for sharded state, save async to object storage every N steps; test restore before long runs.
6. Scale and re-profile: verify near-linear throughput scaling; fix communication bottlenecks.
7. Stop instance: confirm instance termination; verify storage persistence; check billing.

Quick Reference

Parallelism Deep Dive

Data Parallelism (DDP)

Each worker holds a full model replica. Forward + backward runs independently per GPU. AllReduce synchronizes gradients. Scales well up to ~64 GPUs before communication becomes the bottleneck. Memory cost: full model + optimizer state on every GPU.

# PyTorch DDP minimal setup
model = DistributedDataParallel(model, device_ids=[local_rank])

FSDP2 (Fully Sharded Data Parallel)

PyTorch-native. Shards parameters, gradients, and optimizer state across all workers. Use FSDP2 (fully_shard) for all new work — the original FullyShardedDataParallel (FSDP1, FlatParameter-based) is deprecated as of PyTorch 2.11. FSDP2 shards each parameter individually as a DTensor (Shard(dim=0)), giving simpler/inspectable sharded state dicts, cleaner composition with TP/PP/CP via DeviceMesh, and tight torch.compile integration.

ZeRO-stage equivalents map onto reshard_after_forward:

• reshard_after_forward=False → keep params gathered after forward (ZeRO-2-like: shard grads + optimizer state, trade memory for fewer all-gathers)
• reshard_after_forward=True (default) → re-shard params after forward (ZeRO-3-like: shard params + grads + optimizer state)

# FSDP2 (PyTorch >=2.11). Shard each transformer block, then the root.
from torch.distributed.fsdp import fully_shard, MixedPrecisionPolicy

mp = MixedPrecisionPolicy(param_dtype=torch.bfloat16, reduce_dtype=torch.float32)
for block in model.layers:
    fully_shard(block, mp_policy=mp)
fully_shard(model, mp_policy=mp)

FSDP1 (FullyShardedDataParallel + ShardingStrategy.FULL_SHARD/SHARD_GRAD_OP/NO_SHARD) still appears in older tutorials; migrate to FSDP2. Checkpoints are compatible across the two, but the construction API is not.

DeepSpeed ZeRO Stages

ZeRO-Infinity extends stage 3 to NVMe offload. Use only when GPU memory is genuinely exhausted — disk bandwidth becomes the bottleneck.

Tensor Parallelism (Megatron-LM style)

Splits weight matrices across GPUs within a node (column/row parallel linear). Requires high-bandwidth NVLink. Megatron-LM implements Transformer-specific tensor parallel (TP) with sequence parallel (SP) for activation memory reduction. Best for models that cannot fit even with full sharding, or where communication budget allows.

Pipeline Parallelism

Splits model layers across nodes (or GPU groups). Interleaved schedules (1F1B) reduce pipeline bubble overhead. Adds complexity: microbatch sizing, bubble fraction tuning. Typically combined with TP and DP in 3-D parallelism (Megatron-LM, nanotron).

DualPipe (DeepSeek-V3, 2024) is a bidirectional pipeline schedule that fully overlaps forward/backward compute with communication, driving the bubble toward zero — the reference design for large MoE training where cross-node all-to-all would otherwise dominate.

Expert Parallelism (MoE)

Mixture-of-Experts models activate only a few experts per token, so total params (e.g. 1T) vastly exceed activated params (e.g. 32B). Expert parallelism (EP) places different experts on different GPUs; the router dispatches each token to its experts via all-to-all communication (dispatch), then a second all-to-all gathers results (combine). EP composes with DP/TP/PP/CP as an extra mesh dimension.

Key concerns specific to MoE training:

• Load balancing: an auxiliary load-balancing loss (or DeepSeek-V3's auxiliary-loss-free bias-update scheme) keeps tokens spread across experts; without it, a few experts saturate and the rest idle.
• All-to-all is the bottleneck, not all-reduce. It scales with cross-node bandwidth — keep EP inside the NVLink domain where possible, and overlap it with compute (DualPipe). DeepSeek-V3 trained a 671B MoE with no tensor parallelism, relying on EP + DualPipe + fp8 instead.
• Token dropping vs capacity factor: a capacity factor caps tokens per expert; overflow is dropped or rerouted. Tune to balance throughput against quality.
• Frameworks: Megatron-Core, DeepSpeed-MoE, and nanotron implement EP; torch.distributed provides the all-to-all primitives.

Memory-Efficient Attention

FlashAttention (Dao et al., 2022/2024): reorders attention computation to avoid materializing the full N×N attention matrix. Result: O(N) memory vs O(N²), significant speedup on A100/H100.

# PyTorch ≥2.3: select the Flash backend via the current API
from torch.nn.attention import sdpa_kernel, SDPBackend
with sdpa_kernel(SDPBackend.FLASH_ATTENTION):
    out = F.scaled_dot_product_attention(q, k, v)
# (torch.backends.cuda.sdp_kernel(...) is the deprecated pre-2.3 form)

FlashAttention-3 (2024) targets H100 with further hardware-specific optimizations. xformers provides memory_efficient_attention as an alternative with broader GPU support.

Mixed Precision at Scale

bf16 (bfloat16) is the safe default for A100+ and H100. Same exponent range as fp32 (avoids the overflow spikes common in fp16), 16-bit mantissa precision. torch.cuda.amp.autocast(dtype=torch.bfloat16) or pass torch_dtype=torch.bfloat16. Gradient scaler (torch.cuda.amp.GradScaler) is needed for fp16 but not for bf16.

fp8 is now production-proven on Hopper (H100), not just emerging. DeepSeek-V3 trained at fp8 with fine-grained scaling — per-token 1×128 / per-block 128×128 tiles plus high-precision CUDA-core accumulation — keeping the loss within ~0.25% of bf16. Use TransformerEngine or torchao float8 for the linear layers; keep a bf16/fp32 master copy of weights and the optimizer state. Validate loss-vs-bf16 on your workload before committing a long run.

nvfp4 / fp4 arrives with Blackwell. The B200/GB200 add hardware FP4 (including NVIDIA's NVFP4, 16-element micro-scaled blocks with e4m3 scales, vs MXFP4's 32-element UE8M0 blocks). It has moved past pure research: NVIDIA pre-trained a 12B model on 10T tokens with NVFP4 matching the fp8 baseline (arXiv 2509.25149), and MXFP4 needed ~36% more tokens to reach the same loss — so NVFP4 is the stronger FP4 format. Still validate against bf16/fp8 on your own workload before a long run; the recipe (which tensors stay higher-precision, scaling, outlier handling) is less battle-tested than fp8.

Hardware Tiers (mid-2026)

• A10G / L4 — cheap debug and architecture validation. Native bf16 on L4.
• A100 80GB — bf16 workhorse; still common and cost-effective on spot.
• H100 — bf16 + fp8 (TransformerEngine), FlashAttention-3, NVLink/NVSwitch domains. Now the mainstream production tier, not the frontier.
• Blackwell B200 / GB200 NVL72 — mainstream frontier: fp4/nvfp4 hardware, ~2–3× faster training than H100, and a 72-GPU NVLink domain (NVL72) that lets EP/TP span a whole rack at NVLink bandwidth. Widely available on major clouds by mid-2026.
• Rubin / Vera Rubin NVL72 — newest generation: entered production ~June 2026, with cloud/neocloud availability (AWS, GCP, Azure, CoreWeave, Lambda, Nebius) rolling out through H2 2026. Treat as capacity-constrained and premium-priced for now; verify current availability and quoted pricing before planning around it. Reserve any Blackwell/Rubin tier for frontier-scale runs, not 124M debugging.

torch.compile

torch.compile(model) (TorchInductor) fuses kernels and is essential for competitive MFU on modern hardware. It composes with FSDP2 and is on by default in torchtitan. Compile once outside the training loop; expect a warm-up cost on the first steps. Pair with bf16/fp8 — most of the published MFU numbers assume compile is on.

Activation / Gradient Checkpointing

torch.utils.checkpoint.checkpoint(function, *args) recomputes activations during the backward pass instead of storing them. Reduces activation memory by ~sqrt(layers), adds ~33% compute overhead. Use on every transformer block for large models.

Selective checkpointing (checkpointing only the memory-expensive ops like attention) is supported in FSDP via checkpoint_wrapper.

Optimizers at Scale

AdamW remains the default and the best-understood choice. Its memory cost (two fp32 moments ≈ 2× params) is what ZeRO/FSDP optimizer-state sharding targets.

Muon / MuonClip is the notable frontier shift since 2024. Muon (Keller Jordan, originating in modded-nanoGPT speedruns) applies Newton–Schulz orthogonalization to 2-D matmul weight matrices, treating each weight as a matrix rather than a flat vector. It delivers roughly 1.3–1.5× token efficiency over AdamW on pre-training and holds the modded-nanoGPT GPT-2 speed records. "Muon is Scalable for LLM Training" (arXiv 2502.16982) supplied the two fixes — weight decay and per-parameter update-scale adjustment — that make it work at scale without bespoke tuning. By 2026 it trains trillion-param MoE in production: MuonClip (a stability-clamped variant) pre-trained Moonshot's Kimi K2 (1T-param MoE, 15.5T tokens; arXiv 2507.20534), and Muon now also underpins DeepSeek-V4 (arXiv 2606.19348 names Muon explicitly for faster convergence and stability) and GLM-4.5/GLM-5 (Zhipu/Z.ai; GLM-5's paper, arXiv 2602.15763, adds a "Muon Split" per-head orthogonalization plus a zero-redundancy distributed Muon implementation), plus Karpathy's nanochat. It is a serious AdamW replacement at frontier scale, not a speedrun curiosity. Re-verify against the primary report before quoting exact figures — these are recent releases and details can be revised.

Practical notes:

• Apply Muon only to 2-D matmul parameters; keep embeddings, the LM head, biases, and norm/scalar params on AdamW (a hybrid optimizer).
• Muon's per-step orthogonalization adds compute but less optimizer memory than Adam's two moments — a useful trade under memory pressure.
• For distributed use, see the DeepSpeed Muon integration; sharding Muon's update across DP ranks needs care.

Pre-Training Frameworks

Reproducing GPT-2 124M (Reference Run)

Target: ~3.28 loss on FineWeb/Hellaswag after ~10B tokens.

Using llm.c or modded-nanoGPT:

1. Download FineWeb-edu 10B token sample.
2. Set batch size to fill GPU memory (gradient accumulation for logical large batch).
3. Enable FlashAttention, bf16, gradient checkpointing.
4. Run for ~10B tokens; monitor loss curve and MFU (model FLOP utilization).
5. Cost estimate: ~$50-150 on 4×A100 80 GB on RunPod spot (8-12 hours).

Full GPT-2 124M with full convergence checks: ~$100-300 depending on GPU type and provider.

Checkpointing at Scale (DCP)

For sharded training (FSDP2, TP, PP), a single-rank state_dict is the wrong pattern — it forces an all-gather of the full model onto one rank and serializes saving. Use torch.distributed.checkpoint (DCP):

• Each rank saves its own shard in parallel; DCP handles resharding on load (save on 8 GPUs, resume on 16).
• dcp.async_save offloads the write so training continues while the checkpoint flushes to storage — critical on spot instances where every minute of stall is wasted cost.
• Save model + optimizer + dataloader/RNG state together so a resume is bit-for-bit resumable, not just architecturally loadable.

import torch.distributed.checkpoint as dcp
state = {"model": model, "optim": optimizer}        # FSDP2 DTensors handled natively
dcp.async_save(state, checkpoint_id=f"s3://bucket/run/step-{step}")

Cost Estimation

cost ≈ $/GPU-hr × num_GPUs × training_hours

Example: 4×A100 80GB at $2.50/GPU-hr × 10 hours = $100

Always check current spot pricing on RunPod, Lambda Labs, Vast.ai, or Modal before budgeting. Spot/interruptible discounts are typically 30-70% off on-demand.

Rented GPU Cost Discipline

• Debug on the smallest GPU that fits (A10G at ~$0.60/hr vs H100 at ~$4/hr).
• Spot / interruptible instances: 30-70% cheaper; checkpoint every N steps (not just every epoch).
• Checkpoint to object storage immediately (S3-compatible): rclone, aws-cli, or provider's SDK.
• Per-second billing: terminate as soon as training ends; do not leave instances idle.
• Validate checkpoint restore before starting a long run on spot.
• Estimate before running: use the formula above; add 20% buffer for profiling/debugging.

Known Traps

• Debugging on an 8xH100 box: expensive and unnecessary; always debug on the smallest GPU first.
• OOM blamed on GPUs when the real cause is the dataloader or precision bug: profile first with torch.profiler; check torch.cuda.memory_summary().
• Forgetting to stop the instance: set a billing alert and calendar reminder; auto-shutdown scripts on training completion.
• Not checkpointing on spot instances: a preemption without a recent checkpoint loses hours of training.
• Using fp16 instead of bf16 on A100+: fp16 is more prone to loss spikes at pre-training scale; bf16 is safer and equally fast on A100/H100.
• Mixing FSDP + DeepSpeed: incompatible; pick one sharding framework per run.
• Skipping profiling and tuning MFU: low MFU (below 30%) means the run is communication or dataloader bound, not compute bound. Fix before scaling.
• Using FSDP1 in a new project: deprecated since PyTorch 2.11; start on FSDP2 (fully_shard) or inherit unmaintained APIs.
• Saving a full state_dict from sharded training: all-gathers the whole model onto one rank and stalls every other GPU. Use DCP (dcp.async_save) instead.
• Loss spikes at scale, then NaN: not always the optimizer. Common causes are fp16 instead of bf16, missing/late LR warmup, no gradient clipping, or unstable attention logits — mitigate with bf16, grad-clip, QK-norm, and (for MoE) z-loss. Save a checkpoint immediately before resuming from a spike.
• MoE without a load-balancing loss: a few experts saturate while the rest idle, tanking effective throughput and quality. Use an aux load-balancing loss or DeepSeek-V3's bias-update scheme.
• fp8/nvfp4 without a bf16 baseline: low-precision training can silently degrade loss. Always validate against bf16 on your own workload before a long run.

Common Anti-Patterns

• Adding more GPUs before understanding the current bottleneck.
• Using ZeRO-3 or tensor parallelism before the model is too large for simpler strategies.
• Training without a checkpoint on spot instances.
• Leaving an idle GPU instance running while reviewing results.
• Treating a DeepSpeed config from a blog post as production-ready without profiling on your workload.
• Using full fp32 training at scale (memory waste; use bf16 + loss scaling instead).

Core Principles

1. Profile before you scale: identify the bottleneck (compute, memory, communication, or dataloader) at small scale before spending on more GPUs.
2. Smallest GPU that fits for dev: use A10G or L4 for debugging and architecture validation; reserve H100 for production runs.
3. Understand one parallelism strategy fully before combining: DDP → FSDP → 3-D parallel; do not combine before each layer is mastered.
4. Checkpoint often on interruptible: save to durable object storage every N steps; test restore before long runs.
5. Stop the instance: training end ≠ task end; terminate, verify persistence, confirm billing stopped.

Navigation: Core References

• FSDP vs ZeRO Comparison - side-by-side tradeoffs, when to pick each, config examples
• Parallelism Strategies - DDP, tensor, pipeline, 3-D parallel decision guide
• Rented GPU Cost Guide - provider comparison, spot strategies, cost estimation

External Sources

See data/sources.json for curated primary sources across:

• FlashAttention 1/2/3 papers and implementation
• DeepSpeed ZeRO documentation and ZeRO paper
• PyTorch FSDP2 tutorial and torchtitan paper
• Megatron-LM (tensor parallel) and activation-recomputation papers
• DeepSeek-V3 (DualPipe / expert parallelism / fp8) and Ring Attention (context parallel)
• Muon / Kimi K2, DeepSeek-V4, and GLM-5 optimizer reports
• Karpathy's GPT-2 reproduction projects (llm.c, modded-nanoGPT)
• Rented GPU provider pricing and docs

Fact-Checking

• GPU pricing, spot availability, and provider features change frequently. Always verify against current provider pricing pages before budgeting.
• FlashAttention version support varies by GPU architecture. Check the official repo for your target hardware.
• DeepSpeed ZeRO-Infinity NVMe offload performance depends heavily on NVMe bandwidth; benchmark before relying on it.
• Framework releases (torchtitan, nanotron, litgpt) move fast; verify current API against the repo's main branch.
• Known bugs, regressions, and framework footguns must be verified against current primary sources before being treated as fact.

Learnings Loop

Before applying this skill on a non-trivial task, read learnings.consolidated.md in this directory (and learnings.md if present).

After applying it, if you encountered a pattern worth remembering, a mistake worth preventing, or a domain fact that surprised you, append one dated bullet to learnings.md via agents-skills-feedback-loop/scripts/append_learning.py. Do not modify SKILL.md itself.