nemo-mbridge-perf-activation-recompute

Activation Recompute

Stable docs: @docs/training/activation-recomputation.md Card: @skills/nemo-mbridge-perf-activation-recompute/card.yaml

Activation recompute (activation checkpointing) trades additional forward work during backward for lower retained-activation memory. The useful checkpoint boundary depends on the model architecture, attention backend, parallelism, and the tensor that actually drives the per-rank peak.

Quick Decision Guide

1. Confirm the pressure is real allocation, not allocator fragmentation. Compare max_memory_allocated() with max_memory_reserved() on every rank.
2. Keep an explicit no-recompute control when the workload fits. Under selective granularity, recompute_modules=[] is valid and useful for this comparison.
3. Select the first boundary from the architecture and observed peak:
   • Standard attention: core_attn is the common first candidate. It is strongest when unfused attention materializes score/probability tensors. With Transformer Engine fused or Flash Attention, compare it against [] because those backends already rematerialize attention internals.
   • Multi-Latent Attention (MLA): start with mla_up_proj when expanded Q/K/V projections dominate. Add core_attn only when the attention-core state still matters.
   • Grouped MoE: start with moe_act when the expert intermediate activation dominates; add layernorm when norm outputs are material. Use whole moe recompute only after accounting for the extra expert compute and communication it replays.
   • Dense FFN: mlp can save the whole dense-MLP activation region, but it usually costs more compute than a narrow output-discard boundary.
4. Change one label at a time. Record per-rank allocated/reserved peaks plus steady-state step time or throughput; do not infer a global module ranking from one recipe.
5. Use full-layer recompute only when targeted selective boundaries do not make the workload fit. Full recompute has the broadest memory effect and the largest replay cost.
6. Treat CUDA graphs, FP8, context-parallel communication, and overlap features as compatibility constraints, not afterthoughts.

Megatron Core's cpu_offloading=True is an alternative when PCIe/NVLink transfer overhead is preferable to replayed compute. It cannot be combined with activation recompute and is not compatible with pipeline parallelism greater than one.

Enablement

Selective recompute

cfg.model.recompute_granularity = "selective"
cfg.model.recompute_modules = ["core_attn"]  # Common standard-attention candidate, not a universal default.

Use the decision table below to replace or extend that list for MLA, MoE, dense-MLP, or GDN workloads.

Full-layer recompute

cfg.model.recompute_granularity = "full"
cfg.model.recompute_method = "uniform"
cfg.model.recompute_num_layers = 1

• uniform: checkpoint fixed groups of recompute_num_layers transformer layers.
• block: checkpoint the first recompute_num_layers layers on each pipeline stage, with virtual-pipeline-aware distribution.

Selective Module Decision Table

The currently pinned Megatron Core accepts these labels. A development branch can add model-specific labels, so validate against the exact target revision rather than copying a list across branches.

For example, DeepSeek V4 configurations can use the model-specific mhc label only with their required Megatron Core development branch. It is not a portable label for the pinned revision and therefore is not included in the table above.

Common performance configurations consequently fall into several patterns rather than one universal list:

• standard transformer recipes often use core_attn;
• MLA recipes often use mla_up_proj, sometimes with mlp;
• grouped-MoE recipes often use moe_act or layernorm plus moe_act;
• higher-pressure MoE recipes sometimes use broader combinations such as moe plus layernorm.

These are candidate patterns, not an ordering guarantee. Peak attribution and matched measurements decide the final list.

Measurement Contract

For every candidate, capture:

• exact Bridge and Megatron Core revisions;
• model, sequence length, micro/global batch sizes, precision, attention backend, and parallelism;
• the exact recompute_granularity, module list, method, and layer count;
• per-rank max_memory_allocated() and max_memory_reserved();
• steady-state step time or throughput after warmup;
• a short convergence or numerical-sanity check appropriate to the task.

Use a matched no-recompute control and change one recompute choice at a time. Peak memory from different jobs, backends, or parallel layouts is not a module-ranking benchmark.

Do not call a candidate successful merely because it advances farther than the control. Run through optimizer-state initialization and multiple steady-state steps: selective recompute can move the memory wall from forward into gradient synchronization or the optimizer without making the workload viable.

Matched H100 Evidence: Moonlight 16B

A 2026-08-12 short-run study used the exact Bridge revision 600d069b824dd5ce50367a311a5a3244478faf22 and Megatron Core revision 24bad8e677d22625d86ef2a54c9506b6e4992c93. The Moonlight 16B BF16 pretraining recipe ran on 8 H100 80GB GPUs with sequence length 4096, MBS=1, GBS=4, TP=2, PP=1, CP=1, EP=8, mock data, and 20 steps. This model mixes one dense layer with 26 MLA+MoE layers. Each row changed only recompute_modules; all 20 losses were finite with zero skipped or NaN iterations.

Peak allocated memory is the maximum post-optimizer value reported after iteration 2. Time and throughput are means over iterations 11--20.

For this exact workload, moe_act is the best first boundary: it recovered nearly as much allocated memory as mla_up_proj for less replay cost. mla_up_proj is the next candidate if its roughly 39 MB additional reduction matters. Adding mlp to mla_up_proj or layernorm to moe_act did not improve the observed peak and made steps slower. Explicit core_attn added cost without material memory benefit under fused attention.

Maximum reserved memory stayed near 40 GB and did not fall monotonically. That is allocator caching, not contrary evidence: boundary selection in this study is based on allocated memory and successful end-to-end steps.

Matched H100 Evidence: Nemotron 3 Nano

The same 2026-08-12 study used the native 16-H100 BF16 performance recipe for the 52-layer hybrid Mamba/fused-attention MoE model. The matched short-run configuration used sequence length 8192, MBS=1, GBS=16, TP=1, PP=1, CP=1, EP=8, DP=16, expert-DP=2, HybridEP, grouped GEMM, TE CUDA graphs for attention and Mamba, mock data, and 12 steps. Each row changed only recompute_modules.

Both successful rows had finite losses and zero skipped or NaN iterations. For this exact capacity-limited recipe, whole-moe recompute is the smallest tested passing boundary. Adding layernorm recovered another 1.014 GB (1.57%) of rank-0 peak at 3.07% higher step time, so the recipe's broader combination is justified when that headroom is required. Narrow moe_act produced real activation relief but did not make the whole training step viable.

An exploratory native 8-H100 layout failed during FP32 optimizer-state initialization even at sequence length 4096. That is optimizer capacity, not a selective-boundary throughput baseline; no timing comparison from those runs is used here.

Cross-model conclusion

These measurements do not define one ranking. Moonlight fit with an empty control and favored narrow moe_act; Nemotron required broad whole-moe recompute; historical dense Llama evidence found whole-mlp replay costly and lacked an empty control. The correct first candidate is therefore the narrowest boundary implicated by the architecture and peak, followed by broader replay only when the narrow choice does not pass the complete step.

Compatibility and Validation

Configuration semantics

• recompute_granularity="selective" uses recompute_modules; an empty list is accepted as an explicit control.
• recompute_granularity="full" uses recompute_method and recompute_num_layers; selective labels do not apply.
• Full granularity supersedes selective module choices rather than composing with them.
• Unknown labels fail Megatron Core validation. Labels may differ on development branches, so use the exact revision's TransformerConfig validator as the source of truth.

Attention backend and context parallelism

• TE fused and Flash Attention already use internal rematerialization. Explicit core_attn may still change retained inputs/outputs, but it must earn its place in a matched [] comparison.
• Under context parallelism, an attention checkpoint can replay communication as well as compute. Include CP size and topology in the measurement record.

MoE restrictions

• Whole-moe recompute is incompatible with expert-parallel overlap because backward replay would repeat the overlapped routing/communication region.
• shared_experts recompute is incompatible with shared-expert overlap.
• moe_act applies to grouped-GEMM experts and is the narrower choice when only the expert activation needs to be discarded.
• mlp targets dense MLPs and is a no-op on MoE layers; mixed dense/MoE models can still benefit on their dense layers.

FP8 restrictions

• moe_act and layernorm recompute are not supported with FP8 delayed scaling and require a compatible Transformer Engine version.
• Absorbed MLA paths have additional FP8/FP4 restrictions. Validate the exact model/provider path before selecting mla_up_proj.

CUDA graphs

• Selective recompute is valid only when a checkpointed module lies wholly inside or wholly outside the selected graph scope. A checkpoint boundary that straddles a graph boundary is invalid.
• Capture/warmup can bypass checkpoint wrappers, so verify the final graph scope and replay path rather than assuming eager behavior carries over.
• Full recompute with CUDA graphs requires cuda_graph_impl="full_iteration" in the pinned Megatron Core. Otherwise disable CUDA graphs; scoped/local graph capture is not a substitute for full-iteration capture here.

Historical Measurement: Context, Not a Module Ranking

Historical H100 measurements from Bridge PR #3107 used Llama 3 70B SFT on 32 H100 80GB GPUs with FP8 current scaling, sequence length 4096, micro-batch size 1, global batch size 32, TP=4, PP=4, VPP=5, and DP=2:

Limitations of this evidence:

• it did not include a matched no-recompute row;
• the golden row was not a paired module-only comparison;
• the measurements cover one dense Llama workload, not MLA or MoE;
• the table supports the local memory/throughput tradeoff only and must not be used to rank all recompute labels.

Code Anchors

• Selective-label validation and cross-feature checks: 3rdparty/Megatron-LM/megatron/core/transformer/transformer_config.py
• Checkpoint implementations: 3rdparty/Megatron-LM/megatron/core/tensor_parallel/random.py
• Standard-attention checkpoint boundary: 3rdparty/Megatron-LM/megatron/core/transformer/attention.py
• MLA up-projection boundary: 3rdparty/Megatron-LM/megatron/core/transformer/multi_latent_attention.py
• Layernorm, dense-MLP, and outer-MoE placement: 3rdparty/Megatron-LM/megatron/core/transformer/transformer_layer.py
• Grouped expert activation boundary: 3rdparty/Megatron-LM/megatron/core/transformer/moe/experts.py
• Shared-expert and whole-MoE paths: 3rdparty/Megatron-LM/megatron/core/transformer/moe/moe_layer.py
• GDN normalization boundary: 3rdparty/Megatron-LM/megatron/core/ssm/gated_delta_net/gdn.py

Failure Diagnosis

Known Limitations

• A module list is not portable across model families, attention backends, parallel layouts, or Megatron Core revisions.
• Memory savings are nonlinear when boundaries overlap or nest; additive arithmetic is unreliable.
• Full recompute changes RNG execution paths; dropout workloads need a numerical/convergence check.
• Activation recompute does not address parameter, optimizer-state, or allocator-fragmentation pressure.
• The correct result is the smallest measured replay cost that satisfies the per-rank memory target, not the longest module list.

Further Reading

• docs/performance-guide.md
• skills/nemo-mbridge-perf-memory-tuning/SKILL.md
• skills/nemo-mbridge-perf-cuda-graphs/SKILL.md
• skills/nemo-mbridge-perf-cpu-offloading/SKILL.md
• Megatron Core activation recomputation guide: https://docs.nvidia.com/megatron-core/developer-guide/latest/api-guide/index.html