rich7421/fastvideo-wan-l40s-nsys

fastvideo / Wan2.1-T2V-1.3B — nsys profiles on 4× L40S (no NVLink)

This dataset hosts NVIDIA Nsight Systems profiles of fastvideo running Wan-AI/Wan2.1-T2V-1.3B-Diffusers text-to-video inference on a 4× L40S node (PCIe Gen4 x16, no NVLink), plus the static plots and measurement summary used to discuss the trace.

The trace shows a bandwidth-bound, same-stream-serialized workload: compute kernels and NCCL SendRecv collectives share a single CUDA stream, so there is no compute/comm overlap to exploit — every NCCL byte directly inflates forward time.

Contents

profiles/
  perf.nsys-rep            (605 MB) — non-compile baseline
  perf.sqlite              (2.1 GB) — non-compile baseline, exported
  perf_compile.nsys-rep    (506 MB) — torch.compile enabled
  perf_compile.sqlite      (1.8 GB) — torch.compile enabled, exported

plots/
  1_iter_duration_over_time.png    — per-iter latency vs wall time (warmup shaded)
  2_iter_duration_histogram.png    — steady-state distribution (σ = 21 ms / 0.66 %)
  3_stream7_one_iter.png           — stream-7 timeline showing compute / NCCL alternation
  4_per_device_walltime.png        — 47 % compute / 34 % NCCL / 18 % idle per GPU
  5_nccl_kernel_duration_hist.png  — bimodal at 13 ms / 33 ms (two AllToAll4D sizes)
  README.md                        — mentor-question Q&A walkthrough

analysis_measurements.md           — raw §1–§9 measurements from perf_compile.sqlite

Headline numbers (perf_compile.sqlite, full 677 s span, device 0)

Metric	Value
compute_only_ms	311 516 ms (46.0 %)
nccl_only_ms	271 564 ms (40.1 %)
idle_ms	94 265 ms (13.9 %)
overlap_ms	0.02 ms (0.0 %)
same_stream_compute_pct	100 % on stream 7
same_stream_nccl_pct	100 % on stream 7
Total NCCL payload	8 936 GiB
NCCL message p50 / p99	13.18 / 39.55 MiB
Slowest iter	iter 311 / 3 087 ms (compile); iter 321 / 3 292 ms (no-compile)
Steady-state σ	17.7 ms (compile) / 21.2 ms (no-compile)

How to reproduce the measurements

pip install nsys-ai

# Health + topology + bandwidth ceiling
nsys-ai analyze profile_health_manifest    perf_compile.sqlite
nsys-ai analyze overlap_breakdown          perf_compile.sqlite
nsys-ai analyze nccl_payload_breakdown     perf_compile.sqlite
nsys-ai analyze iteration_timing           perf_compile.sqlite
nsys-ai analyze nvtx_layer_breakdown       perf_compile.sqlite

# Or just open the timeline in the browser
nsys-ai timeline-web perf_compile.sqlite

The plots in plots/ were generated from perf.sqlite via a matplotlib script that queries the parquet cache nsys-ai builds on first load (no LLM, no API).

Hardware / software

• 4× NVIDIA L40S, PCIe Gen4 x16, no NVLink (intra-node bus only)
• NCCL aggregate bus BW ≈ 24 GB/s (2 channels × 12 GB/s) measured during the same session
• PyTorch + fastvideo, Ulysses-style sequence parallelism (all_to_all_4D scatter=2/gather=1 pre-attn, scatter=1/gather=2 post-attn)
• Two passes per denoising step (CFG cond + uncond) — 90 denoising steps × 2 = 180 real forwards

License

Profile traces and derived artifacts are released under CC-BY-4.0.