Benchmark and Profiling

Benchmark and Profiling#

Benchmark#

  • Benchmark the latency of running a single static batch without a server. The arguments are the same as for launch_server.py. Note that this is a simplified test script without a dynamic batching server, so it may run out of memory for a batch size that a real server can handle. A real server truncates the prefill into several batches, while this simplified script does not.

    • Without a server (do not need to launch a server)

      python -m sglang.bench_one_batch --model-path meta-llama/Meta-Llama-3.1-8B-Instruct --batch 32 --input-len 256 --output-len 32

    • With a server (please use sglang.launch_server to launch a server first and run the following command.)

      python -m sglang.bench_one_batch_server --base-url http://127.0.0.1:30000 --model-path meta-llama/Meta-Llama-3.1-8B-Instruct --batch-size 32 --input-len 256 --output-len 32

  • Benchmark offline processing. This script will start an offline engine and run the benchmark.

    python3 -m sglang.bench_offline_throughput --model-path meta-llama/Meta-Llama-3.1-8B-Instruct --num-prompts 10

  • Benchmark online serving. Please use sglang.launch_server to launch a server first and run the following command.

    python3 -m sglang.bench_serving --backend sglang --num-prompt 10

Profile with PyTorch Profiler#

Pytorch Profiler is a convenient basic tool to inspect kernel execution time, call stack, and kernel overlap and occupancy.

Profile a server with sglang.bench_serving#

# set trace path
export SGLANG_TORCH_PROFILER_DIR=/root/sglang/profile_log

# start server
python -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct

# send profiling request from client
python -m sglang.bench_serving --backend sglang --model meta-llama/Llama-3.1-8B-Instruct --num-prompts 10 --sharegpt-output-len 100 --profile

Please make sure that the SGLANG_TORCH_PROFILER_DIR should be set at both server and client side, otherwise the trace file cannot be generated correctly . A secure way will be setting SGLANG_TORCH_PROFILER_DIR in the .*rc file of shell (e.g. ~/.bashrc for bash shells).

For more details, please refer to Bench Serving Guide.

Profile In PD Disaggregation Mode#

When profiling in PD disaggregation mode, prefill and decode workers must be profiled separately due to torch profiler limitations. The bench_serving command provides dedicated options for this:

Profile Prefill Workers#

# set trace path
export SGLANG_TORCH_PROFILER_DIR=/root/sglang/profile_log

# start prefill and decode servers (see PD disaggregation docs for setup)
python -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct --disaggregation-mode prefill
python -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct --disaggregation-mode decode --port 30001 --base-gpu-id 1

# start router
python -m sglang_router.launch_router --pd-disaggregation --prefill http://127.0.0.1:30000 --decode http://127.0.0.1:30001 --host 0.0.0.0 --port 8000

# send profiling request targeting prefill workers
python -m sglang.bench_serving --backend sglang --model meta-llama/Llama-3.1-8B-Instruct --num-prompts 10 --sharegpt-output-len 100 --profile --pd-separated --profile-prefill-url http://127.0.0.1:30000

Profile Decode Workers#

# send profiling request targeting decode workers
python -m sglang.bench_serving --backend sglang --model meta-llama/Llama-3.1-8B-Instruct --num-prompts 10 --sharegpt-output-len 100 --profile --pd-separated --profile-decode-url http://127.0.0.1:30001

Important Notes#

  • --profile-prefill-url and --profile-decode-url are mutually exclusive - you cannot profile both at the same time

  • Both options support multiple worker URLs for multi-instance setups:

    # Profile multiple prefill workers
python -m sglang.bench_serving --backend sglang --model meta-llama/Llama-3.1-8B-Instruct --num-prompts 10 --profile --pd-separated --profile-prefill-url http://127.0.0.1:30000 http://127.0.0.1:30002

# Profile multiple decode workers
python -m sglang.bench_serving --backend sglang --model meta-llama/Llama-3.1-8B-Instruct --num-prompts 10 --profile --pd-separated --profile-decode-url http://127.0.0.1:30001 http://127.0.0.1:30003

  • Make sure SGLANG_TORCH_PROFILER_DIR is set on all worker nodes before starting the servers

  • For more details on setting up PD disaggregation, see PD Disaggregation Guide

Profile a server with sglang.bench_offline_throughput#

export SGLANG_TORCH_PROFILER_DIR=/root/sglang/profile_log

# profile one batch with bench_one_batch.py
# batch size can be controlled with --batch argument
python3 -m sglang.bench_one_batch --model-path meta-llama/Llama-3.1-8B-Instruct --batch 32 --input-len 1024 --output-len 10 --profile

# profile multiple batches with bench_offline_throughput.py
python -m sglang.bench_offline_throughput --model-path meta-llama/Llama-3.1-8B-Instruct --dataset-name random --num-prompts 10 --profile --mem-frac=0.8

Profile a server with sglang.profiler#

When the server is running (e.g., processing a decoding request), you can start live profiling immediately by sending a profile request to the server.

You can do this by running python3 -m sglang.profiler. For example:

# Terminal 1: Send a generation request
python3 -m sglang.test.send_one

# Terminal 2: Before the above request finishes, quickly launch the following command in a separate terminal.
# It will generate a profile of the above request for several decoding batches.
python3 -m sglang.profiler

You can also combine the above operations into a single command

python3 -m sglang.test.send_one --profile

Profile a server with HTTP API endpoints#

SGLang provides HTTP API endpoints to control profiling on a running server. This allows you to start and stop profiling programmatically, which is useful for capturing specific workload patterns.

Using /start_profile endpoint#

The /start_profile endpoint starts profiling on the server. You can control when profiling begins and how long it runs using the following parameters:

Basic usage:

# Start profiling immediately for 10 steps
curl -X POST http://127.0.0.1:30000/start_profile \
  -H "Content-Type: application/json" \
  -d '{
    "num_steps": 10
  }'

Parameters:

  • output_dir (optional): Directory where profile traces will be saved. If not specified, uses SGLANG_TORCH_PROFILER_DIR environment variable, or /tmp as the default

  • num_steps (optional): Number of steps to profile. If not specified, profiling continues until manually stopped with /end_profile

  • start_step (optional): Step number at which to start profiling (inclusive). Useful for skipping warmup iterations

  • activities (optional): List of activities to profile, e.g., ["CPU", "GPU"]. Default is ["CPU", "GPU"]

  • merge_profiles (optional): Whether to merge distributed traces. Default is false

Note on step ranges: Profiling starts at start_step (inclusive) and continues for num_steps iterations. For example, with start_step=3 and num_steps=10, profiling captures steps 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 (10 steps total, starting from step 3).

Advanced usage with start_step:

# Wait 5 steps (warmup), then profile for 10 steps
curl -X POST http://127.0.0.1:30000/start_profile \
  -H "Content-Type: application/json" \
  -d '{
    "output_dir": "/tmp/profiles",
    "start_step": 5,
    "num_steps": 10,
    "activities": ["CPU", "GPU"]
  }'

Continuous profiling (manual stop):

# Start profiling without num_steps - must manually stop with /end_profile
curl -X POST http://127.0.0.1:30000/start_profile

Using /end_profile endpoint#

The /end_profile endpoint stops an ongoing profiling session and saves the trace file.

# Stop profiling and save traces
curl -X POST http://127.0.0.1:30000/end_profile

This is only needed when you start profiling without specifying num_steps. If num_steps is specified, profiling will automatically stop after that many steps.

Example workflow#

# Terminal 1: Start the server
export SGLANG_TORCH_PROFILER_DIR=/tmp/profiles
python -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct

# Terminal 2: Start continuous profiling
curl -X POST http://127.0.0.1:30000/start_profile \
  -H "Content-Type: application/json" \
  -d '{
    "start_step": 3
  }'

# Terminal 3: Send requests to generate load
python -m sglang.bench_serving --backend sglang --num-prompts 100

# Terminal 2: Stop profiling when done
curl -X POST http://127.0.0.1:30000/end_profile

Profiler Trace Merger for Distributed Traces#

SGLang now supports automatic merging of profiling traces from distributed setups with multiple parallelism types (TP, DP, PP, EP). This feature is particularly useful for analyzing performance across distributed runs.

Multi-Node Profiling and Shared Storage Considerations#

Single-node profiler output merging is completely supported. When profiling in distributed environments spanning multiple nodes, shared storage (e.g., NFS, Lustre) should be accessible by all nodes for the output directory to enable merging of trace files.

If there is no shared storage accessible across nodes, automatic merging of trace files during profiling is not supported directly as of now.

HTTP API Usage#

# Start profiling with automatic trace merging enabled
curl -X POST <BASE_URL>/start_profile \
  -H "Content-Type: application/json" \
  -d '{
    "output_dir": "/tmp/profiles", # where to store profile traces
    "num_steps": 10,
    "activities": ["CPU", "GPU"],
    "merge_profiles": true # optional argument to merge profile traces (default=False)
  }'

Command Line Usage#

# Start profiling with merge enabled
python -m sglang.profiler \
  --num-steps 10 \
  --cpu \
  --gpu \
  --output-dir /tmp/profiles \
  --merge-profiles # optional argument to merge profile traces (default=False)

Output Files#

The profile merger generates:

  • Individual rank trace files: {profile_id}-TP-{tp}-DP-{dp}-PP-{pp}-EP-{ep}.trace.json.gz

  • Merged trace file: merged-{profile_id}.trace.json.gz

Possible PyTorch bugs#

If in any cases you encounter the following error (for example, using qwen 2.5 VL):

RuntimeError: !stack.empty() INTERNAL ASSERT FAILED at "/pytorch/torch/csrc/autograd/profiler_python.cpp":983, please report a bug to PyTorch. Python replay stack is empty.

This is likely a PyTorch Bug reported in Bug: vLLM Profiler and Bug: torch.profiler.profile. As a workaround, you may disable with_stack with an environment variable such as follows:

export SGLANG_PROFILE_WITH_STACK=False
python -m sglang.bench_offline_throughput --model-path meta-llama/Llama-3.1-8B-Instruct --dataset-name random --num-prompts 10 --profile --mem-frac=0.8

View traces#

Trace files can be loaded and visualized from:

  1. https://ui.perfetto.dev/ (any browser)

  2. chrome://tracing (Chrome browser only)

If browser cannot open trace file due to its large size, client can generate a small trace file (<100MB) by controlling number of prompts and lengths of prompt outputs. For example, when profiling a server,

python -m sglang.bench_serving --backend sglang --model meta-llama/Llama-3.1-8B-Instruct --num-prompts 2 --sharegpt-output-len 100 --profile

This command sets the number of prompts to 2 with --num-prompts argument and limits the length of output sequences to 100 with --sharegpt-output-len argument, which can generate a small trace file for browser to open smoothly.

Additionally, if you want to locate the SGLang Python source code through the cuda kernel in Trace, you need to disable CUDA Graph when starting the service. This can be done by using the --disable-cuda-graph parameter in the command to start the service.

Profile with Nsight#

Nsight systems is an advanced tool that exposes more profiling details, such as register and shared memory usage, annotated code regions and low-level CUDA APIs and events.

  1. Prerequisite:

    Install using apt, or run inside a NVIDIA Docker container or SGLang Docker container.

    # install nsys
# https://docs.nvidia.com/nsight-systems/InstallationGuide/index.html
apt update
apt install -y --no-install-recommends gnupg
echo "deb http://developer.download.nvidia.com/devtools/repos/ubuntu$(source /etc/lsb-release; echo "$DISTRIB_RELEASE" | tr -d .)/$(dpkg --print-architecture) /" | tee /etc/apt/sources.list.d/nvidia-devtools.list
apt-key adv --fetch-keys http://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64/7fa2af80.pub
apt update
apt install nsight-systems-cli

  2. To profile a single batch, use

    nsys profile --trace-fork-before-exec=true --cuda-graph-trace=node python3 -m sglang.bench_one_batch --model meta-llama/Meta-Llama-3-8B --batch-size 64 --input-len 512

  3. To profile a server, e.g.

    # launch the server, set the delay and duration times according to needs
# after the duration time has been used up, server will be killed by nsys

nsys profile --trace-fork-before-exec=true --cuda-graph-trace=node -o sglang.out --delay 60 --duration 70 python3 -m sglang.launch_server --model-path meta-llama/Llama-3.1-8B-Instruct --disable-radix-cache

# client
python3 -m sglang.bench_serving --backend sglang --num-prompts 1000 --dataset-name random --random-input 1024 --random-output 512

    In practice, we recommend users to set --duration argument to a large value. Whenever user wants the server to stop profiling. Firstly run:

    nsys sessions list

    to get the session id in the form of profile-XXXXX, then run:

    nsys stop --session=profile-XXXXX

    to manually kill the profiler and generate nsys-rep files instantly.

  4. Use NVTX to annotate code regions, e.g. to see their execution time.

    # install nvtx
pip install nvtx

    # code snippets
import nvtx
with nvtx.annotate("description", color="color"):
    # some critical code

Layer-wise NVTX Profiling with Nsight Systems#

SGLang provides built-in layerwise NVTX annotations that can be combined with the CUDA Profiler for detailed per-layer profiling in Nsight Systems. This is particularly useful for identifying performance bottlenecks at the layer level.

Using --enable-layerwise-nvtx-marker with Nsight Systems and /start_profile#

The --enable-layerwise-nvtx-marker flag automatically adds NVTX markers to every layer in your model. This is particularly powerful when combined with Nsight Systems profiling to see detailed per-layer performance.

Method 1: Using /start_profile with CUDA_PROFILER (for programmatic control)

This method allows you to control exactly when profiling starts/stops via HTTP API while Nsight Systems is running.

  1. Launch the server with layerwise NVTX enabled under Nsight Systems:

    # Terminal 1: Start server with nsys and capture-range option
nsys profile --trace-fork-before-exec=true \
  --cuda-graph-trace=node \
  --capture-range=cudaProfilerApi \
  --capture-range-end=stop \
  -o layerwise_profile \
  python -m sglang.launch_server \
    --model-path meta-llama/Llama-3.1-8B-Instruct \
    --enable-layerwise-nvtx-marker \
    --disable-cuda-graph

    Note: NVTX markers are not emitted for kernel launches captured by CUDA graphs. Use --disable-cuda-graph to ensure all layerwise NVTX markers are emitted in the trace.

  2. In another terminal, control profiling via /start_profile with CUDA_PROFILER activity:

    # Terminal 2: Wait for server to be ready, then start CUDA profiling
# Wait 3 steps for warmup, then profile for 10 steps
curl -X POST http://127.0.0.1:30000/start_profile \
  -H "Content-Type: application/json" \
  -d '{
    "start_step": 3,
    "num_steps": 10,
    "activities": ["CUDA_PROFILER"]
  }'

  3. Send requests to generate load:

    # Terminal 3: Generate workload
python -m sglang.bench_serving --backend sglang --num-prompts 100

  4. Profiling will automatically stop after 10 steps (due to num_steps: 10). If you hadn’t specified num_steps, you would need to manually stop it:

    # Terminal 2: Only needed if num_steps was not specified
curl -X POST http://127.0.0.1:30000/end_profile

The --capture-range=cudaProfilerApi option tells Nsight Systems to only capture data between cudaProfilerStart() and cudaProfilerStop() calls (triggered by /start_profile and /end_profile), reducing overhead and file size. The start_step parameter skips the first 3 steps to avoid capturing warmup overhead.

Method 2: Simpler approach without /start_profile API

For simpler use cases where you don’t need fine-grained control over profiling start/stop, you can profile with Nsight Systems capturing the entire workload:

# Terminal 1: Start server with layerwise NVTX
# Note: --disable-cuda-graph ensures all NVTX markers are emitted
python -m sglang.launch_server \
  --model-path meta-llama/Llama-3.1-8B-Instruct \
  --enable-layerwise-nvtx-marker \
  --disable-cuda-graph

# Terminal 2: Profile the benchmarking client
nsys profile --trace-fork-before-exec=true \
  --cuda-graph-trace=node \
  -o layerwise_profile \
  python -m sglang.bench_serving --backend sglang --num-prompts 10

This approach profiles the entire client execution, including all server interactions. The layerwise NVTX markers will be visible in the Nsight Systems timeline.

Viewing the profiling results:

Open the generated .qdrep file with Nsight Systems:

nsys-ui layerwise_profile.qdrep

In the Nsight Systems GUI, you’ll see:

  • NVTX ranges: Each layer appears as a labeled range in the timeline with detailed information in the marker metadata

  • CUDA kernels: All GPU kernels are shown alongside the layer annotations

  • Layer hierarchy: The full module path (e.g., meta-llama/Meta-Llama-3.1-8B-Instruct.model.layers.0.self_attn.qkv_proj) helps identify specific layers. The prefix uses the full model path from --model-path.

  • Tensor shapes: Input/output dimensions and parameter shapes are included in the NVTX marker data

Benefits of layerwise NVTX profiling:

  • Granular visibility: See exactly which layers are taking the most time

  • Memory tracking: Identify layers with large memory allocations

  • Bottleneck identification: Quickly locate inefficient operations

  • Communication overhead: In multi-GPU setups, see per-layer communication costs

  • Development debugging: Validate that model architecture changes have the expected performance impact

Other tips#

  1. You can benchmark a model using dummy weights by only providing the config.json file. This allows for quick testing of model variants without training. To do so, add --load-format dummy to the above commands and then you only need a correct config.json under the checkpoint folder.

  2. You can benchmark a model with modified configs (e.g., less layers) by using --json-model-override-args. For example, you can benchmark a model with only 2 layers and 2 kv heads using:

    python -m sglang.bench_one_batch --model-path meta-llama/Meta-Llama-3.1-8B-Instruct --batch 32 --input-len 256 --output-len 32 --load-format dummy --json-model-override-args '{"num_hidden_layers": 1, "num_key_value_heads": 1}'

  3. You can use --python-backtrace=cuda to see python call stack for all CUDA kernels, as in PyTorch Profiler. (Caveat: this can cause inaccurately long kernel runtimes for CUDA event based timing)

  4. For more arguments see Nsight Systems User Guide.

Contents