perf-nsight-systems

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Nsight Systems (nsys) CLI for system-level timeline profiling. Use when the user wants to run nsys profile, analyze .nsys-rep reports, use nsys stats/analyze/recipe commands, diagnose GPU idle time from timeline traces, or profile distributed training with NCCL overlap analysis. NOT for kernel-level metrics like SOL%, occupancy, or roofline (use perf-nsight-compute-analysis for ncu). NOT for writing or generating kernels. NOT for applying optimizations like CUDA Graphs.

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Sourcenvidia/skills

Added on2026-05-22

NPX Install

npx skill4agent add nvidia/skills perf-nsight-systems

SKILL.md Content

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Nsight Systems Profiling

NVIDIA Nsight Systems (

nsys

) is a system-level performance analysis tool that captures CPU/GPU activity timelines, API traces, and OS-level events. Unlike Nsight Compute (kernel-level),

nsys

shows the big picture — how kernels, memory transfers, communication, and CPU work overlap in time.

When to Use

Reach for this skill when you encounter:

Triggers: User wants to profile a training script end-to-end, analyze GPU utilization, find pipeline bottlenecks, check communication/compute overlap, or interpret
```
.nsys-rep
```
reports
Symptoms: Training slower than expected, GPU idle between iterations, need to understand where time is spent across CPU and GPU, poor scaling in distributed training
Keywords: "nsys", "nsight systems", "GPU timeline", "GPU utilization", "kernel launch overhead", "training profiling", "NCCL overlap", "nsys-rep", "cuda trace", "GPU idle", "pipeline stall", "data loading bottleneck"

Do NOT use this skill for:

Kernel-level optimization (use Nsight Compute /
```
ncu
```
instead)
GPU hardware metrics like SM throughput, cache hit rates (use
```
ncu
```
)
GPU monitoring without profiling (use
```
nvidia-smi
```
)
Non-CLI usage (GUI workflows, IDE integration) — consult official docs

Requirements

Dependency	Version	Notes
CUDA Toolkit	>=11.0	Includes `nsys`
`nsys` binary	Match CUDA version	Verify with `nsys -v`
NVIDIA GPU	Any supported

Permissions:

nsys

may require

sudo

CAP_SYS_ADMIN

for system-wide tracing and GPU metrics. In containers, use

--privileged

--cap-add=SYS_ADMIN

Reporting Principles

Every number must have an authoritative source. When presenting timing data, kernel counts, API call durations, or any quantitative metric, always cite the source:

nsys stats

report output,

nsys analyze

rule output, exported SQLite query result, recipe CSV, or raw command output. Show the actual command and its output before interpreting. Never synthesize, estimate, or extrapolate numbers that did not come from a tool output.

Use
nsys stats
for structured analysis, not raw trace data. Always extract metrics via targeted

nsys stats -r <report>

commands rather than trying to read or interpret

.nsys-rep

files directly. Stats reports produce compact, tabular summaries; raw trace data can be enormous (especially with backtraces or verbose API tracing). Run the smallest set of reports needed for the task, then request additional reports only if the initial results raise questions.

Workflows

Workflow 1: Profile a DL Training Script

Goal: Capture a clean, focused profile of steady-state training iterations.

Step 1 — Add profiler markers to your training script to skip warmup:

python

# In training script
for i, batch in enumerate(train_loader):
    if i == warmup_iters:
        torch.cuda.cudart().cudaProfilerStart()
    train_step(model, batch)
    if i == warmup_iters + profile_iters:
        torch.cuda.cudart().cudaProfilerStop()
        break

Step 2 — Profile with
cudaProfilerApi
capture range:

bash

nsys profile -c cudaProfilerApi \
    -t cuda,nvtx,cudnn,cublas \
    --pytorch=autograd-nvtx \
    -o train_profile -- python train.py

This captures only steady-state iterations — no warmup, no initialization noise.

Note:

-t cuda,nvtx,cudnn,cublas

enables API-specific tracing. By default,

-t cuda

only traces the CUDA runtime/driver layer — you see kernel names and launch times but cannot attribute them to higher-level libraries. Adding

cudnn

and

cublas

traces the library-level API calls, letting you distinguish convolution time (cuDNN) from GEMM time (cuBLAS) and measure library overhead separately from raw kernel execution.

Step 3 — Quick summary:

bash

nsys stats -r cuda_gpu_kern_sum,cuda_api_sum,cuda_gpu_mem_time_sum \
    train_profile.nsys-rep

When you traced library APIs (

cudnn

cublas

-t

), also run the library-specific reports to see API-level overhead (workspace allocation, algorithm selection) separately from raw kernel execution:

bash

nsys stats -r cudnn_api_sum,cublas_api_sum train_profile.nsys-rep

Step 4 — Detect anti-patterns:

bash

nsys analyze -r all train_profile.nsys-rep

Step 5 — Dig deeper based on findings. See Tier 2 references.

Workflow 2: Diagnose GPU Idle Time

Goal: Find why the GPU is idle between training iterations.

Step 1 — Profile with OS runtime tracing:

bash

nsys profile -t cuda,nvtx,osrt \
    --pytorch=autograd-nvtx \
    -o idle_debug -- python train.py

Step 2 — Check GPU gaps and utilization:

bash

nsys analyze -r gpu_gaps,gpu_time_util idle_debug.nsys-rep

Step 3 — Check kernel launch phases:

bash

nsys stats -r cuda_kern_exec_sum idle_debug.nsys-rep

High queue time = GPU was busy (not the issue). Near-zero queue time for all kernels = GPU was starved (host not submitting work fast enough).

Step 4 — Common causes and fixes:

GPU idle cause	Evidence	Fix
Slow data loading	CPU busy in DataLoader during gaps	Increase `num_workers` , use `pin_memory=True`
Synchronous memcpy	`cuda_memcpy_sync` rule fires	Use `non_blocking=True` transfers
Over-synchronization	Frequent `cudaDeviceSynchronize` in trace	Remove unnecessary sync calls
Host-side computation	CPU sampling shows compute during gaps	Move to GPU or overlap with async ops
Python GIL contention	GIL trace shows contention	Use multiprocessing, reduce Python overhead

Workflow 3: Profile Distributed Training

Goal: Profile multi-GPU/multi-node training with communication analysis.

Step 1 — Collect per-rank profiles:

bash

nsys profile -t cuda,nvtx,mpi,ucx \
    --pytorch=autograd-nvtx \
    -o profile_%q{RANK} \
    -- torchrun --nproc_per_node=8 train.py

Step 2 — Analyze NCCL communication/compute overlap:

bash

nsys recipe nccl_gpu_overlap_trace -- profile_*.nsys-rep
nsys recipe nccl_gpu_time_util_map -- profile_*.nsys-rep

Step 3 — Check per-rank utilization:

bash

nsys recipe cuda_gpu_time_util_map -- profile_*.nsys-rep

Step 4 — Check for stragglers:

Compare

cuda_gpu_kern_sum

across ranks. If one rank is slower, check its network and data loading patterns.

Workflow 4: Analyze Iteration Time Consistency

Goal: Check whether training iterations are stable or have outliers.

bash

# Profile with NVTX iteration markers
nsys profile --pytorch=autograd-nvtx -t cuda,nvtx \
    -o iter_check -- python train.py

# Check iteration timing distribution
nsys stats -r nvtx_pushpop_sum iter_check.nsys-rep

# Check GPU projection per NVTX range
nsys stats -r nvtx_gpu_proj_sum iter_check.nsys-rep

# Visual pace analysis
nsys recipe nvtx_pace -- iter_check.nsys-rep

High StdDev in iteration duration indicates inconsistency — investigate outlier iterations on the timeline.

Workflow 5: Attribute Kernels to Source Code via Stack Traces

Goal: Identify which Python function or code path triggers expensive GPU kernels.

Step 1 — Profile with backtrace collection:

bash

nsys profile -t cuda,nvtx \
    --backtrace=cuda \
    --python-backtrace=lbr \
    --pytorch=autograd-nvtx \
    -o stacktrace_profile -- python train.py

```
--backtrace=cuda
```
: Captures CUDA API call stacks (C/C++ frames) so each
```
cudaLaunchKernel
```
shows the host-side call chain that triggered it.
```
--python-backtrace=lbr
```
: Captures Python-level call stacks, correlating GPU work back to specific Python functions (e.g.,
```
compute_attention
```
vs
```
compute_ffn
```
).

Step 2 — Get kernel summary and NVTX attribution:

Use targeted stats reports to identify top kernels and their NVTX context:

bash

# Top kernels by total GPU time
nsys stats -r cuda_gpu_kern_sum stacktrace_profile.nsys-rep

# Kernels attributed to NVTX ranges (maps kernels to annotated code regions)
nsys stats -r nvtx_kern_sum stacktrace_profile.nsys-rep

The

nvtx_kern_sum

report (requires

--pytorch=autograd-nvtx

or manual NVTX annotations) maps each kernel to its enclosing NVTX range, directly showing which Python function or autograd op launched it. This is more efficient than manually cross-referencing raw backtrace data.

Step 3 — For PyTorch models,

--pytorch=autograd-nvtx

automatically wraps each autograd op in an NVTX range. Combined with backtrace, this maps: GPU kernel → CUDA API call → Python function → PyTorch autograd op.

When to use: Workloads with multiple code paths launching similar kernels (e.g., attention vs FFN both calling GEMM). Stack traces disambiguate which caller is responsible for the dominant kernel time.

Output Formats

Report files (

.nsys-rep

): Binary format, viewable in GUI or processed with

nsys stats

nsys analyze

nsys export

nsys recipe

Stats output formats:

column

(terminal),

csv

json

table

tsv

hdoc

htable

Export formats:

sqlite

(SQL queries),

arrow

parquetdir

(Pandas/Dask),

hdf

jsonlines

text

Recipe output: Directory with CSV/Parquet data + Plotly HTML visualizations

```
.nsys-analysis
```
(Jupyter notebook).

Key stats report columns:

Report	Key columns
`cuda_gpu_kern_sum`	Time%, Total Time, Instances, Kernel Name
`cuda_api_sum`	Time%, Total Time, Num Calls, API Name
`cuda_kern_exec_sum`	API Time, Queue Time, Kernel Time
`cuda_gpu_mem_time_sum`	Time%, Total Time, Operations, Direction
`nvtx_gpu_proj_sum`	Projected Duration, Original Duration, GPU Op Count

Examples

Example 1: Quick DL Profile and Summary

bash

# Profile
nsys profile -t cuda,nvtx,cudnn,cublas \
    --pytorch=autograd-nvtx --stats=true \
    -o quick_profile -- python train.py

# Auto-generates stats at the end of profiling

Example 2: Detect Sync Memcpy in DataLoader

bash

nsys profile -t cuda,nvtx -o dataloader_check -- python train.py
nsys analyze -r cuda_memcpy_sync,cuda_memcpy_async dataloader_check.nsys-rep

If flagged, fix with:

python

loader = DataLoader(dataset, pin_memory=True, num_workers=4)
tensor_gpu = tensor_cpu.to(device, non_blocking=True)

Example 3: Multi-Node NCCL Analysis

bash

# Collect
nsys profile -t cuda,nvtx,mpi -o rank_%q{RANK} \
    -- torchrun --nproc_per_node=8 train.py

# Analyze overlap
nsys recipe nccl_gpu_overlap_trace -- rank_*.nsys-rep

# Visualize
nsys recipe nccl_gpu_time_util_map -- rank_*.nsys-rep

Example 4: API-Level Breakdown (cuDNN vs cuBLAS)

bash

# Profile with library-level tracing
nsys profile -t cuda,nvtx,cudnn,cublas \
    -o api_breakdown -- python model.py

# cuDNN API summary (convolution calls)
nsys stats -r cudnn_api_sum api_breakdown.nsys-rep

# cuBLAS API summary (GEMM calls)
nsys stats -r cublas_api_sum api_breakdown.nsys-rep

# Compare with kernel-level view
nsys stats -r cuda_gpu_kern_sum api_breakdown.nsys-rep

The API-level reports (

cudnn_api_sum

cublas_api_sum

) show time spent in library calls including overhead (workspace allocation, algorithm selection), while

cuda_gpu_kern_sum

shows only raw GPU kernel execution. The difference reveals library-side overhead.

Error Handling

Error	Cause	Fix
`nsys: command not found`	Not in PATH	`export PATH=$PATH:/usr/local/cuda/bin`
`Permission denied` or `requires root`	Needs elevated privileges	`sudo nsys ...` or `--cap-add=SYS_ADMIN` in containers
No CUDA activity captured	App didn't use GPU during collection window	Adjust `--delay` / `--duration` , or use `cudaProfilerApi` capture range
Report file very large	Long profile with many APIs traced	Use focused capture ( `-c cudaProfilerApi` ), reduce `--duration`
`--pytorch` has no effect	Wrong nsys version or Python env	Verify nsys version supports `--pytorch` ; check Python is in PATH
`nsys stats` shows empty reports	No matching activity in report	Check `--trace` flags included the right APIs
MPI rank profiles out of sync	Clock skew between nodes	Use NTP sync; analyze per-rank independently
`cudaProfilerStart` not captured	Missing `-c cudaProfilerApi` flag	Add `--capture-range=cudaProfilerApi`
Recipe fails with import error	Missing Python dependencies	Install recipe dependencies: `pip install pandas plotly`

Finding More Information

Tier 1: This File (SKILL.md)

You are reading it now. The workflows and error table above cover the most common DL profiling tasks. Search this file first.

Tier 2: references/ Directory

Grep for keywords across

references/

— headers are grep-friendly:

```
references/cli-profiling.md
```
— Complete
```
nsys profile
```
flags for DL

references/cli-post-collection.md

—

nsys stats

analyze

export

recipe

commands

```
references/app-preparation.md
```
— Focused profiling, NVTX markers, PyTorch patterns
```
references/stats-reports.md
```
— CUDA statistical report columns and meanings
```
references/expert-systems.md
```
— Expert system rules, anti-pattern detection
```
references/recipes-dl.md
```
— DL-relevant advanced recipes with examples
```
references/nvtx-analysis.md
```
— NVTX statistical reports for annotated code

How to search:

```
Grep
```
for your keyword across
```
references/
```
```
Read
```
only the file that Grep points to

Tier 3: Official Documentation

If Tiers 1-2 don't answer:

User Guide — Full CLI reference, all tracing options
Analysis Guide — Stats reports, expert systems, recipes

WebFetch or WebSearch these URLs for the latest content. Consider distilling new findings back into

references/

perf-nsight-systems

NPX Install

Tags

SKILL.md Content

Nsight Systems Profiling

When to Use

Requirements

Reporting Principles

Workflows

Workflow 1: Profile a DL Training Script

Workflow 2: Diagnose GPU Idle Time

Workflow 3: Profile Distributed Training

Workflow 4: Analyze Iteration Time Consistency

Workflow 5: Attribute Kernels to Source Code via Stack Traces

Output Formats

Examples

Example 1: Quick DL Profile and Summary

Example 2: Detect Sync Memcpy in DataLoader

Example 3: Multi-Node NCCL Analysis

Example 4: API-Level Breakdown (cuDNN vs cuBLAS)

Error Handling

Finding More Information

Tier 1: This File (SKILL.md)

Tier 2: references/ Directory

Tier 3: Official Documentation