Cuda

CUDA Programming Skill

Core Philosophy

Measure before guessing. GPU performance is deeply counterintuitive. Profile first, hypothesize second, change third, verify fourth.

Small, isolated changes. CUDA bugs compound. Make one change, test it, commit it. Resist the urge to "fix everything at once."

printf is your strongest tool. When debuggers fail, when tools produce inscrutable output, printf in device code reveals truth. Don't be embarrassed to use it extensively.

Sometimes, stare at the diff. Inscrutable segfaults are common. Tools often don't help. The human approach: minimize the diff, read it carefully, see the bug. This is legitimate and often faster than tooling.

Debugging Workflow

First Response to a Bug

1. Reproduce minimally — Isolate the failing kernel with smallest possible input
2. Add printf — Before any tool, add printf in device code to trace execution
3. Run compute-sanitizer — Catch memory errors non-interactively:

   compute-sanitizer --tool memcheck ./your_program
   compute-sanitizer --tool racecheck ./your_program  # for race conditions
   compute-sanitizer --tool initcheck ./your_program  # uninitialized memory
   

4. If still stuck, try cuda-gdb non-interactively for backtrace:

   cuda-gdb -batch -ex "run" -ex "bt" ./your_program
   

5. When tools fail — Minimize the diff between working and broken code. Read it. The bug is in the diff.

printf in Device Code

__global__ void myKernel(float* data, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx == 0) {  // Limit output
        printf("Kernel launched, n=%d, data[0]=%f\n", n, data[0]);
    }
    // ... kernel logic ...
    if (idx < 10) {  // Sample a few threads
        printf("Thread %d: result=%f\n", idx, someValue);
    }
}

Key patterns:

• Guard with if (idx == 0) or if (idx < N) to avoid output flood
• Print at kernel entry to confirm launch
• Print intermediate values at suspected failure points
• Flush is automatic at kernel completion

compute-sanitizer Quick Reference

Common gotcha: "Invalid shared write... out of bounds" usually means insufficient dynamic shared memory allocation in the kernel launch, not wrong array indexing. Check <<<grid, block, smem_size>>>.

# Memory errors (most common)
compute-sanitizer --tool memcheck ./program

# Other tools: racecheck, initcheck, synccheck
# For detailed options, see references/debugging-tools.md

cuda-gdb Non-Interactive

# Get backtrace on crash
cuda-gdb -batch -ex "run" -ex "bt" ./program

# For breakpoints, thread inspection, see references/debugging-tools.md

Compile with debug info:

nvcc -g -G -lineinfo program.cu -o program

cuobjdump for Binary Inspection

# Dump PTX and SASS
cuobjdump -ptx ./program
cuobjdump -sass ./program

# For resource usage, symbol listing, see references/debugging-tools.md

For complete debugging tool reference: See references/debugging-tools.md for detailed compute-sanitizer options, cuda-gdb workflows, and cuobjdump analysis patterns.

Performance Optimization Workflow

Golden Rule

Never optimize without profiling first. Intuition about GPU bottlenecks is almost always wrong. The profile → fix → verify loop is the actual optimization work, not a preliminary step.

Performance Investigation Steps

1. Establish baseline — Time the operation, record it
2. Profile with nsys — Get timeline, identify which kernels matter
3. Deep-dive with ncu — Analyze specific bottleneck kernels
4. Hypothesize — Based on metrics, form specific hypothesis
5. Change one thing — Make a single targeted change
6. Verify — Re-profile, confirm improvement
7. Repeat

nsys (Nsight Systems) — Timeline Profiling

Use nsys for: "Where is time being spent?" — CPU/GPU interaction, kernel launch patterns, memory transfers, overall timeline.

# Basic profile
nsys profile -o report ./program
nsys stats report.nsys-rep --report cuda_gpu_kern_sum

# With NVTX markers
nsys profile --trace=cuda,nvtx -o report ./program

# Key reports: cuda_gpu_kern_sum, cuda_api_sum, cuda_gpu_mem_time_sum, nvtx_sum
# For detailed usage, see references/nsys-guide.md

For detailed nsys analysis patterns: See references/nsys-guide.md for timeline interpretation, identifying common bottlenecks, and analysis workflows.

ncu (Nsight Compute) — Kernel Analysis

Use ncu for: "Why is this kernel slow?" — Detailed metrics, roofline, memory analysis, occupancy.

# Profile specific kernel
ncu --kernel-name "myKernel" -o report ./program

# Quick summary to stdout
ncu --set basic ./program

# Sets: basic, full, memory, launch, roofline
# Sections: ComputeWorkloadAnalysis, MemoryWorkloadAnalysis, Occupancy
# For detailed metrics and interpretation, see references/ncu-guide.md

Warning: ncu expert system recommendations can be misleading. Always verify with actual metrics and experiments.

Scale matters: Optimizations that help at large scale can hurt at small scale. Always profile at your actual problem size, not theoretical maximums.

For detailed ncu metric interpretation: See references/ncu-guide.md for understanding roofline analysis, memory bottlenecks, occupancy limits, and warp scheduling.

NVTX for Custom Instrumentation

When you need finer granularity than kernel-level, use NVTX:

#include <nvtx3/nvToolsExt.h>

nvtxRangePush("Operation Name");
// ... code to profile ...
nvtxRangePop();

Compile: -lnvToolsExt | Profile: nsys profile --trace=cuda,nvtx

For complete patterns: See references/nvtx-patterns.md for nested ranges, colors, and analysis workflows.

Common Performance Patterns

Critical traps: Bank conflicts and memory coalescing issues often dominate performance but aren't obvious without profiling. See references/performance-traps.md for detailed diagnosis and fixes.

Reality check: Budget 80% of optimization time for problems you didn't predict. Profile-driven iteration discovers the real bottlenecks.

Compilation Reference

# Debug build
nvcc -g -G -lineinfo -O0 program.cu -o program_debug

# Release build
nvcc -O3 -lineinfo program.cu -o program

# Specific architecture
nvcc -arch=sm_80 program.cu -o program  # Ampere
nvcc -arch=sm_89 program.cu -o program  # Ada Lovelace
nvcc -arch=sm_90 program.cu -o program  # Hopper

# Generate PTX (inspect it)
nvcc -ptx program.cu

# Verbose compilation (see register usage)
nvcc --ptxas-options=-v program.cu

# With NVTX
nvcc program.cu -lnvToolsExt -o program

Always compile with -lineinfo for production profiling — minimal overhead, enables source correlation.

Local API Documentation

Complete reference documentation available for grep-based search:

PTX ISA 9.1 — references/ptx-docs/ (405 files, 2.3MB)

• Search guide: references/ptx-isa.md
• Use for: Instruction-level optimization, inline PTX, TensorCore operations (WMMA, WGMMA, TMA), memory swizzling

CUDA Runtime API 13.1 — references/cuda-runtime-docs/ (107 files, 0.9MB)

• Search guide: references/cuda-runtime.md
• Use for: Error codes, API parameters, device properties (cudaDeviceProp), memory management, stream behavior

CUDA Driver API 13.1 — references/cuda-driver-docs/ (128 files, 0.8MB)

• Search guide: references/cuda-driver.md
• Use for: Context management (cuCtxCreate), module loading (cuModuleLoad), virtual memory, Driver errors (CUDA_ERROR_*), advanced features

Each search guide contains grep examples, documentation structure, and common usage patterns.

Search strategy: Use grep/ripgrep to search directly in the *-docs/ directories. The search guides (.md files) provide navigation patterns and common queries.

Additional References

• references/performance-traps.md — Bank conflicts, memory coalescing, scale-dependent optimizations
• references/debugging-tools.md — compute-sanitizer, cuda-gdb, cuobjdump detailed usage
• references/nsys-guide.md — nsys timeline analysis and bottleneck identification
• references/ncu-guide.md — ncu metrics, roofline, occupancy interpretation
• references/nvtx-patterns.md — NVTX instrumentation and profiling patterns

Checklist Before Optimizing

• Established reproducible baseline timing
• Profiled with nsys to identify hotspots
• Know which kernel(s) dominate runtime
• Profiled target kernel with ncu
• Identified specific bottleneck (memory? compute? latency?)
• Formed specific, testable hypothesis
• Plan to change ONE thing