art with code

2018-03-12

Unified Interconnect

Playing with InfiniBand got me thinking. This thing is basically a PCIe to PCIe -bridge. The old kit runs at x4 PCIe 3 speeds, the new stuff is x16 PCIe. The next generation is x16 PCIe 4.0 and 5.0.

Why jump through all the hoops? Thunderbolt is x4 PCIe over a USB connector. What if you bundle four of those, throw in some fiber and transceivers for long distance. You get x16 PCIe between two devices.

And once you start thinking of computers as a series of components hanging off a PCIe bus, your system architecture clarifies dramatically. A CPU is a PCIe 3 device with 40 to 64 lanes. DRAM uses around 16 lanes per channel.

GPUs are now hooked up as 16-lane devices, but could saturate 256 to 1024 lanes. Because of that, GPU RAM is on the GPU board. If the GPU had enough lanes, you could hook GPU RAM up to the PCIe bus with 32 lanes per GDDR5 chip. HBM is probably too close to the GPU to separate.

You could build a computer with 1024 lanes, then start filling them up with the mix of GPUs, CPUs, DRAM channels, NVMe and connectivity that you require. Three CPUs with seven DRAM channels? Sure. Need an extra CPU or two? Just plug them in. How about a GPU-only box with the CPU OS on another node. Or CPUs connected as accelerators via 8 lanes per socket as you'd like to use the extra lanes for other stuff. Or a mix of x86 and ARM CPU cards to let you run mixed workloads at max speed and power efficiency.

Think of a rack of servers, sharing a single PCIe bus. It'd be like one big computer with everything hotpluggable. Or a data center, running a single massive OS instance with 4 million cores and 16 PB of RAM.

Appendix, more devices

Then you've got the rest of the devices, and they're pretty well on the bus already. NVMe comes in 4-lane blocks. Thunderbolt 3, Thunderbolt 2 and USB 3.1 are 4, 2 and 1 lane devices. SAS and SATA are a bit awkward, taking up a bit more than 1 lane or bit more than half a lane. I'd replace them with NVMe connectors.

Display connectors could live on the bus as well (given some QoS to keep up the interactivity). HDMI 2.1 uses 6 lanes, HDMI 2 is a bit more than 2 lanes. DisplayPort's next generation might go up to 7-8 lanes.

Existing kit

[Edit] Hey, someone's got products like this already. Dolphin ICS produces a range of PCIe network devices. They've even got an IP-over-PCIe driver.

[Edit #2] Hey, the Gen-Z Interconnect looks a bit like this Gen-Z Interconnect Core Specification 1.0 Published

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