nemo-automodel-distributed-training

Distributed Training in NeMo AutoModel

Purpose

NeMo AutoModel uses PyTorch-native distributed training.
All parallelism is orchestrated through a single MeshContext object that
holds device meshes, strategy configs, and axis names.
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Instructions

For conceptual distributed-training questions, answer directly from the quick
patterns in this skill without inspecting the repository. Start with the
strategy choice, then list only the YAML fields and constraints relevant to the
question.

Use direct action verbs in the final answer: recommend the strategy, show the
minimal YAML, state the sizing constraint, and name the unsupported strategies.
Do not discuss model onboarding, recipes, Slurm, SkyPilot, or checkpointing
unless the user asks.

Examples

TP plus PP for a large multi-node model

Recommend strategy: fsdp2. Mention tp_size, pp_size, cp_size,
ep_size, and the pipeline sub-config. State that dp_size is inferred from
world_size / (tp_size * pp_size * cp_size).

distributed:
  strategy: fsdp2
  tp_size: 8
  pp_size: 4
  cp_size: 1
  ep_size: 1
  pipeline:
    pp_schedule: interleaved1f1b
    pp_microbatch_size: 1

MoE expert parallelism

Recommend strategy: fsdp2 with ep_size > 1. Say this creates a separate
moe_mesh; include the moe sub-config when relevant; state that ep_size
must divide dp_size * cp_size. Do not recommend megatron_fsdp or ddp.

distributed:
  strategy: fsdp2
  ep_size: 8
  moe:
    reshard_after_forward: false

MegatronFSDP limitations

Say no for pipeline parallelism, expert parallelism, and sequence_parallel.
Recommend fsdp2 for PP, EP, or sequence_parallel; mention that DDP is only
simple data parallelism.

Strategy Selection

Three strategies are available, selected via the distributed.strategy YAML key:

Strategy	YAML value	Best for
FSDP2	fsdp2	General use, recommended default. Supports TP, PP, CP, EP, HSDP.
MegatronFSDP	megatron_fsdp	NVIDIA Megatron-style FSDP. No PP, no EP, no sequence_parallel.
DDP	ddp	Simple data parallelism only. No TP, PP, CP, or EP.

Decision tree:

• Single GPU: no distributed config needed (FSDP2Manager skips parallelization when world_size=1).
• Multi-GPU single node: fsdp2 (default). Use ddp only if you need the simplest possible setup.
• Multi-node: fsdp2 with appropriate TP/PP sizing.
• MoE models with expert parallelism: fsdp2 with ep_size > 1 (creates a separate moe_mesh).
• Large models (70B+): fsdp2 with PP + TP.
• Long sequences (8K+): add CP (cp_size > 1).

When answering strategy-selection questions, state the chosen distributed.strategy
first, then enumerate the YAML fields the user must set.

Quick TP + PP answer:

• Use strategy: fsdp2; do not use megatron_fsdp when pipeline parallelism is required.
• Set tp_size for tensor parallelism and pp_size for pipeline parallelism.
• Add a pipeline: sub-config with pp_schedule and pp_microbatch_size.
• Leave dp_size unset or none; it is inferred as world_size / (tp_size * pp_size * cp_size).
• Keep TP inside a fast intra-node domain when possible, and use PP across model depth for 70B+ models.

Quick MoE expert-parallel answer:

• Start with strategy: fsdp2 and ep_size > 1.
• Include a moe: sub-config only when ep_size > 1; it maps to MoEParallelizerConfig.
• Expect a separate moe_mesh for expert parallelism in addition to the main device_mesh.
• Do not recommend megatron_fsdp or ddp for expert parallelism; megatron_fsdp has no EP support.
• Before finishing an MoE EP answer, explicitly state that ep_size must divide dp_size * cp_size and that megatron_fsdp does not support EP, PP, or sequence_parallel.

YAML Config Structure

The distributed section in the recipe YAML maps directly to
parse_distributed_section() in recipes/_dist_utils.py:

distributed:
  strategy: fsdp2           # fsdp2 | megatron_fsdp | ddp
  dp_size: none             # auto-calculated from world_size / (tp * pp * cp)
  dp_replicate_size: none   # FSDP2-only, for HSDP
  tp_size: 1
  pp_size: 1
  cp_size: 1
  ep_size: 1

  # Strategy-specific flags (forwarded to the strategy dataclass):
  sequence_parallel: false
  activation_checkpointing: false
  defer_fsdp_grad_sync: true   # FSDP2 only

  # Sub-configs (optional):
  pipeline:
    pp_schedule: 1f1b
    pp_microbatch_size: 1
    # ... see PipelineConfig fields

  moe:
    reshard_after_forward: false
    # ... see MoEParallelizerConfig fields

The dp_size is always inferred:

dp_size = world_size / (tp_size * pp_size * cp_size)

Infrastructure Flow

initialize_distributed()                       [components/distributed/init_utils.py]
    -> initializes torch.distributed process group and returns DistInfo
YAML distributed section + DistInfo.world_size
    -> parse_distributed_section()          [recipes/_dist_utils.py]
    -> create_distributed_setup_from_config()              [recipes/_dist_utils.py]
        -> DistributedSetup.build()         [components/distributed/config.py]
    -> instantiate_infrastructure()         [_transformers/infrastructure.py]
        -> _instantiate_distributed()       -> FSDP2Manager / MegatronFSDPManager / DDPManager
        -> _instantiate_pipeline()          -> AutoPipeline (if pp_size > 1)
        -> parallelize_fn                   -> MoE parallelizer (if ep_size > 1) or PP wrapper
    -> apply_model_infrastructure()         [_transformers/infrastructure.py]
        -> _shard_pp() or _shard_ep_fsdp()  (applies sharding to the model)

FSDP2 Configuration

Basic FSDP2 (data parallelism only)

distributed:
  strategy: fsdp2
  tp_size: 1
  cp_size: 1

This auto-calculates dp_size = world_size and applies fully_shard() per
transformer block via DTensor-based sharding.

FSDP2 with Tensor Parallelism

Keep TP within a single NVLink domain (typically one node):

distributed:
  strategy: fsdp2
  tp_size: 4        # 2, 4, or 8 -- must divide GPUs per node
  sequence_parallel: true

The TP plan is auto-selected based on the model type. Pass a custom plan via
the Python API if needed:

config = FSDP2Config(sequence_parallel=True, tp_plan=my_custom_plan)

FSDP2 with Pipeline Parallelism

distributed:
  strategy: fsdp2
  pp_size: 2
  pipeline:
    pp_schedule: interleaved1f1b   # 1f1b, gpipe, interleaved_1f1b, etc.
    pp_microbatch_size: 4
    scale_grads_in_schedule: false

The model must have a _pp_plan attribute (set on the HF model class) for
AutoPipeline to know how to split layers across stages. Models without
_pp_plan are not compatible with PP.

FSDP2 with HSDP (Hybrid Sharded Data Parallel)

Intra-node full sharding + inter-node replication via a 2D DeviceMesh:

distributed:
  strategy: fsdp2
  dp_replicate_size: 2   # must divide dp_size

Constraint: dp_replicate_size < dp_size (pure replication with no sharding
is not supported by FSDP2).

Activation Checkpointing

Trades compute for memory by recomputing activations during backward:

distributed:
  activation_checkpointing: true

This is a model-build/training behavior flag, not mesh topology. Dense
strategies read it from the strategy config; EP/MoE paths pass the recipe-level
flag directly into model infrastructure.

Gradient Sync Deferral

FSDP2 defers gradient sync to the final micro-batch by default for
communication overlap:

distributed:
  defer_fsdp_grad_sync: true   # default

Mixed Precision

FSDP2Config defaults to bfloat16 for all three precision knobs via
MixedPrecisionPolicy(param_dtype=bf16, reduce_dtype=bf16, output_dtype=bf16, cast_forward_inputs=True). Override via the Python API:

from torch.distributed.fsdp import MixedPrecisionPolicy
config = FSDP2Config(
    mp_policy=MixedPrecisionPolicy(param_dtype=torch.float16, reduce_dtype=torch.float32),
)

Pipeline Parallelism

Requirements

1. Model class must define _pp_plan (a dict mapping module FQNs to stages).
2. pp_size > 1 in the distributed section.
3. A pipeline sub-config with schedule and microbatch size.

Supported schedules

Defined in PipelineConfig.pp_schedule:

• 1f1b (one-forward-one-backward, default)
• gpipe
• interleaved_1f1b / interleaved1f1b
• looped_bfs
• dfs
• v_schedule
• zero_bubble

Example (8B model on 8 GPUs, PP=2 + DP=4)

distributed:
  strategy: fsdp2
  pp_size: 2

  pipeline:
    pp_schedule: interleaved1f1b
    pp_microbatch_size: 4
    scale_grads_in_schedule: false

checkpoint:
  model_save_format: safetensors
  save_consolidated: final

How it works

AutoPipeline.build() calls pipeline_model() which splits the model into
stages using the model's _pp_plan, creates PipelineStage objects, and
builds the schedule. During training, schedule.step() drives forward and
backward through the pipeline.

Context Parallelism

Use CP for long sequences (8K+). CP shards Q/K/V on the sequence dimension
as DTensors.

Config

distributed:
  strategy: fsdp2
  cp_size: 2   # or 4, 8

Requirements

• SDPA (Flash Attention or Efficient Attention backend) or Transformer Engine
  attention. SDPBackend.MATH is not compatible with DTensor.
• Attention masks are automatically stripped; is_causal=True is set via
  forward pre-hooks registered by attach_context_parallel_hooks().

How it works

1. After model sharding, apply_model_infrastructure() calls
   attach_context_parallel_hooks() on each model part (for non-TE models).
2. At each training step, make_cp_batch_and_ctx() creates a CP context
   manager that shards the batch along the sequence dimension and sets up
   context_parallel() from torch.distributed.tensor.experimental.
3. For TE attention models, make_cp_batch_for_te() uses THD format and
   TE's thd_get_partitioned_indices for sharding.

CP with Sequence Packing

CP works with packed sequences. The packed_sequence_size must be divisible
by cp_size. When using TE, chunks are sharded per-chunk via
_shard_thd_chunk_for_te().

Sequence Packing

Packing multiple sequences into a single training sample for efficiency.

Config

packed_sequence:
  packed_sequence_size: 4096   # 0 = disabled

step_scheduler:
  local_batch_size: 1          # must be 1 for packed sequences

When packed_sequence_size > 0, the dataset collator packs sequences up to
that length. local_batch_size must be 1 because each "sample" is already a
packed batch.

MoE Distributed Training

Expert Parallelism

Set ep_size > 1 to distribute experts across GPUs. This creates a separate
moe_mesh alongside the main device_mesh:

distributed:
  strategy: fsdp2
  ep_size: 8
  activation_checkpointing: true

The moe_mesh shape is (pp_size, ep_shard_size, ep_size) with dimension
names ("pp", "ep_shard", "ep").

Constraint: dp_cp_size (= dp_size * cp_size) must be divisible by
ep_size.

MoE sub-config

distributed:
  strategy: fsdp2
  ep_size: 8
  activation_checkpointing: true

  moe:
    reshard_after_forward: false
    ignore_router_for_ac: false
    wrap_outer_model: true

The moe sub-section maps to MoEParallelizerConfig and is only
instantiated when ep_size > 1.

Full MoE example (Qwen3-30B-A3B on 8 GPUs)

distributed:
  strategy: fsdp2
  tp_size: 1
  cp_size: 1
  pp_size: 1
  ep_size: 8
  sequence_parallel: false
  activation_checkpointing: true

MegatronFSDP limitations

Despite its name, megatron_fsdp does not support expert parallelism
(ep_size > 1), pipeline parallelism (pp_size > 1), or
sequence_parallel. Use fsdp2 for these features.

Parallelism Sizing Guidelines

Dense models

Model size	TP	PP	CP	Strategy
< 3B	1	1	1	FSDP2 (DP only)
3-13B	2-4	1	1	FSDP2 + TP
13-70B	4-8	2-4	1	FSDP2 + TP + PP
70B+	8	4-8	1	FSDP2 + TP + PP
Any + long seq (8K+)	as above	as above	2-8	add CP

MoE models

MoE models need less TP than dense models of similar total parameter count
because only a fraction of parameters are active per token. EP is the primary
scaling dimension:

Model	TP	PP	EP	Notes
Small MoE (<10B total)	1	1	8	EP only
Medium MoE (10-30B total)	1-2	1	8	small TP for shared layers
Large MoE (100B+ total)	1-2	4+	8-64	PP for depth, EP for experts

Hardware topology rules

• TP must stay within a single NVLink domain (one node, typically 8 GPUs).
• Use PP or DP for cross-node scaling.
• TP across InfiniBand degrades throughput severely.

Programmatic API (from_pretrained / from_config)

When not using YAML recipes, configure distributed training via Python:

from nemo_automodel.components.distributed import (
    DistributedSetup,
    FSDP2Config,
    ParallelismSizes,
    initialize_distributed,
)

dist_env = initialize_distributed("nccl")
distributed_setup = DistributedSetup.build(
    strategy=FSDP2Config(sequence_parallel=True),
    parallelism_sizes=ParallelismSizes(tp_size=2),
    activation_checkpointing=True,
    world_size=dist_env.world_size,
)

Or pass directly to from_pretrained:

from nemo_automodel import NeMoAutoModelForCausalLM

model = NeMoAutoModelForCausalLM.from_pretrained(
    "meta-llama/Llama-3.2-1B",
    distributed_setup=distributed_setup,
)

Code Anchors

Strategy config dataclasses:

components/distributed/config.py
    FSDP2Config       -- sequence_parallel, tp_plan, mp_policy, offload_policy,
                         activation_checkpointing, defer_fsdp_grad_sync
    MegatronFSDPConfig -- zero_dp_strategy, overlap_grad_reduce, overlap_param_gather, etc.
    DDPConfig          -- activation_checkpointing only

MeshContext (single source of truth for parallelism):

components/distributed/mesh.py
    MeshContext  -- device_mesh, moe_mesh
                    Properties: tp_size, pp_size, cp_size, ep_size, dp_size, dp_replicate_size
    MeshAxisName -- PP, DP, DP_REPLICATE, DP_SHARD, DP_SHARD_CP, DP_CP, CP, TP, EP, EP_SHARD

Mesh context and raw mesh creation:

components/distributed/config.py
    DistributedSetup.build()      -- builds MeshContext from strategy + parallelism
components/distributed/mesh_utils.py
    _create_device_meshes()       -- routes to FSDP2/MegatronFSDP/DDP raw mesh creation
    _create_fsdp2_device_mesh()   -- shape (pp, dp_replicate, dp_shard, cp, tp) + flattened submeshes
    _create_megatron_fsdp_device_mesh() -- shape (dp, cp, tp)

Distributed managers:

components/distributed/fsdp2.py          -- FSDP2Manager.parallelize()
components/distributed/megatron_fsdp.py  -- MegatronFSDPManager.parallelize()
components/distributed/ddp.py            -- DDPManager

Pipeline parallelism:

components/distributed/pipelining/config.py        -- PipelineConfig dataclass
components/distributed/pipelining/autopipeline.py  -- AutoPipeline orchestrator
components/distributed/pipelining/functional.py    -- pipeline_model(), schedule creation
components/distributed/pipelining/hf_utils.py      -- HF model validation for PP

Context parallelism:

components/distributed/context_parallel/utils.py
    make_cp_batch_and_ctx()            -- creates CP context manager + shards batch
    create_context_parallel_ctx()      -- wraps torch.distributed.tensor.experimental.context_parallel
    attach_context_parallel_hooks()    -- strips attention_mask, sets is_causal=True
    make_cp_batch_for_te()             -- TE-specific CP batch sharding (THD format)

Infrastructure orchestration:

_transformers/infrastructure.py
    instantiate_infrastructure()    -- config objects -> runtime objects
    apply_model_infrastructure()    -- applies sharding, PEFT, checkpoints to model
    _shard_pp()                     -- pipeline parallel path
    _shard_ep_fsdp()                -- EP + FSDP path (non-PP)

YAML parsing:

recipes/_dist_utils.py
    parse_distributed_section()  -- YAML dict -> typed configs + sizes
    create_distributed_setup_from_config()  -- recipe adapter: parse + create DistributedSetup; does not init process group

MoE config:

components/distributed/config.py
    MoEParallelizerConfig  -- reshard_after_forward, ignore_router_for_ac, wrap_outer_model, etc.
components/moe/config.py
    MoEConfig              -- n_routed_experts, n_activated_experts, score_func, etc.

Pitfalls

1. TP across nodes destroys throughput. Always keep TP within a single
   NVLink domain. Use PP or DP for cross-node scaling.

2. PP requires _pp_plan on the model class. Not all HF models have this.
   Check validate_hf_model_for_pipeline_support() before enabling PP.

3. PP bubbles reduce GPU utilization. Use interleaved schedules
   (interleaved_1f1b) and smaller microbatches to reduce bubble time.

4. FSDP2 requires DTensor-aware state dict saving. Use safetensors with
   save_consolidated: final for final HF export, or save_consolidated: false
   plus the generated model/consolidate.sh helper for offline export.

5. CP requires compatible attention. SDPA (Flash Attention or Efficient
   Attention) or TE attention only. SDPBackend.MATH is not compatible with
   DTensor.

6. MoE EP size must evenly divide dp_size * cp_size. The device mesh
   creation asserts dp_cp_size % ep_size == 0.

7. MegatronFSDP is more limited than FSDP2. It does not support PP
   (pp_size > 1), EP (ep_size > 1), or sequence_parallel. The
   MeshContext validation raises on these combinations.

8. DDP supports nothing beyond data parallelism. No TP, PP, CP, EP, or
   HSDP. Validation raises on any of these.

9. Activation checkpointing increases compute. It saves memory by
   recomputing activations during backward, but adds ~30% compute overhead.

10. Mixed precision policy must match model expectations. The default
    bfloat16 policy works for most models. FP16 models may need a custom
    MixedPrecisionPolicy.

11. packed_sequence_size must be divisible by cp_size when using CP
    with packed sequences.

12. dp_replicate_size is FSDP2-only. Passing it with megatron_fsdp
    or ddp raises a ValueError.

Verification

Run the smallest recipe that exercises the requested strategy. Success means
exit code 0, finite loss, no NCCL timeout, and log output matching the expected
TP/PP/CP/EP sizes.