[benchmark] add flashinfer_allreduce_fusion benchmark (#9937)

benchmark/kernels/flashinfer_allreduce_fusion/README.md

+# FlashInfer Fused AllReduce + RMSNorm Benchmark
+
+This benchmark script is modified from the [original implementation](https://github.com/vllm-project/vllm/blob/237e1fb887c7f5a579420fa0295097f24b006594/benchmarks/kernels/benchmark_fused_collective.py) by the vLLM community. It aims to compare the performance differences between FlashInfer fused operators in SGLang (trtllm_allreduce_fusion: AllReduce + Residual Add + RMSNorm + optional quantization) and conventional implementations (standard `tensor_model_parallel_all_reduce` + separate RMSNorm/quantization). Specifically, this script tests the timing performance of two implementation paths: 1) Standard AllReduce and RMSNorm executed separately; 2) FlashInfer's fused operator combining AllReduce, Residual Add, RMSNorm, and optional quantization operations.
+
+This benchmark script helps us tune the ipc workspace size of the `flashinfer_allreduce_residual_rmsnorm` operator in SGLang and prepare for applications with FP8/FP4 quantized fused operators.
+
+Script path: `benchmark/kernels/flashinfer_allreduce_fusion/benchmark_fused_collective.py`
+
+## Feature Overview
+
+- Compare average execution time (ms) and calculate speedup ratios for the following paths:
+  - standard_allreduce_rmsnorm (Standard AllReduce + RMSNorm)
+  - flashinfer_fused_allreduce_rmsnorm (Fused AllReduce + RMSNorm), including oneshot and twoshot modes
+  - Optionally compare FP8/FP4 quantized fused paths with standard paths
+- Use CUDA Graph capture and batch replay to reduce measurement noise
+- Automatically select the faster "standard baseline" (native/compiled version) as the denominator for speedup calculation
+- Optionally export results in Markdown format
+
+## Runtime Environment and Prerequisites
+
+- At least 2 GPUs, and launch multi-process distributed training using `torchrun` (NCCL backend)
+- Properly install/compile sglang along with sgl-kernel and custom operators
+
+## Quick Start (Command Examples)
+
+The following examples use world_size=2. You can modify `--nproc_per_node` and parameters according to your machine:
+
+- Regular paths only (no quantization):
+```
+torchrun --nproc_per_node=2 \
+benchmark/kernels/flashinfer_allreduce_fusion/benchmark_fused_collective.py \
+--no-quant --hidden-dim 1024 --seq-lens 512 1024 2048 4096 --trials 100
+```
+
+- FP8 quantization paths only:
+```
+torchrun --nproc_per_node=2 \
+benchmark/kernels/flashinfer_allreduce_fusion/benchmark_fused_collective.py \
+--quant-fp8 --hidden-dim 1024 --seq-lens 512 1024 2048 4096 --trials 100
+```
+
+- FP4 quantization paths only:
+```
+torchrun --nproc_per_node=2 \
+benchmark/kernels/flashinfer_allreduce_fusion/benchmark_fused_collective.py \
+--quant-fp4 --hidden-dim 1024 --seq-lens 512 1024 2048 4096 --trials 100
+```
+
+- Larger hidden dimensions:
+```
+torchrun --nproc_per_node=2 \
+benchmark/kernels/flashinfer_allreduce_fusion/benchmark_fused_collective.py \
+--no-quant  --hidden-dim 4096 --seq-lens 512 1024 2048 4096 --trials 100
+```
+
+## Parameter Description
+- `--seq-lens`: List of sequence lengths to test (default: 128 512 1024 2048)
+- `--hidden-dim`: Hidden dimension (default: 8192)
+- `--dtypes`: Data type list, `float16|bfloat16|float32` (default: bfloat16)
+- `--no-residual`: Only test "no residual" scenarios (default tests both "with/without residual")
+- Mutually exclusive quantization options:
+  - `--no-quant`: No quantization testing
+  - `--quant-fp8`: Only FP8 quantization testing
+  - `--quant-fp4`: Only FP4 quantization testing
+  - `--quant-all`: Test all (default)
+- FlashInfer related:
+  - `--disable-oneshot`: Disable oneshot mode (default enables oneshot and tests twoshot simultaneously)
+- Runtime configuration:
+  - `--warmup`: Warmup count before graph capture and before graph replay (default 5)
+  - `--trials`: Benchmark iteration count (default 20; internally each `graph.replay()` will batch replay multiple times)
+  - `--output-file`: Save results as Markdown file (only rank0 takes effect)
+
+## Output Example
+
+Each configuration group prints a table showing average execution time and relative speedup ratios (baseline is the faster standard implementation). For example:
+```
+================================================================================
+Results: seq_len=1024, hidden_dim=1024
+dtype=torch.bfloat16, residual=yes, quant_mode=none
+================================================================================
+Operation                                          Time (ms)    Speedup
+--------------------------------------------------------------------------------
+standard_allreduce_rmsnorm                         0.024        0.98x
+standard_allreduce_rmsnorm_native_compiled         0.023        baseline
+flashinfer_fused_allreduce_rmsnorm_oneshot         0.011        2.19x
+flashinfer_fused_allreduce_rmsnorm_twoshot         0.041        0.57x
+```
+
+If `--output-file` is specified, all configurations will be summarized in Markdown tables in that file.
+
+## Important Notes and Recommendations
+
+- Distributed: The script uses `torchrun` environment variables to initialize distributed training and binds tensors/communication groups to the current rank's corresponding device.
+- World size: Requires `WORLD_SIZE > 1` to perform communication operator benchmarks. Otherwise, the script will error and prompt.
+- FlashInfer:
+  - If not installed or interfaces are missing, the script will only run standard paths and provide prompts in the logs.
+  - The fused operator internally uses "oneshot"/"twoshot" two trigger methods; oneshot is enabled by default and twoshot is tested simultaneously.
+- FP8/FP4:
+  - FP8 uses sglang's FP8 tools and dtype, with underlying platform selection of `e4m3`/`e4m3fnuz` etc.
+  - FP4 uses sgl-kernel's `scaled_fp4_quant`, requiring corresponding platform support.
+- CUDA Graph:
+  - Uses sglang's `graph_capture()` to prepare capture-ready state for communication, then uses `torch.cuda.graph` to capture kernels, reducing measurement jitter.