Qwen3-VL Usage

Qwen3-VL Usage#

Qwen3-VL is Alibaba’s latest multimodal large language model with strong text, vision, and reasoning capabilities. SGLang supports Qwen3-VL Family of models with Image and Video input support.

Launch commands for SGLang#

Below are suggested launch commands tailored for different hardware / precision modes

FP8 (quantised) mode#

For high memory-efficiency and latency optimized deployments (e.g., on H100, H200) where FP8 checkpoint is supported:

python3 -m sglang.launch_server \
  --model-path Qwen/Qwen3-VL-235B-A22B-Instruct-FP8 \
  --tp 8 \
  --ep 8 \
  --host 0.0.0.0 \
  --port 30000 \
  --keep-mm-feature-on-device

Non-FP8 (BF16 / full precision) mode#

For deployments on A100/H100 where BF16 is used (or FP8 snapshot not used):

python3 -m sglang.launch_server \
  --model-path Qwen/Qwen3-VL-235B-A22B-Instruct \
  --tp 8 \
  --ep 8 \
  --host 0.0.0.0 \
  --port 30000 \

Hardware-specific notes / recommendations#

  • On H100 with FP8: Use the FP8 checkpoint for best memory efficiency.

  • On A100 / H100 with BF16 (non-FP8): It’s recommended to use --mm-max-concurrent-calls to control parallel throughput and GPU memory usage during image/video inference.

  • On H200 & B200: The model can be run “out of the box”, supporting full context length plus concurrent image + video processing.

Sending Image/Video Requests#

Image input:#

import requests

url = f"http://localhost:30000/v1/chat/completions"

data = {
    "model": "Qwen/Qwen3-VL-30B-A3B-Instruct",
    "messages": [
        {
            "role": "user",
            "content": [
                {"type": "text", "text": "What’s in this image?"},
                {
                    "type": "image_url",
                    "image_url": {
                        "url": "https://github.com/sgl-project/sglang/blob/main/test/lang/example_image.png?raw=true"
                    },
                },
            ],
        }
    ],
    "max_tokens": 300,
}

response = requests.post(url, json=data)
print(response.text)

Video Input:#

import requests

url = f"http://localhost:30000/v1/chat/completions"

data = {
    "model": "Qwen/Qwen3-VL-30B-A3B-Instruct",
    "messages": [
        {
            "role": "user",
            "content": [
                {"type": "text", "text": "What’s happening in this video?"},
                {
                    "type": "video_url",
                    "video_url": {
                        "url": "https://github.com/sgl-project/sgl-test-files/raw/refs/heads/main/videos/jobs_presenting_ipod.mp4"
                    },
                },
            ],
        }
    ],
    "max_tokens": 300,
}

response = requests.post(url, json=data)
print(response.text)

Important Server Parameters and Flags#

When launching the model server for multimodal support, you can use the following command-line arguments to fine-tune performance and behavior:

  • --mm-attention-backend: Specify multimodal attention backend. Eg. fa3(Flash Attention 3)

  • --mm-max-concurrent-calls <value>: Specifies the maximum number of concurrent asynchronous multimodal data processing calls allowed on the server. Use this to control parallel throughput and GPU memory usage during image/video inference.

  • --mm-per-request-timeout <seconds>: Defines the timeout duration (in seconds) for each multimodal request. If a request exceeds this time limit (e.g., for very large video inputs), it will be automatically terminated.

  • --keep-mm-feature-on-device: Instructs the server to retain multimodal feature tensors on the GPU after processing. This avoids device-to-host (D2H) memory copies and improves performance for repeated or high-frequency inference workloads.

  • SGLANG_USE_CUDA_IPC_TRANSPORT=1: Shared memory pool based CUDA IPC for multi-modal data transport. For significantly improving e2e latency.

Example usage with the above optimizations:#

SGLANG_USE_CUDA_IPC_TRANSPORT=1 \
SGLANG_VLM_CACHE_SIZE_MB=0 \
python -m sglang.launch_server \
  --model-path Qwen/Qwen3-VL-235B-A22B-Instruct \
  --host 0.0.0.0 \
  --port 30000 \
  --trust-remote-code \
  --tp-size 8 \
  --enable-cache-report \
  --log-level info \
  --max-running-requests 64 \
  --mem-fraction-static 0.65 \
  --chunked-prefill-size 8192 \
  --attention-backend fa3 \
  --mm-attention-backend fa3 \
  --enable-metrics
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