MLM vs Bridge Training

For how they differ, the arg mapping tables, gotchas, and translation script, see:

@docs/megatron-lm-to-megatron-bridge.md

Correlation Testing

Use

vanilla_gpt_pretrain_config

for loss-correlation testing. This recipe uses bare

GPTModelProvider

defaults (LayerNorm, GeLU, learned_absolute position embeddings,

vocab_size

inherited from tokenizer) — matching MLM

pretrain_gpt.py

defaults with no args.

MLM Correlation Run (2L/256H, 1 GPU)

bash

PYTHONPATH=3rdparty/Megatron-LM:$PYTHONPATH \
uv run python -m torch.distributed.run --nproc_per_node=1 \
  3rdparty/Megatron-LM/pretrain_gpt.py \
  --num-layers 2 --hidden-size 256 --num-attention-heads 4 \
  --ffn-hidden-size 1024 --seq-length 512 --max-position-embeddings 512 \
  --micro-batch-size 4 --global-batch-size 32 \
  --train-iters 10 --eval-iters 2 --eval-interval 10 \
  --mock-data --bf16 --use-mcore-models \
  --tokenizer-type NullTokenizer --vocab-size 32000 \
  --lr 3e-4 --min-lr 3e-5 --seed 1234 --log-interval 1

Bridge Correlation Run (same config, 1 GPU)

bash

rm -rf nemo_experiments && \
uv run python -m torch.distributed.run --nproc_per_node=1 \
  scripts/training/run_recipe.py \
  --recipe vanilla_gpt_pretrain_config \
  model.num_layers=2 model.hidden_size=256 \
  model.num_attention_heads=4 model.ffn_hidden_size=1024 \
  model.seq_length=512 dataset.sequence_length=512 \
  train.train_iters=10 train.global_batch_size=32 train.micro_batch_size=4 \
  validation.eval_interval=10 validation.eval_iters=2 \
  optimizer.lr=3e-4 optimizer.min_lr=3e-5 \
  scheduler.lr_warmup_iters=1 scheduler.lr_decay_iters=10 \
  rng.seed=1234 logger.log_interval=1

Verification

With matched parameters the LM losses should be nearly identical at each iteration. Compare

lm loss

values from both logs — they should agree to within BF16 rounding.

Multi-GPU Examples

MLM 2-GPU with TP=2

bash

PYTHONPATH=3rdparty/Megatron-LM:$PYTHONPATH \
uv run python -m torch.distributed.run --nproc_per_node=2 \
  3rdparty/Megatron-LM/pretrain_gpt.py \
  --tensor-model-parallel-size 2 --sequence-parallel \
  --num-layers 4 --hidden-size 256 --num-attention-heads 4 \
  --seq-length 1024 --max-position-embeddings 1024 \
  --micro-batch-size 2 --global-batch-size 16 \
  --train-iters 10 --eval-iters 2 --eval-interval 10 \
  --mock-data --bf16 --use-mcore-models \
  --tokenizer-type NullTokenizer --vocab-size 1024 \
  --lr 1e-4 --log-interval 1

Bridge 2-GPU with TP=2

bash

rm -rf nemo_experiments && \
uv run python -m torch.distributed.run --nproc_per_node=2 \
  scripts/training/run_recipe.py \
  --recipe vanilla_gpt_pretrain_config \
  model.tensor_model_parallel_size=2 model.sequence_parallel=true \
  model.num_layers=4 model.hidden_size=256 \
  model.num_attention_heads=4 model.ffn_hidden_size=1024 \
  model.seq_length=1024 dataset.sequence_length=1024 \
  train.train_iters=10 train.global_batch_size=16 train.micro_batch_size=2 \
  validation.eval_interval=10 validation.eval_iters=2 \
  scheduler.lr_warmup_iters=2 scheduler.lr_decay_iters=10 \
  logger.log_interval=1

Available Recipes

Common recipes (use with

--recipe

```
vanilla_gpt_pretrain_config
```
— Minimal GPT (bare GPTModelProvider defaults, ideal for correlation testing and custom configs)
```
llama32_1b_pretrain_config
```
— Llama 3.2 1B (16L, 2048H, GBS=512, seq=8192)
```
llama3_8b_pretrain_config
```
— Llama 3 8B
```
qwen3_8b_pretrain_config
```
— Qwen3 8B
```
deepseek_v2_lite_pretrain_config
```
— DeepSeek-V2-Lite 16B MoE

SFT/PEFT variants use

_sft_config

_peft_config

suffix.

Megatron-Core Submodule

For what the submodule is and why two versions exist, see @docs/megatron-lm-to-megatron-bridge.md.

Check current version

bash

./scripts/switch_mcore.sh status

Switch to dev for testing newer MCore features

bash

./scripts/switch_mcore.sh dev

# uv sync (without --locked) since lockfile is for main
uv sync

Switch back to main

bash

./scripts/switch_mcore.sh main

After pulling latest main

When you pull the latest Bridge main branch, the submodule pointer may have been updated. Re-sync the submodule:

bash

git submodule update --init 3rdparty/Megatron-LM

Pitfalls

Always
rm -rf nemo_experiments
before a fresh correlation run. Bridge auto-resumes from stale checkpoints silently.

uv run
required: Always use

uv run python -m torch.distributed.run

(not bare

torchrun

python

MLM PYTHONPATH: Must include
```
3rdparty/Megatron-LM
```
so
```
gpt_builders.py
```
is importable.
Scheduler overrides: When overriding
```
train.train_iters
```
to a small value, also set
```
scheduler.lr_warmup_iters
```
and
```
scheduler.lr_decay_iters
```
or you get an assertion error.

Use
dataset.sequence_length
in CLI overrides, not

dataset.seq_length

MoE OOM: Large MoE models require full activation recomputation and typically multi-node EP. TP does NOT reduce per-GPU expert memory.
uv sync --locked
fails after switching to dev: The lockfile is generated against the main MCore commit. Use
```
uv sync
```
(without
```
--locked
```
) when on dev.

mlm-bridge-training

NPX Install

Tags

SKILL.md Content

MLM vs Bridge Training

Correlation Testing

MLM Correlation Run (2L/256H, 1 GPU)

Bridge Correlation Run (same config, 1 GPU)

Verification

Multi-GPU Examples

MLM 2-GPU with TP=2

Bridge 2-GPU with TP=2

Available Recipes

Megatron-Core Submodule

Check current version

Switch to dev for testing newer MCore features

Switch back to main

After pulling latest main

Pitfalls