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The 4090 indeed does have ecc support

Yes, but ECC is inline, so it costs bandwidth and memory capacity.

Doesn't it always. (Except sometimes on some hw you can't turn it off)

I believe the cards that are intended for compute instead of GPU default to ECC being on and report memory performance with the overheads included.

Anything with DDR5 or above has built in limited ECC... it's required by the spec. https://www.corsair.com/us/en/explorer/diy-builder/memory/is...

Sure, but it's very limited. It doesn't detect or fix errors in the dimm (outside the chips), motherboard traces, CPU socket, or CPU.