Apple Silicon with Metal - Sglang

This document describes how run SGLang on Apple Silicon using Metal (MLX). If you encounter issues or have questions, please open an issue.

Prerequisites

Building the native Metal kernels in sgl-kernel requires the Apple toolchain (clang++, the Metal framework headers, and xcrun). These ship with the Xcode Command Line Tools, which cannot be installed via pip:

bash

xcode-select --install

If you have the full Xcode app installed, the Command Line Tools are already available. You can verify with xcode-select -p && xcrun --find metal.

Install SGLang

You can install SGLang using one of the methods below.

Install from Source

bash

# Use the default branch
git clone https://github.com/sgl-project/sglang.git
cd sglang

# Create and activate a virtual environment
uv venv -p 3.12 sglang-metal
source sglang-metal/bin/activate

# (Optional) Compile sgl-kernel
uv pip install --upgrade pip
uv run sgl-kernel/setup_metal.py install

# Install sglang python package along with diffusion support
rm -f python/pyproject.toml && mv python/pyproject_other.toml python/pyproject.toml
uv pip install -e "python[all_mps]"

Launch of the Serving Engine

Launch the server with:

bash

SGLANG_USE_MLX=1 python -m sglang.launch_server \
  --model <MODEL_ID_OR_PATH> \
  --disable-cuda-graph \
  --host 0.0.0.0

Key Parameters Explained:

SGLANG_USE_MLX=1 - Enables the use of MLX as the SGLang runtime backend (if disabled, SGLang will fall back to torch.mps, which has less support)
--disable-cuda-graph - Disables usage of CUDA graph, which is not relevant for Apple Metal.
--disable-overlap-schedule - Disables overlap scheduling (enabled/not present by default) achieved using MLX's async_eval()

Quantization

The MLX backend supports two quantization paths on Apple Silicon:

Pre-quantized HF repos. Any mlx-community/<model>-4bit (or -8bit) repo loads directly through mlx_lm.load(...) — no extra flag needed.
bash
```
SGLANG_USE_MLX=1 python -m sglang.launch_server \
  --model-path mlx-community/Qwen3-0.6B-4bit \
  --disable-cuda-graph
```
On-the-fly quantization. For any fp16 model, pass --quantization mlx_q4 or --quantization mlx_q8 to have sglang quantize the weights at load time via mlx_lm.utils.quantize_model (group size 64, the mlx-community default). The quantized weights stay in process memory; the on-disk model is untouched.
bash
```
SGLANG_USE_MLX=1 python -m sglang.launch_server \
  --model-path Qwen/Qwen3-0.6B \
  --quantization mlx_q4 \
  --disable-cuda-graph
```
Expected log line:
```
Quantizing MLX model on-the-fly: bits=4 group_size=64 (preset=mlx_q4)
Quantization complete in 0.13s — active mem: 1.11 GB -> 0.31 GB (71.9% reduction)
```
The MLX backend silently ignores --quantization mlx_q4 when the model is already quantized in its HF config (path 1), so the same flag is safe to pass either way.

Benchmarking with Requests

sglang.benchmark_one_batch calls the synchronous prefill/decode methods directly without going through the scheduler and the overlap code path.

sglang.benchmark_offline_throughput can toggle overlap scheduling as it uses the scheduler and the overlap code path by using the flag --disable-overlap-schedule.

Throughput Testing

Basic synchronous one batch throughput:

bash

SGLANG_USE_MLX=1 python -m sglang.bench_one_batch \
  --model-path <MODEL_ID_OR_PATH> \
  --disable-cuda-graph \
  --tp-size 1 \
  --batch-size 1 \
  --input-len 60 \
  --output-len 10

Synchronous offline throughput:

bash

SGLANG_USE_MLX=1 python -m sglang.bench_offline_throughput \
  --model-path <MODEL_ID_OR_PATH> \
  --disable-cuda-graph \
  --num-prompts 1 \
  --disable-overlap-schedule

Asynchronous offline throughput:

bash

SGLANG_USE_MLX=1 python -m sglang.bench_offline_throughput \
  --model-path <MODEL_ID_OR_PATH> \
  --disable-cuda-graph \
  --num-prompts 1