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The darwin vllm-cpp image built the Metal backend with vllm.cpp's native MSL
GEMM only. vllm.cpp also ships an optional MLX provider for the dense GEMM,
kept OFF upstream because it costs a ~19 MB libmlx.dylib plus a ~105 MB
mlx.metallib, on the stated position that it must earn that cost by
measurement.
Measured on an Apple M4 (16 GiB, macOS 26.5.2) it does. One binary, arms
toggled with VT_OP_PROVIDER_DISABLE=mlx so there is no build-difference
confound, Qwen3-1.7B-bf16 p=512 g=128, 2 reps, arm order alternated per rep:
B=1 5.79 vs 3.08 agg tok/s (1.88x) TTFT 3.32 s vs 7.68 s
B=8 25.70 vs 13.69 (1.88x) TTFT 13.95 s vs 34.38 s
B=16 38.65 vs 17.69 (2.19x) TTFT 18.33 s vs 54.48 s
Peak RSS is unchanged (6.65 to 7.50 GB in both arms) and the output is
bit-identical: vllm.cpp's three-way parity test measures mlx-vs-msl NMSE of 0
on all six shapes, and mlx-vs-cpu equal to msl-vs-cpu, against a 5e-4 bar. MLX
serves the dense GEMM alone; paged attention stays vllm.cpp's own kernel
because MLX has no paged-KV primitive. Full disposition, including the
INDICATIVE status and the isolation actually achieved, is in vllm.cpp
docs/BENCHMARKS.md "MLX GEMM provider A/B on Apple M4".
Build: MLX comes from the pinned prebuilt pip wheel (MLX_VERSION, default
0.29.3) into a venv under the backend dir. Building MLX from source needs
`xcrun metal`, i.e. a full Xcode the macOS runners do not have, while the wheel
ships include/, lib/libmlx.dylib and the compiled metallib ready to link. The
install is a stamp FILE rather than a phony target, because a phony
prerequisite is always newer than libvllm and would re-link it every
invocation. VLLM_CPP_MLX=off restores the previous Metal build.
Packaging vendors libmlx.dylib, mlx.metallib and MLX's MIT license into
package/lib/. Three things this had to get right, each verified on the M4
before it was written rather than after:
1. libvllm.dylib links @rpath/libmlx.dylib and its build-time LC_RPATH points
inside the build venv, a path no user has. Every build rpath is deleted
and replaced with @loader_path/lib.
2. MLX loads its metallib from beside its OWN dylib, so both files must land
in the same directory or every Metal op fails with "Failed to load the
default metallib".
3. install_name_tool invalidates the code signature and macOS refuses to load
an arm64 image with a stale one, so the patched library is re-signed
ad-hoc.
Verified end to end on the M4 by building through this Makefile and running the
packaged artifact: `DYLD_PRINT_LIBRARIES` resolves libmlx from package/lib/,
`codesign -v` passes, no build-venv path survives in the load commands, and a
real generation runs with the provider selected (op=65 selected=mlx) and zero
metallib failures. A missing rpath now fails the build instead of the user's
first inference.
Cost: the darwin vllm-cpp image grows by about 124 MB.
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude Code:claude-opus-5 [ClaudeCode]