# Benchmark and Profiling

## Benchmark

SGLang provides four benchmark tools that operate at different levels of the stack. The table below summarizes their key differences:

| Tool                       | HTTP Server                                   | Scheduler                               | Use Case                                                                   |
| -------------------------- | --------------------------------------------- | --------------------------------------- | -------------------------------------------------------------------------- |
| `bench_serving`            | Yes (async HTTP client to a running server)   | Yes (indirectly, via server)            | Realistic online serving benchmarks with latency metrics (TTFT, TPOT, ITL) |
| `bench_one_batch_server`   | Yes (sends HTTP requests to a running server) | Yes (indirectly, via server)            | End-to-end single-batch latency including HTTP and scheduler overhead      |
| `bench_offline_throughput` | No                                            | Yes (directly uses `Engine` in-process) | Maximum throughput measurement without HTTP overhead                       |
| `bench_one_batch`          | No                                            | No (directly calls `ModelRunner`)       | Kernel-level latency profiling of a single static batch                    |

Use `bench_serving` by default unless there are specific needs.

**`bench_serving`** is an async HTTP load-testing client that sends requests at controlled rates with configurable concurrency to a running server. It measures realistic online serving metrics including time-to-first-token (TTFT), time-per-output-token (TPOT), inter-token latency (ITL), and throughput. Use `num-prompts >= 5 * max-concurrency` to measure steady-state performance. Launch a server with `sglang.launch_server` first.

  ```bash
  python3 -m sglang.bench_serving --backend sglang --max-concurrency 16 --num-prompts 80 --random-input-len 256 --random-output-len 32 --dataset-name random
  ```

**`bench_one_batch_server`** sends a single batch as one HTTP request to a running server. Due to only having a single batch, the server is never in a steady-state and metrics will be biased. Launch a server with `sglang.launch_server` first.

  ```bash
  python3 -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
  ```

**`bench_offline_throughput`** directly instantiates the `Engine` object in-process (no HTTP server) and submits all requests at once via `engine.generate()`. The engine's scheduler handles batching and execution. This measures maximum achievable throughput without any network overhead.

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

**`bench_one_batch`** is the lowest-level tool. It directly instantiates a `ModelRunner` and calls `extend()` / `decode()` on a fixed static batch, bypassing the scheduler entirely. The prefill and decode phases are run separately, making profiling easier but rendering the metrics unrealistic. Because there is no dynamic batching, it may run out of memory for batch sizes that a real server can handle (a real server chunks prefill into smaller batches). This is best suited for profiling individual kernel performance.

  ```bash
  python3 -m sglang.bench_one_batch --model-path meta-llama/Meta-Llama-3.1-8B-Instruct --batch-size 32 --input-len 256 --output-len 32
  ```

## Profile with PyTorch Profiler

[Pytorch Profiler](https://pytorch.org/tutorials/recipes/recipes/profiler_recipe.html) is a convenient basic tool to inspect kernel execution time, call stack, and kernel overlap and occupancy.

### Profile a server with `sglang.bench_serving`

```bash
# 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
```

The `SGLANG_TORCH_PROFILER_DIR` environment variable must be set on both the server and client side; otherwise, the trace file will not be generated correctly. A secure way to do this is by setting it in your shell's resource file (e.g., `~/.bashrc` for bash).

For more details, please refer to [Bench Serving Guide](./bench_serving.md).

### 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

```bash
# 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

```bash
# 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:
  ```bash
  # 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](../advanced_features/pd_disaggregation.md)

### Profile a server with `sglang.bench_offline_throughput`
```bash
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:**

```bash
# 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`:**

```bash
# 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):**

```bash
# 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.

```bash
# 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

```bash
# 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

```bash
# 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

```bash
# 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):
```bash
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](https://github.com/vllm-project/vllm/issues/18240) and [Bug: torch.profiler.profile](https://github.com/pytorch/pytorch/issues/101632). As a workaround, you may disable `with_stack` with an environment variable such as follows:
```bash
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,

```bash
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](https://docs.nvidia.com/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](https://catalog.ngc.nvidia.com/orgs/nvidia/containers/pytorch/tags) or [SGLang Docker container](https://github.com/sgl-project/sglang/tree/main/docker).

   ```bash
   # 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

   ```bash
   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.

   ```bash
   # 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:

   ```bash
   nsys sessions list
   ```

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

   ```bash
   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.

   ```bash
   # install nvtx
   pip install nvtx
   ```

   ```python
   # 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:

   ```bash
   # 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:

   ```bash
   # 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:

   ```bash
   # 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:

   ```bash
   # 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:

```bash
# 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:

```bash
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:

   ```bash
   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](https://docs.nvidia.com/nsight-systems/UserGuide/index.html).