Dual-GPU on AM5 delivers 0.1GB/s instead of 8GB/s: Promontory bridge blamed
Ed-2-Zero-9 · reddit · 2026-08-28
A Reddit user tried to run dual GPUs on AM5 (AMD R9700 32GB + 9070 XT 16GB, for local inference). Slot 2 should run at PCIe 4x for roughly 8GB/s, but measured only 0.1GB/s. Per Claude's analysis, ASMedia's Promontory 21 bridge chips on AM5 boards don't implement AtomicOp routing, with two of them daisy-chained — nothing above that layer can fix it, and Windows would hit the same wall.
The author concludes dual-GPU on AM5 is likely a no-go and asks the community for expert confirmation. Replies may add more real-world dual-card platform experience.
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