32 Gb DDR4 RAM module has a bandwidth of 25 Gb/s and costs $160. If you buy 8 of these, you get 256 Gb RAM with 200 Gb/s bandwidth at $1280. And if you buy 16 x 16 Gb modules (each at $60) then you can get 400 Gb/s of bandwidth for $960.

The only problem, you need 8 or 16 memory controllers. Memory controllers are not that expensive: Intel Core i3-14100F has 2 channel controller and costs $110, so we can estimate that 16-channel controller should cost not more than $880, and 8-channel controller should cost $440.

So isn't it better to make a cheap CPU with 16 DRAM controllers instead of this $4K gear having only 128 Gb? Or maybe 2 CPUs each having 8 RAM channels?

DDR5 costs 2 times more ($360 for 32 Gb) while not even having 2 times the bandwidth so it is not worth buying. It is more reasonable to make more RAM channels and stuff them with DDR4.

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wmf 35 minutes ago | parent | next [-]

If you want Epyc go for it. The motherboards can be quite expensive though.

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codedokode an hour ago | parent | prev [-]

So what I am trying to say, industry took a wrong turn. Instead of moving to over-priced DDR5, they should just make even cheapest CPUs support 8/16 DDR4 channels. Because a 32Gb DDR5-4800 module costs $360, and two 32Gb DDR4-3200 modules cost $320, so you get twice more size, more bandwidth and it costs you less. DDR5 is just a rip off.

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icedchai 32 minutes ago | parent | next [-]

In the alternate reality where this happened, wouldn't the price of DDR4 still be sky high? We'll ignore any costs for CPU, chip set, and motherboard redesign. You're just pushing the demand somewhere else.

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bradfa an hour ago | parent | prev | next [-]

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.

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tbrownaw 44 minutes ago | parent | prev [-]

> just make even cheapest CPUs support 8/16 DDR4 channels

Isn't adding pins kind of expensive?

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arnitdo 32 minutes ago | parent [-]

Well to be honest, there are a lot of NOOP pins on CPUs, but using them basically means fabbing a new die altogether, which is basically making a whole new CPU altogether.

	▲	bronson a minute ago \| parent [-]
		This is an odd comment. Pins are seriously expensive. Companies don't just throw them in for fun. Usually they're necessary for signal integrity or packaging constraints.