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capture-nsys-profile

Testing & Quality
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Capture a Nsight Systems (.nsys-rep) profile of a short PithTrain run for performance analysis. Use when the user asks to "capture an nsys profile", "profile training", or "grab an nsys trace", or wants to inspect kernel timelines / pipeline behavior / all-to-all overheads. Adaptive over pipeline-parallel (PP), expert-parallel (EP), context-parallel (CP), and sequence length; size the global batch so the pipeline reaches steady state without producing a multi-GB .nsys-rep. Run 5 warmup steps + 1 profiled step from a released checkpoint.

QUICK START

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Capture Nsys Profile

Capture a single-step Nsight Systems (.nsys-rep) trace of PithTrain training, loaded from a released HuggingFace checkpoint so MoE load balancing is representative. The profile configuration (PP / EP / CP / sequence length) is specified at launch time; micro-batch size is hardcoded to 1.

Prerequisites

  • Python environment: activate .venv in the repo root (source .venv/bin/activate).
  • Nsight Systems CLI: nsys --version must work on every compute node.
  • Hardware: enough GPUs to satisfy world_size >= PP * CP * EP (with at least DP >= 1). See Step 2.
  • Benchmark inputs set up: the setup-benchmark-inputs skill has run for the target model (produces the tokenized corpus and DCP checkpoint capture loads).

Step 1: Choose parallelism

The user typically specifies PP and EP (and sometimes CP / sequence length). Confirm the numbers before launching:

  • --model: one of the supported models
  • --pipeline-parallel-size: PP
  • --expert-parallel-size: EP
  • --context-parallel-size: CP (default 1)
  • --sequence-length: sequence length in tokens (default 2048)

If the user is vague ("just profile DeepSeek-V2-Lite"), ask for PP and EP before launching. Different parallelism splits surface different bottlenecks, so the right config depends on what they want to see.

Step 2: Determine node count

Target DP=1 (smallest world that satisfies the parallelism plan). Assuming 8 GPUs per node:

ConfigPP * CP * EPMin nodes (8 GPUs/node)
pp=2 cp=1 ep=241 (half of an 8-GPU node)
pp=2 cp=1 ep=8162
pp=4 cp=1 ep=8324

If running under SLURM

If $SLURM_JOB_ID is set, use the launch-with-slurm skill to read the allocation's node count and compare it to the minimum above. If the allocation is short, surface that to the user instead of launching.

Step 3: Capture the profile

# Single-node, minimum GPUs (DeepSeek-V2-Lite, pp=2 ep=2)
bash .agents/skills/capture-nsys-profile/scripts/launch_capture.sh --model deepseek-v2-lite --pipeline-parallel-size 2 --expert-parallel-size 2

# Multi-node via SLURM (Qwen3-30B-A3B, pp=2 ep=8 -> 2 nodes)
srun -N 2 -W 0 .agents/skills/capture-nsys-profile/scripts/launch_capture.sh --model qwen3-30b-a3b --pipeline-parallel-size 2 --expert-parallel-size 8

# Custom sequence length (Qwen3-30B-A3B, pp=4 ep=8 -> 4 nodes, seq=4096)
srun -N 4 -W 0 .agents/skills/capture-nsys-profile/scripts/launch_capture.sh --model qwen3-30b-a3b --pipeline-parallel-size 4 --expert-parallel-size 8 --sequence-length 4096

Output

Each node produces one .nsys-rep at workspace/capture-nsys-profile/pithtrain_node<N>.nsys-rep, containing traces for all ranks on that node (nsys attaches to torchrun's child processes). Analysis (GUI inspection, nsys stats, etc.) is out of scope for this skill; that belongs to a separate analyze-nsys-profile skill.

Common Issues

WORLD_SIZE is not divisible by pp*cp*ep

The allocation doesn't have enough GPUs for the requested parallelism. Stop; tell the user their allocation is short (report current world_size and required pp * cp * ep) and ask whether to reduce PP/EP/CP or request more nodes. Do not silently adjust on their behalf.

No .nsys-rep produced after the run

Check the per-node log for nsys errors. Common causes: nsys not on PATH inside the srun step, or the output directory not writable.