[bradfa]

Each memory controller interface is a not-insignificant number of PCB traces. Increasing the number of memory controllers may dramatically increase the number of PCB layers (or may not, it really depends on the CPU pinout) but it definitely will increase the number of pins on the CPU socket.

This is one of the main reasons (the other is the number of PCIe lanes) why high end desktop and server CPUs have like double the number of pins and so much bigger sockets as compared to consumer desktop CPUs.

[codedokode]

Then what's about using 4-8 cheapest motherboards with 64Gb DDR4 and a cheap CPU, and connecting them via PCIE x16 sockets?

And as for DRAM channels, typical cheap motherboard has 2 channels and 4 slots, it should not be super difficult to add 2 more channels.

[bradfa]

To get memory mapping across systems you'd want to look into infiniband (https://en.wikipedia.org/wiki/InfiniBand) but the adapters generally aren't cheap. Common to find in super computer clusters.

If you have a desktop CPU with 2 memory channels and 2 DIMMs per channel, then the 2 DIMMs on each single channel share all of the address and data lines, there's just a chip select difference to pick which DIMM you're addressing. There is some extra loading on the lines, since you have 2 DIMMs, hence the supported speeds are usually slightly lower, but you only add 1-2 more actual PCB traces to have 2 DIMMs per channel vs 1 DIMM per channel. In contrast, adding another memory controller would add upwards of almost 100 additional PCB traces from the CPU.

[icedchai]

Don't you need a new CPU to support more channels? It doesn't sound like a cheap, trivial change to upgrade my Ryzen to a Threadripper.