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Infinity fabric isn't serial. Wikichip [1] says there are two versions of infinity fabric (which is a super-set of HyperTransport), one optimised for on-packag

by phire 6y ago

Infinity fabric isn't serial. Wikichip [1] says there are two versions of infinity fabric (which is a super-set of HyperTransport), one optimised for on-package communication that is 32 bits wide, and one optimised for inter-socket communication that is 16bits wide. I'm not a hardware person, just a software person who dabbles in hardware, so I don't know if the term "SerDes" strongly implies serial and AMD have misused it here, or if SerDes is generic enough to apply to any SERialisation/DESerialisation. But still, I think you are on the right track. The investment and development into low power, high-speed, and low-latency on-package links is probably what has enabled chiplets to become relevant now. Because multi-chip modules have always existed. [1] https://en.wikichip.org/wiki/amd/infinity_fabric https://en.wikichip.org/wiki/amd/infinity_fabric

deleted 6y ago
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temac 6y ago
IFIS multiplexes with PCIe so it probably qualifies as quite serial. IFOP a little less, but that's blurry at this point.
dfox 6y ago
Probably all modern fast interconnects work by interleaving some kind of "physical frames" (often bytes with some 8B10B encoding) across multiple distinct serial links that are synchronized as if these frames were bits of parallel interface. The primary reason for this design is that at current frequencies it is essentially impossible to manufacture the physical parallel interface with equal-enough wire lengths. Interestingly, memory interfaces (like DDR4) use opposite approach: the interface is still mostly parallel, but memory controller measures the delays and mismatches of the physical wires and compensates for that in its timing.
- cogman10 6y ago
  > the interface is still mostly parallel Really? That's crazy! I thought DDR was serial connections. In fact, I thought parallel connections had mostly gone the way of the dodo. Serial is just so much less complicated.

by phire 6y ago

deleted 6y ago

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temac 6y ago

IFIS multiplexes with PCIe so it probably qualifies as quite serial. IFOP a little less, but that's blurry at this point.

dfox 6y ago

Probably all modern fast interconnects work by interleaving some kind of "physical frames" (often bytes with some 8B10B encoding) across multiple distinct serial links that are synchronized as if these frames were bits of parallel interface. The primary reason for this design is that at current frequencies it is essentially impossible to manufacture the physical parallel interface with equal-enough wire lengths. Interestingly, memory interfaces (like DDR4) use opposite approach: the interface is still mostly parallel, but memory controller measures the delays and mismatches of the physical wires and compensates for that in its timing.

cogman10 6y ago
> the interface is still mostly parallel Really? That's crazy! I thought DDR was serial connections. In fact, I thought parallel connections had mostly gone the way of the dodo. Serial is just so much less complicated.