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Comment by ownagefool | Hacker News Reader

What's this trade off about?

I thought it was a simple 2 dims are probably better than 4, but unsure how you'd ever land on 48?

DRAM chips aren't always manufactured in power of two sizes. It's been common for years to have non power of two capacities for LPDDR used in phones, and has started to show up in other DRAM types with the current generation standards: DDR5 for desktops/servers and GDDR7 for GPUs. That's how there have been 24GB single-rank DIMMs and 48GB dual-rank DIMMs for desktops and 96GB RDIMMs for servers for a few years, and how a mobile RTX 5090 has 24GB VRAM vs mobile RTX 5080 having only 16GB VRAM despite both GPUs being different bins of the same silicon and both configurations using a 256-bit memory bus.

Not that simple. 4 dimms were getting higher clocks on 2 CCD Ryzen models (12 & 16 cores) compared to those with one CCD. Motherboard topology is a factor too.

But there is no single configuration where having 4 DIMMs populated gives higher speeds than when 4 DIMMs are populated on the same configuration. This is because while the higher end parts tend to have the higher binned components they still inly have 1 shared memory die between the CCD and the motherboard topology is either it has 4 slots or it doesn't, but no matter how they are ran it's still better to only use 1 rank of each channel.

More capacity is also harder to drive, even on the same number of channels, but needing to go from 2 to 4 channels is also a (bigger) drag.

You can go up to 64 GB per DIMM on the current consumer offerings (max of 256 GB total across 4 DIMMs). So you could could 128 GB over 2 DIMMs, but it's still going to perform worse than 2x24 GB or 2x16 GB.

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