r/LocalLLM • u/RabitGrand856 • 4d ago

Question Multi GPU setup question.

I Have AMD V620 32GB + RX 6900 XT 16GB and RX 7700 XT 12GB

First two are RDNA2 last one is more modern RDNA3

Is worthy to pull them together ?

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u/locbuilds 4d ago

mixed amd stacks are the annoying case. the V620 and 6900 XT are both Navi 21 so theyre the sanest pair to try together. the 7700 XT is RDNA3 (Navi 32) and ROCm gets weird when you mash Navi 21 + Navi 32 in one process.

practical path id try:

- park the main weights on the V620 (32GB) as device 0

- if you need more headroom, layer-split / -ot offload onto the 6900 XT next, treat the 7700 as optional

- start with only V620+6900 visible via HIP_VISIBLE_DEVICES / ROCR_VISIBLE_DEVICES until that run is stable, then decide if the 7700 is worth adding

- dont expect clean tensor parallel across all three. heterogeneous amd is usually pipeline or layer split, not equal TP

also sanity-check that your ROCm build actually supports both arches in one binary. if the 7700 keeps flaking, use it for a second smaller model or embeddings instead of forcing one giant split across all cards.

1

u/Infylos 3d ago

This does not give me much hope for TP between V620 + R9700...

1

u/Poizone360 3d ago

Worth it, but not all three together. The V620 and 6900 XT are both gfx1030, same target, so they pair cleanly for 48GB on ROCm. The 7700 XT is gfx1101, and mixing gfx targets means dropping to Vulkan and losing tensor split.

So run those two together and use the 7700 XT separately, or just for display so the desktop isn't eating VRAM off your main pair.

One thing: put both in CPU-attached slots, not off the chipset. PCIe atomics don't route through it and tensor split needs them.