this post was submitted on 02 Apr 2025
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Is there any technical reason for some memory bus widths only showing up in cut-down GPUs? (lemmy.world)
submitted 1 year ago by flavonol@lemmy.world to c/hardware@lemmy.world
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There are some bus widths that are used in full discrete GPU dies and some that I've only ever seen in cut-down GPUs. Full-die GPU memory bus widths seem to always be:
64-bit × 2^n^ × (1 or 0.75)
where n is a whole number. Cut-down models can feature pretty much any multiple of 32 bits.

Examples:

  • 128-bit bus: common in entry-level GPUs (RTX 4060, RX 6600 XT)
  • 160-bit bus: occasionally shows up in cut-down designs (Arc B570)
  • 192-bit bus: common in midrange & entry-level GPUs (RTX 5070, Arc B580)
  • 256-bit bus: common in midrange GPUs (RX 9070 XT, RTX 3070 Ti)
  • 320-bit bus: occasionally shows up in cut-down designs (RX 7900 XT, RTX 3080)
  • 352-bit bus: Appeared in the RTX 2080 Ti, which was cut down
  • 384-bit bus: common in upper-midrange & high-end GPUs (RX 7900 XTX, RTX 4090)

Any insights into why this is? As a layperson, it seems like having a full die with perhaps a 160-bit bus for the entry level or a 224-bit bus for the midrange would at least occasionally make sense.

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[–] UnfortunateShort@lemmy.world 1 points 1 year ago

If I'm not mistaken, most memory chips use 32-bit-wide interfaces, meaning you are stuck with multiples of that. They come in multiple capacities, I think around 1 or 2 GB ones (including 1920 MB / 2 GiB / whatever) are most common these days. The 2 GB ones are higher density, but more expensive, so you need to decide what you care about first. You also don't want to let to bandwidth go too low, so your performance doesn't tank.

You then decide much capacity you want on your card. That decision is primarily driven by cost and demand, but you also don't want the gaps between SKUs to be too small. It just doesn't make sense to have tons of SKUs very close together, neither for you, nor your competitors. So the industry came up with a difference of 2 chips ≈ 2 or 4 GB as the minimum difference.

We see 4, 8 12, 16 and so on GB cards, but rarely something like 5 or 10, because for 8+ GB, you would typically use the 2 GB chips, so you have 4 GB jumps there, and for the sub-8-GB ones you have 1 GB chips, hence 2 GB jumps. This leads to 64 bit jumps in bus width, so 128, 192, 256, 320... bit bus-widths.

128 is already very low in terms of bandwidth, I don't know if you can actually find this on any modern cards, but that's about the reasoning here.

And I guess we don't start at 3 or 5 GB because it's just weird, I don't know :D