| name | moe-comm-overlap |
| description | MoE expert-parallel communication overlap in Megatron Bridge. Use when the user asks about overlap_moe_expert_parallel_comm, MoE dispatch overlap, flex dispatcher, DeepEP overlap, or expert wgrad scheduling. |
MoE Communication Overlap
For what MoE communication overlap is and when to use it, see:
docs/training/communication-overlap.md
card.yaml (co-located)
Enablement
cfg.comm_overlap.overlap_moe_expert_parallel_comm = True
cfg.model.moe_delay_wgrad_compute = True
cfg.model.moe_shared_expert_overlap = False
Prerequisites
expert_model_parallel_size > 1
num_moe_experts > 1
moe_token_dispatcher_type must be "alltoall" or "flex"
- Precision: BF16 or FP16
- If PP is used, VPP (
virtual_pipeline_model_parallel_size) must be set (non-None)
Flex dispatcher activation
Setting moe_flex_dispatcher_backend alone does not activate flex dispatch.
You must also set moe_token_dispatcher_type = "flex".
Code Anchors
- Overlap validation:
src/megatron/bridge/training/comm_overlap.py
- Flex dispatcher backend:
src/megatron/bridge/training/flex_dispatcher_backend.py
- Config:
src/megatron/bridge/training/config.py
- Unit tests:
tests/unit_tests/training/test_comm_overlap.py
- DeepEP tests:
tests/unit_tests/training/test_deepep.py
Pitfalls
-
Shared expert overlap conflict: moe_shared_expert_overlap and
overlap_moe_expert_parallel_comm can conflict. Disable shared expert
overlap when using the dispatch overlap path.
-
PP without VPP: MoE overlap requires VPP when pipeline parallelism is
active. Without it, the overlap scheduling cannot interleave correctly.
-
Flex != backend flag: moe_flex_dispatcher_backend="deepep" alone
does nothing if moe_token_dispatcher_type is still "alltoall".
-
Conservative recipe defaults: Most public recipes leave MoE overlap
disabled. You need to explicitly enable it via overrides.
Verification
Look for overlap-related log messages during initialization. The comm overlap
validation in comm_overlap.py will raise if prerequisites are not met, so a
clean startup confirms the feature is active.