Perf Nsight Compute Analysis

Nsight Compute Analysis

NVIDIA Nsight Compute (ncu) profiles individual CUDA kernels to determine why they are slow and what to optimize. It measures GPU throughput as a percentage of theoretical peak (Speed of Light / SOL%), enabling systematic bottleneck classification and targeted optimization.

When to Use

Reach for this skill when you encounter:

• Triggers: User wants to profile a CUDA kernel, analyze ncu output, interpret .ncu-rep reports, or optimize GPU kernel performance
• Symptoms: Kernel running slower than expected, low GPU utilization, need to classify compute-bound vs memory-bound, occupancy issues
• Keywords: "ncu", "nsight compute", "SOL%", "speed of light", "kernel profiling", "compute-bound", "memory-bound", "latency-bound", "occupancy", "roofline", "warp stalls", "cache hit rate", "ncu-rep"

Do NOT use this skill for:

• System-level profiling (use Nsight Systems / nsys instead)
• CUDA API tracing or CPU-GPU timeline analysis (use nsys)
• GPU monitoring without profiling (use nvidia-smi)

Requirements

Permissions: ncu may require sudo, CAP_SYS_ADMIN, or --privileged in containers. Check with ncu -v first.

Principles

Data Integrity

This is a data-driven analysis system. Every number you present must have an authoritative source. Follow these rules without exception:

1. Quote before you interpret. When presenting metrics from ncu output, always show the actual ncu command you ran AND the relevant raw output (CSV lines, metric values) before stating any numeric conclusion.
2. Never fabricate metrics. If ncu fails, returns unexpected output, or you cannot run it, say so explicitly. Do not invent plausible-looking numbers. An honest "profiling failed" is better than fabricated data.
3. Attribute every value. For each metric you cite (SOL%, duration, occupancy, throughput), the reader must be able to trace it back to a specific line in the raw ncu output you showed.

SOL% Mental Model

Speed of Light (SOL%) measures how close a kernel runs to the GPU's theoretical peak:

• Compute SOL% = actual compute throughput / peak compute throughput
• Memory SOL% = actual memory throughput / peak memory throughput

A kernel cannot saturate both simultaneously. The higher metric reveals the bottleneck type. Use this as the primary classification signal.

Classification Thresholds

Additional signals:

• Duration <10us with many launches -> Launch-overhead bound (use nsys first)
• Both <40% but occupancy >50% -> Instruction-bound (check InstructionStats)

SOL% Performance Levels

Section-First Profiling

Always use targeted --section flags instead of bulk --set collection. Individual sections are faster and more surgical. Only escalate to --set basic or --set detailed when broad exploration is needed.

ncu vs nsys

Use nsys first to identify top kernels by GPU time, then ncu for deep analysis of those specific kernels.

Workflow

Choose your path based on the request:

• Knowledge query (what metrics to use, --section vs --set, how to filter kernels): Answer directly from Principles, Command Reference, and References below. Do NOT run ncu.
• Quick diagnosis (classify bottleneck, check SOL%): Step 1 only. Escalate if user wants more.
• Specific diagnosis (bank conflicts, register pressure, occupancy): Quick SOL% check (Step 1), then go directly to the relevant section in Step 2.
• Deep analysis (detailed report, optimization recommendations): Full Steps 1-5. Present the complete structured report with all key metrics (SOL%, duration, occupancy) in your final response — do not split the report across messages or replace it with a brief summary.

Step 0: Verify ncu

ncu -v
# Or: $NCU -v

If not found, ensure CUDA toolkit is installed or set NCU env var to the binary path.

Step 1: SOL% Diagnosis

Always start with SpeedOfLight to classify the bottleneck:

ncu --section SpeedOfLight --csv \
    --kernel-name regex:"KERNEL" \
    --launch-skip 5 --launch-count 3 \
    -- COMMAND

Read Compute (SM) Throughput and Memory Throughput from the output. Classify using the thresholds above.

Step 2: Escalate with Targeted Sections

Based on Step 1 classification, add sections:

Always include LaunchStats and Occupancy when diagnosing latency-bound kernels. These reveal register pressure, shared memory limits, and block size issues.

Example -- memory-bound deep dive:

ncu --section SpeedOfLight --section MemoryWorkloadAnalysis --csv \
    --kernel-name regex:"embedding_lookup" \
    --launch-count 3 \
    -- python script.py

Example -- compute-bound deep dive:

ncu --section SpeedOfLight --section ComputeWorkloadAnalysis --csv \
    --kernel-name regex:"gemm" \
    --launch-count 3 -- python script.py

Example -- occupancy investigation:

ncu --section SpeedOfLight --section LaunchStats --section Occupancy --csv \
    --kernel-name regex:"small_kernel" \
    -- python script.py

Step 3: Roofline Analysis (Optional)

For visual understanding of compute vs memory balance:

ncu --section SpeedOfLight_RooflineChart \
    --kernel-name regex:"KERNEL" -- COMMAND

For precision-specific hierarchical roofline:

# FP16 kernels
ncu --section SpeedOfLight_HierarchicalHalfRooflineChart \
    --kernel-name regex:"KERNEL" -- COMMAND

# Tensor core kernels
ncu --section SpeedOfLight_HierarchicalTensorRooflineChart \
    --kernel-name regex:"KERNEL" -- COMMAND

Interpretation: kernel left of ridge point = memory-bound; right = compute-bound; far below both roofs = latency/occupancy issue. See references/roofline-analysis.md.

Step 4: Interpret and Optimize

1. Identify the dominant bottleneck from SOL% classification
2. Look up detailed analysis and optimization strategies in references/bottleneck-guide.md
3. Apply highest-impact optimization first
4. Re-profile to validate improvement and detect bottleneck shifts

Step 5: Validate

Re-profile the same kernel after optimization:

ncu --section SpeedOfLight --csv \
    --kernel-name regex:"optimized_kernel" \
    --launch-count 3 \
    -- python optimized_script.py

Compare: Did throughput % increase? Did duration decrease? Did the bottleneck type shift?

Profiling JIT-Compiled Kernels (Triton/cuTile/CuTeDSL)

JIT-compiled kernels trigger autotuning on first invocation. Isolate the actual execution:

1. Warmup first: Run the kernel 3-5 times to complete JIT compilation and autotuning, then torch.cuda.synchronize().
2. Use profiler markers: Bracket the measured region with cudaProfilerStart()/cudaProfilerStop().
3. Use --profile-from-start off so ncu only captures the marked region:

# Warmup (JIT + autotuning)
for _ in range(5):
    result = kernel(inputs)
torch.cuda.synchronize()

# Profile only steady-state
torch.cuda.cudart().cudaProfilerStart()
for _ in range(3):
    result = kernel(inputs)
    torch.cuda.synchronize()
torch.cuda.cudart().cudaProfilerStop()

ncu --profile-from-start off --section SpeedOfLight --csv \
    --kernel-name regex:"target_kernel" \
    --launch-count 3 -- python script.py

Alternative: use --launch-skip N to skip autotuning launches. See references/advanced-profiling.md for NVTX range and replay mode alternatives.

Programmatic Report Analysis

Extract metrics from .ncu-rep files using the ncu_report Python module (in extras/python/ of the Nsight Compute installation):

import ncu_report

ctx = ncu_report.load_report("report.ncu-rep")
for rng in ctx:
    for action in rng:
        name = action.name()
        compute = action["sm__throughput.avg.pct_of_peak_sustained_elapsed"].as_double()
        memory = action["dram__throughput.avg.pct_of_peak_sustained_elapsed"].as_double()
        duration = action["gpu__time_duration.sum"].as_uint64()

        if compute > 60:
            classification = "compute-bound"
        elif memory > 60:
            classification = "memory-bound"
        else:
            classification = "latency-bound"

        print(f"{name}: {classification} (compute={compute:.1f}%, mem={memory:.1f}%, {duration}ns)")

See references/python-report-api.md for the full API (IContext, IRange, IAction, IMetric classes).

Output Formats

CSV output (for scripting and automated analysis):

ncu --csv --section SpeedOfLight --kernel-name regex:"KERNEL" -- COMMAND
ncu --csv --page raw --section SpeedOfLight -- COMMAND   # All metrics flat

Report files (for later analysis):

ncu -o report --section SpeedOfLight -- COMMAND
ncu --import report.ncu-rep --csv --page raw            # Export to CSV

Key CSV columns:

Success indicators:

• SOL% values present in output -> profiling succeeded
• Duration values reasonable (not 0 or extremely large)
• Multiple launches captured when --launch-count > 1

Examples

Example: Classify a GEMM Kernel

ncu --section SpeedOfLight --csv \
    --kernel-name regex:"gemm" \
    --launch-skip 5 --launch-count 3 \
    -- python train.py

Output:

"Kernel Name","Duration","Compute (SM) Throughput","Memory Throughput"
"ampere_fp16_gemm",1250000,78.5,35.2

Interpretation: compute-bound (78.5% compute, 35.2% memory). Next step: check tensor core usage with --section ComputeWorkloadAnalysis.

Example: Diagnose a Memory-Bound Embedding Kernel

ncu --section SpeedOfLight --section MemoryWorkloadAnalysis --csv \
    --kernel-name regex:"embedding" \
    --launch-count 3 -- python train.py

Check L1/L2 cache hit rates and coalescing efficiency in output. Low hit rates suggest poor data locality; low coalescing efficiency suggests scattered access.

Error Handling

Finding More Information

Tier 1: This File (SKILL.md)

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

Tier 2: references/ Directory

Grep for keywords across references/ -- headers are grep-friendly:

• references/cli-reference.md -- Complete CLI options, filtering, output formats
• references/metrics-guide.md -- Hardware model, metric naming, key metrics
• references/sections-guide.md -- All --section names, when to use each
• references/bottleneck-guide.md -- Per-bottleneck root causes and optimization
• references/memory-analysis.md -- Memory hierarchy, cache analysis, coalescing
• references/roofline-analysis.md -- Roofline charts and interpretation
• references/advanced-profiling.md -- Replay modes, MPI, CUDA graphs, PM sampling, customization
• references/python-report-api.md -- ncu_report Python module API

How to search:

1. Grep for your keyword across references/
2. Read only the file that Grep points to

Tier 3: Official Documentation

If Tiers 1-2 don't answer:

• Profiling Guide -- Metrics, hardware model, analysis concepts
• CLI Reference -- Full CLI options
• Python Report Interface -- ncu_report API
• Customization Guide -- Section files, rules

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