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fix: Default to the CPU device when Metal is not available - #148
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georgeguimaraes wants to merge 2 commits into
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georgeguimaraes wants to merge 2 commits into
georgeguimaraes wants to merge 2 commits into
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EMLX.default_device/0 returned :gpu when :default_device was not configured. On Linux there is no GPU worker, so a jit with compiler: EMLX and no device: option failed. Now default_device/0 returns :cpu when the application did not start a GPU worker. A configured :default_device still wins.
polvalente
reviewed
Oct 20, 2026
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| def default_device do | ||
| Application.get_env(:emlx, :default_device, :gpu) | ||
| case Application.get_env(:emlx, :default_device) do | ||
| nil -> | ||
| # EMLX.Application starts a GPU worker only when Metal is available. | ||
| if :persistent_term.get({EMLX, :default_worker, :gpu}, nil), do: :gpu, else: :cpu | ||
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| device -> | ||
| device | ||
| end | ||
| end |
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I think this is not the correct solution and should be solved at the runtime config level.
As this stands, there can be a race condition where an app runs EMLX.default_device before the default worker is started and therefore gets cpu instead of gpu.
We should expose mlx::core::is_available as a NIF and call it with the result of Application.get_env(:emlx, :default_device, :gpu). If the configured default device is not available, we return :cpu instead
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EMLX.default_device/0returns:gpuwhen:default_deviceis not configured. On Linux,EMLX.Applicationdoes not start a GPU worker, so ajitwithcompiler: EMLXand nodevice:option fails. Users have to passdevice: :cputo every compile, or set the config, even though the CPU is the only device.I found this in CI on Linux, where the same code worked on a Mac.
With this change,
default_device/0returns:cpuwhen the application did not start a GPU worker. A configured:default_devicestill wins, and on a Mac with Metal the default is still:gpu.