Now I'm remembering... They had the phi as a coprocessor in a PCI slot, effectively making it just as issue as a GPU. But the second gen (knights landing) made the phi the host processor, but removed almost all ability for external devices. It had potential I think, but it was a weird transition from v1 to v2.
I actually found the Coprocessor more interesting.
Yeah, NVidia CUDA makes a better coprocessor for deep learning and matrix multiplication. But a CPU-based coprocessor for adding extra cores to a system seems like it'd be better for some class of problems.
SIMD compute is great and all, but I kind of prefer to see different solutions in the computer world. I guess that the classic 8-way socket with Xeon 8180 is more straightforward (though expensive).
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A Xeon Phi on its own motherboard is just competing with regular ol' Xeons. Granted, at a cheaper price... but its too similar to normal CPUs.
Xeon Phi was probably trying to do too many unique things. It used HMC memory instead of GDDR5x or HBM (or DDR4). It was a CPU in a GPU form factor. It was a GPU (ish) running its own OS. It was just really weird. I keep looking at the thing in theory, and wondering what problem it'd be best at solving. All sorts of weird decisions, nothing else was ever really built like it.
Or alternatively, an AMD EPYC (64-cores / 128x PCIe lanes).