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by dragontamer 521 days ago
> And the splitting into CDNA and RDNA comes from the same direction: market segmentation

Not really.

Wave64 on CDNA is provably more throughput. But with most video game code written for NVidia's Wave32, RDNA being reworked to be more NVidia-like and Wave32 is how you reach better practical video game performance.

HPC will prefer the wider execute, 64-bit execution, and other benefits.

Video Gamers will prefer massive amounts of 32MB+ of "Infinity cache", which is used in practice for all kinds of screen-space calculations. But this would NEVER be used for fluid dynamics.
1 comments
markstock 520 days ago
Maybe never by the big players, but RDNA and even fp32 are perfectly fine for a number of CFD algorithms and uses; Stable Fluids-like algorithms and Lagrangian Vortex Particle Methods to name two.
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dragontamer 520 days ago
I'm talking about Wave64.

CDNA executes 64-threads per compute unit per clock tick. RDNA only executes 32-threads. CDNA is smaller, more efficient, more parallel and much higher compute than RDNA.

Furthermore, all ROCm code from GCN (and older) was on Wave64, because historically AMD's architecture from 2010 through 2020 was Wave64. RDNA changed to Wave32 so that they can match NVidia and have slightly better latency characteristics (at the cost of bandwidth).

CDNA has more compute bandwidth and parallelism. RDNA is narrower, faster latency and less parallelism. Building a GPU out of 2048-bit compute (aka: 64-lanes x 32-bit wide/CDNA) is always going to be more bandwidth than 1024-bit compute (aka: 32-lanes x 32-bit wide) like RDNA.

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markstock 520 days ago
I wasn't familiar with the "Wave32" term, but took "RDNA" to mean the smaller wavefront size. I've used both, and wave32 is still quite effective for CFD.
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dragontamer 520 days ago
ROCm support for RDNA took like 2 years, maybe longer.

If you actually were using both, you'd know that CDNA was the only supported platform on ROCm for what felt like an eternity. That's because CDNA was designed to be as similar to GCN so that ROCm could support it easier.

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What I'm saying is that today, now that ROCm works on RDNA and CDNA, the two architectures can finally be unified into UDNA. And everyone should be happy with the state of code moving forward.

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