benchmark-kernel

Tutorial: Benchmarking FlashInfer Kernels

This tutorial shows you how to accurately benchmark FlashInfer kernels.

Goal

Measure the performance of FlashInfer kernels:

• Get accurate GPU kernel execution time
• Compare multiple backends (FlashAttention2/3, cuDNN, CUTLASS, TensorRT-LLM)
• Generate reproducible benchmark results
• Save results to CSV for analysis

Timing Methods

FlashInfer supports two timing methods:

1. CUPTI (Preferred): Hardware-level profiling for most accurate GPU kernel time

   • Measures pure GPU compute time without host-device overhead
   • Requires cupti-python >= 13.0.0 (CUDA 13+)

2. CUDA Events (Fallback): Standard CUDA event timing

   • Automatically used if CUPTI is not available
   • Good accuracy, slight overhead from host synchronization

The framework automatically uses CUPTI if available, otherwise falls back to CUDA events.

Installation

Install CUPTI (Recommended)

For the most accurate benchmarking:

pip install -U cupti-python

Requirements: CUDA 13+ (CUPTI version 13+)

Without CUPTI

If you don't install CUPTI, the framework will:

• Print a warning: CUPTI is not installed. Falling back to CUDA events.
• Automatically use CUDA events for timing
• Still provide good benchmark results

Method 1: Using flashinfer_benchmark.py (Recommended)

Step 1: Choose Your Test Routine

Available routines:

• Attention: BatchDecodeWithPagedKVCacheWrapper, BatchPrefillWithPagedKVCacheWrapper, BatchPrefillWithRaggedKVCacheWrapper, BatchMLAPagedAttentionWrapper
• GEMM: bmm_fp8, gemm_fp8_nt_groupwise, group_gemm_fp8_nt_groupwise, mm_fp4
• MOE: trtllm_fp4_block_scale_moe, trtllm_fp8_block_scale_moe, trtllm_fp8_per_tensor_scale_moe, cutlass_fused_moe

Step 2: Run a Single Benchmark

Example - Benchmark decode attention:

# CUPTI will be used automatically if installed
python benchmarks/flashinfer_benchmark.py \
    --routine BatchDecodeWithPagedKVCacheWrapper \
    --backends fa2 fa2_tc cudnn \
    --page_size 16 \
    --batch_size 32 \
    --s_qo 1 \
    --s_kv 2048 \
    --num_qo_heads 32 \
    --num_kv_heads 8 \
    --head_dim_qk 128 \
    --head_dim_vo 128 \
    --q_dtype bfloat16 \
    --kv_dtype bfloat16 \
    --num_iters 30 \
    --dry_run_iters 5 \
    --refcheck \
    -vv

Example - Benchmark FP8 GEMM:

python benchmarks/flashinfer_benchmark.py \
    --routine bmm_fp8 \
    --backends cudnn cublas cutlass \
    --batch_size 256 \
    --m 1 \
    --n 1024 \
    --k 7168 \
    --input_dtype fp8_e4m3 \
    --mat2_dtype fp8_e4m3 \
    --out_dtype bfloat16 \
    --refcheck \
    -vv \
    --generate_repro_command

Timing behavior:

• ✅ If CUPTI installed: Uses CUPTI (most accurate)
• ⚠️ If CUPTI not installed: Automatically falls back to CUDA events with warning
• 🔧 To force CUDA events: Add --use_cuda_events flag

Step 3: Understand the Output

[INFO] FlashInfer version: 0.6.0
[VVERBOSE] gpu_name = 'NVIDIA_H100_PCIe'
[PERF] fa2            :: median time 0.145 ms; std 0.002 ms; achieved tflops 125.3 TFLOPs/sec; achieved tb_per_sec 1.87 TB/sec
[PERF] fa2_tc         :: median time 0.138 ms; std 0.001 ms; achieved tflops 131.5 TFLOPs/sec; achieved tb_per_sec 1.96 TB/sec
[PERF] cudnn          :: median time 0.142 ms; std 0.001 ms; achieved tflops 127.8 TFLOPs/sec; achieved tb_per_sec 1.91 TB/sec

Key metrics:

• median time: Median kernel execution time (lower is better)
• std: Standard deviation (lower means more consistent)
• achieved tflops: Effective TFLOPS throughput
• achieved tb_per_sec: Memory bandwidth utilization

Step 4: Run Batch Benchmarks

Create a test list file my_benchmarks.txt:

--routine BatchDecodeWithPagedKVCacheWrapper --backends fa2 cudnn --page_size 16 --batch_size 32 --s_kv 2048 --num_qo_heads 32 --num_kv_heads 8 --head_dim_qk 128 --head_dim_vo 128
--routine BatchDecodeWithPagedKVCacheWrapper --backends fa2 cudnn --page_size 16 --batch_size 64 --s_kv 4096 --num_qo_heads 32 --num_kv_heads 8 --head_dim_qk 128 --head_dim_vo 128
--routine bmm_fp8 --backends cudnn cutlass --batch_size 256 --m 1 --n 1024 --k 7168 --input_dtype fp8_e4m3 --mat2_dtype fp8_e4m3 --out_dtype bfloat16

Run all tests:

python benchmarks/flashinfer_benchmark.py \
    --testlist my_benchmarks.txt \
    --output_path results.csv \
    --generate_repro_command \
    --refcheck

Results are saved to results.csv with all metrics and reproducer commands.

Step 5: Common Flags

Flag	Description	Default
--num_iters	Measurement iterations	30
--dry_run_iters	Warmup iterations	5
--refcheck	Verify output correctness	False
--allow_output_mismatch	Continue on mismatch	False
--use_cuda_events	Force CUDA events (skip CUPTI)	False
--no_cuda_graph	Disable CUDA graph	False
-vv	Very verbose output	-
--generate_repro_command	Print reproducer command	False
--case_tag	Tag for CSV output	None

Method 2: Using bench_gpu_time() in Python

For custom benchmarking in your own code:

Step 1: Write Your Benchmark Script

import torch
from flashinfer.testing import bench_gpu_time

# Setup your kernel
def my_kernel_wrapper(q, k, v):
    # Your kernel call here
    return output

# Create test inputs
device = torch.device("cuda")
q = torch.randn(32, 8, 128, dtype=torch.bfloat16, device=device)
k = torch.randn(2048, 8, 128, dtype=torch.bfloat16, device=device)
v = torch.randn(2048, 8, 128, dtype=torch.bfloat16, device=device)

# Benchmark - CUPTI preferred, CUDA events if CUPTI unavailable
median_time, std_time = bench_gpu_time(
    my_kernel_wrapper,
    args=(q, k, v),
    enable_cupti=True,          # Prefer CUPTI, fallback to CUDA events
    num_iters=30,               # Number of iterations
    dry_run_iters=5,            # Warmup iterations
)

print(f"Kernel time: {median_time:.3f} ms ± {std_time:.3f} ms")

# Calculate FLOPS if you know the operation count
flops = ...  # Your FLOP count
tflops = (flops / 1e12) / (median_time / 1000)
print(f"Achieved: {tflops:.2f} TFLOPS/sec")

Note: If CUPTI is not installed, you'll see a warning and the function will automatically use CUDA events instead.

Step 2: Run Your Benchmark

python my_benchmark.py

Output with CUPTI:

Kernel time: 0.145 ms ± 0.002 ms
Achieved: 125.3 TFLOPS/sec

Output without CUPTI (automatic fallback):

[WARNING] CUPTI is not installed. Try 'pip install -U cupti-python'. Falling back to CUDA events.
Kernel time: 0.147 ms ± 0.003 ms
Achieved: 124.1 TFLOPS/sec

Step 3: Advanced Options

# Cold L2 cache benchmarking (optional)
median_time, std_time = bench_gpu_time(
    my_kernel,
    args=(x, y),
    enable_cupti=True,          # Will use CUDA events if CUPTI unavailable
    cold_l2_cache=True,         # Flush L2 or rotate buffers automatically
    num_iters=30
)

# Force CUDA events (skip CUPTI even if installed)
median_time, std_time = bench_gpu_time(
    my_kernel,
    args=(x, y),
    enable_cupti=False,         # Explicitly use CUDA events
    num_iters=30
)

Troubleshooting

CUPTI Warning Message

Warning: CUPTI is not installed. Falling back to CUDA events.

What it means: CUPTI is not available, using CUDA events instead

Impact: Less accurate for very fast kernels (5-50 us) due to synchronization overhead, but becomes negligible for longer-running kernels

Solution (optional): Install CUPTI for best accuracy:

pip install -U cupti-python

If installation fails, check:

• CUDA version >= 13
• Compatible cupti-python version

You can still run benchmarks without CUPTI - the framework handles this automatically.

Inconsistent Results

Problem: Large standard deviation or varying results

Solutions:

1. Increase warmup iterations:

   --dry_run_iters 10
   

2. Increase measurement iterations:

   --num_iters 50
   

3. Use cold L2 cache (in Python):

   bench_gpu_time(..., rotate_buffers=True)
   

4. Disable GPU boost (advanced):

   sudo nvidia-smi -lgc <base_clock>
   

Reference Check Failures

Error: [ERROR] Output mismatch between backends

What it means: Different backends produce different results

Solutions:

1. Allow mismatch and continue:

   --allow_output_mismatch
   

2. Check numerical tolerance: Some backends use different precisions (FP32 vs FP16)

3. Investigate the difference:

   -vv  # Very verbose mode shows tensor statistics
   

Backend Not Supported

Error: [WARNING] fa3 for routine ... is not supported on compute capability X.X

Solution: Check the backend support matrix in benchmarks/README.md or remove that backend from --backends list

Best Practices

1. Install CUPTI for best accuracy (but not required):

   pip install -U cupti-python
   

2. Use reference checking to verify correctness:

   --refcheck
   

3. Use verbose mode to see input shapes and dtypes:

   -vv
   

4. Generate reproducer commands for sharing results:

   --generate_repro_command
   

5. Run multiple iterations for statistical significance:

   --num_iters 30 --dry_run_iters 5
   

6. Save results to CSV for later analysis:

   --output_path results.csv
   

7. Compare multiple backends to find the best:

   --backends fa2 fa3 cudnn cutlass
   

Quick Examples

Decode Attention (H100)

python benchmarks/flashinfer_benchmark.py \
    --routine BatchDecodeWithPagedKVCacheWrapper \
    --backends fa2 fa2_tc cudnn trtllm-gen \
    --page_size 16 --batch_size 128 --s_kv 8192 \
    --num_qo_heads 64 --num_kv_heads 8 \
    --head_dim_qk 128 --head

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