The discussion focuses on the limitations of utilizing both GPUs in a Mac Pro 2013 (MP 6,1) configuration. Users report that under macOS, one GPU is dedicated to display tasks while the other is reserved for compute workloads, leading to inefficient resource utilization. Unlike Windows, which offers basic CrossFire X support, macOS lacks a system-wide equivalent that distributes rendering tasks across both GPUs. Consequently, most applications, including games, only leverage a single GPU unless specifically designed to do so by developers.
PREREQUISITESThis discussion is beneficial for Mac Pro users, game developers, and performance analysts interested in optimizing GPU usage and understanding the limitations of multi-GPU configurations in macOS.
With two GPUs standard in every Mac Pro configuration, there’s obviously OS support for the configuration. Under Windows, that amounts to basic CrossFire X support.
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Under OS X the situation is a bit more complicated. There is no system-wide CrossFire X equivalent that will automatically split up rendering tasks across both GPUs. By default, one GPU is setup for display duties while the other is used exclusively for GPU compute workloads. GPUs are notoriously bad at context switching, which can severely limit compute performance if the GPU also has to deal with the rendering workloads associated with display in a modern OS.
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Due to the nature of the default GPU division under OS X, all games by default will only use a single GPU. It is up to the game developer to recognize and split rendering across both GPUs, which no one is doing at present. Unfortunately firing up two instances of a 3D workload won’t load balance across the two GPUs by default. I ran Unigine Heaven and Valley benchmarks in parallel, unfortunately both were scheduled on the display GPU leaving the compute GPU completely idle.