cuda-graphs

name	cuda-graphs
description	Validate and use CUDA graph capture in Megatron Bridge, including local full-iteration graphs and Transformer Engine scoped graphs for attention, MLP, and MoE modules.

CUDA Graphs

Stable docs: docs/training/cuda-graphs.md Card: card.yaml (co-located)

What It Is

CUDA graphs capture GPU operations once and replay them with minimal host-driver overhead. Bridge supports two implementations:

cuda_graph_impl	Mechanism	Scope support
"local"	MCore FullCudaGraphWrapper wrapping entire fwd+bwd	full_iteration
"transformer_engine"	TE make_graphed_callables() per layer	attn, mlp, moe, moe_router, moe_preprocess, mamba

Enablement

Local full-iteration graph

cfg.model.cuda_graph_impl = "local"
cfg.model.cuda_graph_scope = ["full_iteration"]
cfg.model.cuda_graph_warmup_steps = 3
cfg.model.use_te_rng_tracker = True
cfg.rng.te_rng_tracker = True
cfg.rerun_state_machine.check_for_nan_in_loss = False
cfg.ddp.check_for_nan_in_grad = False

TE scoped graph (dense model)

cfg.model.cuda_graph_impl = "transformer_engine"
cfg.model.cuda_graph_scope = ["attn"]           # or ["attn", "mlp"]
cfg.model.cuda_graph_warmup_steps = 3
cfg.model.use_te_rng_tracker = True
cfg.rng.te_rng_tracker = True

TE scoped graph (MoE model)

cfg.model.cuda_graph_impl = "transformer_engine"
cfg.model.cuda_graph_scope = ["attn", "moe_router", "moe_preprocess"]
cfg.model.cuda_graph_warmup_steps = 3
cfg.model.use_te_rng_tracker = True
cfg.rng.te_rng_tracker = True

Performance harness CLI

python scripts/performance/run_performance_workload.py \
  --cuda_graph_impl transformer_engine \
  --cuda_graph_scope attn moe_router moe_preprocess \
  ...

Valid CLI values live in scripts/performance/argument_parser.py:

• VALID_CUDA_GRAPH_IMPLS: ["none", "local", "transformer_engine"]
• VALID_CUDA_GRAPH_SCOPES: ["full_iteration", "attn", "mlp", "moe", "moe_router", "moe_preprocess", "mamba"]

Required constraints

• use_te_rng_tracker = True (enforced in gpt_provider.py)
• full_iteration scope only with cuda_graph_impl = "local"
• full_iteration scope requires check_for_nan_in_loss = False
• Do not combine moe scope and moe_router scope
• Tensor shapes must be static (fixed seq_length, fixed micro_batch_size)
• MoE token-dropless routing limits graphable scope to dense modules
• With PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True, set NCCL_GRAPH_REGISTER=0 (MCore enforces for local impl on arch < sm_100; TE impl asserts unconditionally)
• CPU offloading is incompatible with CUDA graphs
• moe_preprocess scope requires moe_router scope to also be set

Code Anchors

Bridge config and validation

        # CUDA graph scope validation: check_for_nan_in_loss must be disabled with full_iteration graph
        if self.model.cuda_graph_impl == "local" and CudaGraphScope.full_iteration in self.model.cuda_graph_scope:
            assert not self.rerun_state_machine.check_for_nan_in_loss, (
                "check_for_nan_in_loss must be disabled when using full_iteration CUDA graph. "
                "Set rerun_state_machine.check_for_nan_in_loss=False."
            )
        if self.model.cuda_graph_impl == "none":
            self.model.cuda_graph_scope = []

TE RNG tracker requirement

        if self.cuda_graph_impl != "none":
            assert getattr(self, "use_te_rng_tracker", False), (
                "Transformer engine's RNG tracker is required for cudagraphs, it can be "
                "enabled with use_te_rng_tracker=True'."

Graph creation and capture in training loop

    # Capture CUDA Graphs.
    cuda_graph_helper = None
    if model_config.cuda_graph_impl == "transformer_engine":
        cuda_graph_helper = TECudaGraphHelper(...)
    # ...
    if config.model.cuda_graph_impl == "local" and CudaGraphScope.full_iteration in config.model.cuda_graph_scope:
        forward_backward_func = FullCudaGraphWrapper(
            forward_backward_func, cuda_graph_warmup_steps=config.model.cuda_graph_warmup_steps
        )

TE graph capture after warmup

        # Capture CUDA Graphs after warmup.
        if (
            model_config.cuda_graph_impl == "transformer_engine"
            and cuda_graph_helper is not None
            and not cuda_graph_helper.graphs_created()
            and global_state.train_state.step - start_iteration == model_config.cuda_graph_warmup_steps
        ):
            if model_config.cuda_graph_warmup_steps > 0 and should_toggle_forward_pre_hook:
                disable_forward_pre_hook(model, param_sync=False)
            cuda_graph_helper.create_cudagraphs()
            if model_config.cuda_graph_warmup_steps > 0 and should_toggle_forward_pre_hook:
                enable_forward_pre_hook(model)
                cuda_graph_helper.cuda_graph_set_manual_hooks()

RNG initialization

        _set_random_seed(
            rng_config.seed,
            rng_config.data_parallel_random_init,
            rng_config.te_rng_tracker,
            rng_config.inference_rng_tracker,
            use_cudagraphable_rng=(model_config.cuda_graph_impl != "none"),
            pg_collection=pg_collection,
        )

Delayed wgrad + CUDA graph interaction

            cuda_graph_scope = getattr(model_cfg, "cuda_graph_scope", []) or []
            # ... scope parsing ...
            if wgrad_in_graph_scope:
                assert is_te_min_version("2.12.0"), ...
                assert model_cfg.gradient_accumulation_fusion, ...
                if attn_scope_enabled:
                    assert not model_cfg.add_bias_linear and not model_cfg.add_qkv_bias, ...

Perf harness override helper

def _set_cuda_graph_overrides(
    recipe, cuda_graph_impl=None, cuda_graph_scope=None
):
    # Sets impl, scope, and auto-enables te_rng_tracker

Graph cleanup

def _delete_cuda_graphs(cuda_graph_helper):
    # Deletes FullCudaGraphWrapper and TE graph objects to free NCCL buffers

MCore classes (in 3rdparty/Megatron-LM)

• CudaGraphManager: megatron/core/transformer/cuda_graphs.py
• TECudaGraphHelper: megatron/core/transformer/cuda_graphs.py
• FullCudaGraphWrapper: megatron/core/full_cuda_graph.py
• CudaGraphScope enum: megatron/core/transformer/enums.py

Positive recipe anchors

• scripts/performance/configs/deepseek/deepseek_workload_base_configs.py
• scripts/performance/configs/qwen/qwen3_workload_base_configs.py
• scripts/performance/configs/gpt_oss/gpt_oss_workload_base_configs.py

Tests

File	Coverage
tests/unit_tests/training/test_config.py	full_iteration NaN-check constraint
tests/unit_tests/training/test_comm_overlap.py	delay_wgrad + CUDA graph interaction
tests/unit_tests/models/test_gpt_full_te_layer_autocast_spec.py	TE autocast with CUDA graphs
tests/functional_tests/recipes/test_llama_recipes_pretrain_cuda_graphs.py	End-to-end local and TE graph smoke tests
tests/unit_tests/recipes/kimi/test_kimi_k2.py	TE + CUDA graph recipe config
tests/unit_tests/recipes/gpt/test_gpt3_175b.py	TE + CUDA graph recipe config
tests/unit_tests/recipes/qwen_vl/test_qwen25_vl_recipes.py	VLM CUDA graph settings

Pitfalls

1. TE RNG tracker is mandatory: Setting cuda_graph_impl without use_te_rng_tracker=True and rng.te_rng_tracker=True will assert in the provider.

2. full_iteration requires NaN checks disabled: The entire fwd+bwd is captured, so loss-NaN checking cannot inspect intermediate values.

3. MoE scope restrictions: moe scope and moe_router scope are mutually exclusive. Token-dropless MoE can only graph moe_router and moe_preprocess, not the full expert dispatch.

4. Memory overhead: CUDA graphs pin all intermediate buffers for the graph's lifetime (no memory reuse). TE scoped graphs add a few GB; full-iteration graphs can increase peak memory by 1.5–2×. PP > 1 compounds overhead since each stage holds its own graph.

5. Delayed wgrad interaction: When delay_wgrad_compute=True and attention or MoE router is in cuda_graph_scope, additional constraints apply: TE >= 2.12.0, gradient_accumulation_fusion=True, and no attention bias.

6. Variable-length sequences break graphs: Sequence lengths must be constant across steps. Use padded packed sequences if packing is needed.

7. Graph cleanup is required: CUDA graph objects hold NCCL buffer references. Bridge handles this in _delete_cuda_graphs() at the end of training, but early exits must call it explicitly.

8. Older GPU architectures: On GPUs with compute capability < 10.0 (pre-Blackwell), set NCCL_GRAPH_REGISTER=0 when using PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True. Enforced in MCore CudaGraphManager (cuda_graphs.py:1428) and TECudaGraphHelper (cuda_graphs.py:1697). The TE impl asserts unconditionally regardless of arch.

9. CPU offloading incompatible: CUDA graphs cannot be used with CPU offloading. Enforced in MCore transformer_config.py:1907.

10. MoE recompute + moe_router scope: MoE recompute is not supported with moe_router CUDA graph scope when using cuda_graph_impl = "transformer_engine". Enforced in MCore transformer_config.py:1977.

Verification

Unit tests

uv run python -m pytest \
  tests/unit_tests/training/test_config.py -k "cuda_graph" \
  tests/unit_tests/training/test_comm_overlap.py -k "cuda_graph" \
  tests/unit_tests/models/test_gpt_full_te_layer_autocast_spec.py -k "cuda_graph" -q

Functional smoke test (requires GPU)

uv run python -m pytest \
  tests/functional_tests/recipes/test_llama_recipes_pretrain_cuda_graphs.py -q

Success criteria

• Unit tests pass, covering config validation for both local and transformer_engine implementations.
• Functional test completes training steps with both CUDA graph implementations.
• No NCCL errors or illegal memory access in logs.