Mobos, and 2 channel vs. 4 channel memory

aviation1972 · June 26, 2019

I'm very happy to have access to the knowledge base on this forum. I have a couple of basic questions. I'm trying to learn. Hope you can bear with me. What is the significance, ( the difference ), between motherboards with 4 Dimm slots, versus motherboards with 8 Dimm slots? Can motherboards with 4 Dimm slots use 4 channel memory, or just 2 channel memory? I believe the 8 Dimm slot motherboards can use 4 channel memory, but are not necessarily required to. Is that right? Why is 2 channel memory sufficient? Not everyone chooses to use 4 channel memory. Right? Does 2 channel memory cause lesser, or downgraded performance specs? For gaming purposes, ( given the current, top-of-the-line hardware that's available today ), what would be most advantageous? 2 channel memory, or 4 channel memory? My leanings are toward Intel, because I'm into flight simulation, ( but most folks regard that as gaming ). I'm not against AMD, however. I just want the fastest possible cpu speed, and not a lot of useless cores and threads. I believe my current flight simulation software is not written in a format that interacts with the multiple-cores of cpus. So a single core cpu, ( or a cpu with only a few cores ), is just fine for flight simulation. Just last week, Microsoft announced a newly refreshed version of Microsoft Flight Simulator is about to be released. I have no idea how the new software will be written, and whether it will make use of multi-core cpus in ways that the current software does not. Thanks so much for taking your time, to answer my rudimentary questions. Your answers and info will help me a lot. Thanks.

Jurrunio · June 26, 2019

10 minutes ago, aviation1972 said:

between motherboards with 4 Dimm slots, versus motherboards with 8 Dimm slots?

max memory stick count of 4 versus 8.

10 minutes ago, aviation1972 said:

Can motherboards with 4 Dimm slots use 4 channel memory, or just 2 channel memory?

Depends on the CPU. Those that support quad channel can also run dual channel, but not the opposite way.

10 minutes ago, aviation1972 said:

I believe the 8 Dimm slot motherboards can use 4 channel memory, but are not necessarily required to. Is that right?

Yes

10 minutes ago, aviation1972 said:

Why is 2 channel memory sufficient?

that's what consumer parts currently max out (so LGA 1151 socket CPUs for Intel, AM4 socket CPUs for AMD).

10 minutes ago, aviation1972 said:

Not everyone chooses to use 4 channel memory. Right?

People dont have a choice unless they go HEDT (high end desktop, currently LGA2066 socket CPUs for Intel excluding the quad core CPUs, TR4 socket CPUs for AMD). Those cost so much that quad channel memory is affordable.

10 minutes ago, aviation1972 said:

Does 2 channel memory cause lesser, or downgraded performance specs?

Yes

10 minutes ago, aviation1972 said:

For gaming purposes, ( given the current, top-of-the-line hardware that's available today ), what would be most advantageous? 2 channel memory, or 4 channel memory?

quad channel of course. That said HEDT stuff inherently dont game as well as consumer parts that max out at dual channel memory because they have too many cores to run high frequencies on their cores. Core to core latency is also increased compared to consumer parts.

Fasauceome · June 26, 2019

6 minutes ago, aviation1972 said:

Can motherboards with 4 Dimm slots use 4 channel memory, or just 2 channel memory?

It's not just the motherboard, but the processor inside it that can use dual vs quad channel. Although they have to go hand in hand, a CPU that can use quad channel dropped into a motherboard with 2 dimm slots is of course gone to waste, and a CPU that only has dual channel capabilities will disable ram slots on a quad channel board (though there are extremely few cases of these, like the i7 7740x)

8 minutes ago, aviation1972 said:

I believe the 8 Dimm slot motherboards can use 4 channel memory, but are not necessarily required to

Yep, any number of channels accessible can be dropped, so a quad channel capable CPU can do triple, dual, and single. Six channel can do 5, 4, 3, etc.

9 minutes ago, aviation1972 said:

Why is 2 channel memory sufficient?

For most consumers, that's plenty of memory bandwidth, populate your dimm slots with DDR4 3200 and you'd be hard pressed to see a performance increase past that in most applications.

mariushm · June 26, 2019

It's also about compromises.

A DDR4 slot has 288 contacts... About 150-200 of those are used to transmit signals (data) between the RAM stick and the CPU.

So for every channel, around 200 pins in the cpu socket will be used to connect one or two memory slots to the CPU.

For example, the AM4 socket has 1331 pins.. probably 4-500 pins are just for memory, for the dual channel configuration which allows up to 4 memory slots.

If you add more channels, you have to add more pins which increases the cost of the CPU socket and it also makes it much more difficult for those wires to be routed from under the CPU socket to the actual memory slots. You can't just wire those wires in some random way, going around other components on the motherboard and so on... those wires must be in pairs and the length of the pairs must match to the millimeter otherwise you can't achieve those high frequencies. That's why you often see those wiggles and curves on traces on the circuit boards - the wiggles are added to make some wires longer on the circuit board in order to get all wires the same length at the end.

So if you want more channels, you have to squeeze more such wires on the circuit board. You're making your work easier by placing some memory slots on one side of the motherboard and another set on the other side.

Even so, with more than 4 memory slots, you most likely have to go from using a circuit board with only 4 or 6 layers with connections between them to a motherboard that has 8 or 10 or even more layers. This increases the cost of the circuit board significantly ... think maybe 5-10$ for a mATX motherboard with 4 layers (let's say some budget A320 based motherboard) to 15-20$ for a 6 layer motherboard to maybe 30-50$ for a 8 layer motherboard.

The speed with a single memory channel is something like 15-25 GB/s.. in dual channel mode you're doubling that, getting twice the speed, depending on frequencies and latency with DDR4 you're getting up to around 60-80 GB/s of transfer speed.

Current modern applications simply don't deal with huge chunks of data, so individual pieces of data are most of the time transferred from RAM to CPU caches fast, within a few microseconds. For that, two channels are good enough, and going up to triple and quad channels most of the time won't help ( it's like 100 nanoseconds for quad channel vs 120 nanosecond for dual channel... because it takes 50-80 nanoseconds for the brains in ram sticks to actually go to the locations in the ram chips where data is located and then the data starts streaming to cpu - data is streamed twice as fast in the quad channel configuration compared to dual channel, but with small chunks of data the "setup time" is what takes a lot of time and that's the same regardless of how many channels you have.

You can compare it like pci-e slots. Your graphics card can work in pci-e x16 slot, but could also work in pci-e x8 slot and you'd lose maybe 1-3% of performance.... mostly because you're not transferring hundreds of megabytes of data all that time to make those 16 channels useful to the max.

Well, it's the same with memory ... the dual channel is like running in a pci-e x8 slot... you know there may be some performance increase in some games by going to a pci-e x16 slot but the increase in performance will be minimal.