fournex 0.2.1

Open-source GPU performance profiler and bottleneck analyzer for PyTorch.

Fournex

Open-source GPU performance profiler and bottleneck analyzer for PyTorch.

Fournex wraps your training script, collects GPU telemetry, and tells you exactly what is slowing it down — with ranked, actionable recommendations.

Install

pip install fournex

Quick start

# Profile your workload
frx collect --name my-run -- python train.py

# Analyze and get recommendations
frx analyze runs/run-<id>

# Check your environment
frx doctor

# Validate the pipeline end-to-end
frx smoke-test

Detected bottleneck types

Label	Signal
input_bound	DataLoader wait ≥ 20% of step time
copy_bound	H2D transfer ≥ 15% of step time
sync_bound	Sync wait ≥ 10% of step time
underutilized_gpu	GPU utilization < 35%
memory_pressure	Peak memory ratio ≥ 90%
shape_instability	Shape volatility ≥ 30%
launch_bound	Low utilization + profiler windows, no dominant stall
insufficient_telemetry	No timing or GPU utilization data

Safe config benchmarking

frx tune --safe --max-trials 12 -- python train.py

Fournex sweeps DataLoader and runtime configs, benchmarks each one, and recommends the fastest safe candidate — without changing your code.

Interrupted or repeated tune runs can reuse completed trial artifacts:

frx tune --resume runs/tune-<id> -- python train.py

--resume reuses a trial only when the saved config.yaml, benchmark_window.json, and metrics.json match the current workload command and benchmark settings.

CUDA static analysis

frx analyze accepts .cu and .cuh files directly and reports kernel-level antipatterns, launch configuration issues, and occupancy estimates without requiring a GPU:

frx analyze kernel.cu
frx analyze kernel.cu --gpu-model RTX4090
frx analyze kernel.cu --output-json

Detected antipatterns span five categories across 22 rules:

Category	Example findings
Memory	uncoalesced_access, no_shared_memory_tiling, missing_vectorized_loads
Synchronization	unnecessary_syncthreads, conditional_syncthreads, sync_inside_tight_loop
Control flow	warp_divergence, excessive_branching, bounds_check_inside_hot_loop
Occupancy	large_static_shared_memory, possible_shared_memory_bank_conflict, high_register_pressure
Tensor cores	fp32_only_matmul, missing_wmma_mma_path, dimensions_not_tensor_core_friendly

PTX and Nsight Compute analysis

frx analyze also accepts PTX assembly files and Nsight Compute CSV exports:

frx analyze kernel.ptx
frx analyze profile.csv

Kernel comparison

Compare two CUDA source files across all available evidence layers:

# Source-only diff
frx compare baseline.cu optimized.cu

# With PTX and NCU profiling (requires nvcc + ncu on PATH)
frx compare baseline.cu optimized.cu --with-ptx --with-ncu

# Supply pre-collected NCU CSVs
frx compare baseline.cu optimized.cu --ncu-a baseline.csv --ncu-b optimized.csv

The comparison report scores each kernel across memory, compute, occupancy, and instruction-mix dimensions, with a verdict and reconciled diagnosis from all available layers.

Recommendation validation commands

Every recommendation includes an NCU command to confirm the fix worked:

Validate:
  ncu --metrics l1tex__t_sectors_pipe_lsu_mem_global_op_ld.sum_per_request \
      --csv ./report.csv ./your_app
  <-- Global load sectors/request: drops toward 1-4 (target: 4.0)

Adding antipattern rules

Rules live in fournex/cuda_rules/ as YAML files, one per finding:

id: my_new_rule
scope: kernel          # kernel | launch | occupancy
category: memory
severity: medium       # high | medium | low
confidence: medium
message: "Description with optional {signal_name} interpolation."
conditions:
  strided_or_pitched: true    # bool equality
  for_count_gte: 1            # numeric >=
recommendations:
  - rec_ncu_improve_coalescing
ncu_signals:
  sectors_per_request_gt: 4.0
architecture_overrides: {}

Drop the file into the matching category subfolder — the engine picks it up automatically with no code changes required.

Available signals for scope: kernel conditions include: has_shared, has_sync, has_tc, has_fp16, has_matmul, has_loop, has_thread_indexing, has_bounds_guard, strided_or_pitched, likely_coalesced_1d, vectorized, bank_conflict_risk, tc_unfriendly_dims, warp_divergence_pattern, conditional_syncthreads_pattern, sync_count, for_count, global_access_count, branch_count, bounds_check_count, local_var_count, max_shared_bytes.

Links

• GitHub
• Documentation
• Website