Metal Debugger#

Profiling is a key step for performance optimization. You can build MLX with the MLX_METAL_DEBUG option to improve the Metal debugging and optimization workflow. The MLX_METAL_DEBUG debug option:

  • Records source during Metal compilation, for later inspection while debugging.

  • Labels Metal objects such as command queues, improving capture readability.

To build with debugging enabled in Python prepend CMAKE_ARGS="-DMLX_METAL_DEBUG=ON" to the build call.

The metal.start_capture() function initiates a capture of all MLX GPU work.


To capture a GPU trace you must run the application with MTL_CAPTURE_ENABLED=1.

import mlx.core as mx

a = mx.random.uniform(shape=(512, 512))
b = mx.random.uniform(shape=(512, 512))
mx.eval(a, b)

trace_file = "mlx_trace.gputrace"

# Make sure to run with MTL_CAPTURE_ENABLED=1 and
# that the path trace_file does not already exist.

for _ in range(10):
  mx.eval(mx.add(a, b))


You can open and replay the GPU trace in Xcode. The Dependencies view has a great overview of all operations. Checkout the Metal debugger documentation for more information.


Xcode Workflow#

You can skip saving to a path by running within Xcode. First, generate an Xcode project using CMake.

mkdir build && cd build
cmake .. -DMLX_METAL_DEBUG=ON -G Xcode
open mlx.xcodeproj

Select the metal_capture example schema and run.