Fill all memory slots?

[Thomas4]

I'm in the very early stages of gathering information on the parts for a new build, but there is one final question that I have and haven't been able to find a recommendation on

For video editing I'm planning to use 32Gb RAM and I'm not sure if its better to use 2x 16Gb sticks or 4x 8Gb sticks. Price difference is irrelevant, as I'm wondering if one is better than the other or is it immaterial.

 I'm only concerned about best usage of available RAM, and is it possible that one will give me any sort of speed advantage, the latter not being THAT important, but I am curious.

To answer a few possible questions, I'm planning to use DDR5 type memory..

[BiotechBen]

Unless you have a chip and board that support quad channel memory, the 4 sticks are not going to be a benefit. 

 

 

If you were running database operations, the extra bandwidth of 4 sticks vs 2 may be advantageous, but with more sticks, generally speed decreases due to mem controller limitations. That said, with DDR5 the controller is on the dimm, so that matters less, but the quad channel support still remains the limiting factor.

[RONOTHAN##]

TL;DR: With DDR5, use 2 DIMMs when ever possible. 

 

For the more complete answer, there are a number of factors at play. 

1. DDR5 memory topologies are very weak right now, and 4 DIMM setups for the most part are just kinda broken. Default speeds drop down to something like 4000MT/s or lower (worse than DDR4) if it even works at all. ASUS has seemed to get it to kinda work, but it still doesn't really make sense for most people. 
2. The reason why 4x8GB performed better on DDR4 compared to 2x16GB is because it was a question of memory ranks. More memory ranks leads to more rank interleaving and better performance per clock, though there are usually diminishing returns after 2 ranks (on DDR4, DDR5 is different), so quad rank and triple rank were barely different in performance for given settings and triple rank is only marginally faster than dual rank in some synthetic tasks. 2x16GB DDR4 DIMMs, unless you were very particular with the memory you were buying, was pretty much always single rank towards the end of DDR4's lifespan, and since ranks only needed to be present in the memory channel, running 2 DIMMs per channel (4 DIMMs total on a consumer motherboard) was the easiest way to guarantee effectively dual rank operation. Single rank operation was still preferred in a lot of applications though for quite a while, because the more ranks a memory controller has to control the more stress is on the memory controller and, depending on the memory controller, it can significantly reduce the max frequency achievable (5GT/s RAM, for example, is not super difficult to achieve on a single rank setup on a Ryzen 3000/5000 memory controller when desynchronized with a good motherboard and RAM kit, but with dual rank the memory speed drops to about 4GT/s if you're lucky). Dual rank does still make sense in a lot of situations, 10th gen and later Intel DDR4 memory controllers and Ryzen 3000/5000 series chips do perform best for daily with dual rank memory since either the memory controller can either handle running high speeds (Intel) or there just isn't much of a reason to be running that high a frequency in general (Ryzen). 
3. With DDR5, the rank discussion is a bit different. The 8GB DDR5 DIMMs are all guaranteed to be 1Rx16, which (for a couple of reasons) is kinda like half rank memory (not exactly but close enough) and has significantly less performance than 16GB DDR5 DIMMs. Running 4x8GB DDR5 would be basically running the same as running 2x16GB DDR5, but with all of the downsides of running 4 DIMMs. 
4. Even when running 4x16GB or 2x32GB setups (both of which are proper dual rank), DDR5 doesn't show nearly the same amount of benefit from rank interleaving that DDR4 did, so the performance difference between single rank DDR5 and dual rank DDR5 is very similar to the difference between dual rank DDR4 and quad rank DDR4 (it exists, but is only really noticeable in 1-2 benchmarks). Unless you need the capacity, there is no real performance benefit by going for 64GB of DDR5 compared to 32GB of DDR5, especially since, as with DDR4, it puts a lot more stress on the memory controller and would reduce the max speed you can reasonably expect (if you're just running XMP of 6200MT/s or slower though, that doesn't really matter since they should both be about equal for that). 

[xg32]

2 hours ago, Thomas4 said:

.

only use 2x16 with ddr5

[Mister Woof]

Do they sell dummy RGB sticks for ddr5 het? Because a very valid reason to use all 4 slots is cuz it looks GUD

[Thomas4]

Thank you all for the rapid responses. With a special thanks of appreciation to RONTHAN## for a more detailed explanation of the situation.

As I said, I'm going to be fundamentally video editing so 2x 16 it will be.

I wish when building a computer, tech builders would talk about such things, Or maybe Linus could even do a talk about it as I think that more people may be have the same question in the back of their minds.