AMD Big Navi and RDNA 2 GPUs: Everything We Know

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AMD Big Navi and RDNA 2 GPUs: Everything We Know

AMD Big Navi, RX 6000, Navi 2x, RDNA 2. Whatever the name, AMD’s latest GPUs promise big performance and efficiency gains, along with feature parity with Nvidia in terms of ray tracing support. Team Red finally puts up some serious competition in our GPU benchmarks hierarchy and provides several of the best graphics cards, going head to head with the Nvidia Ampere architecture.

AMD officially unveiled Big Navi on October 28, 2020, including specs for the RX 6900 XT, RTX 6800 XT, and RTX 6800. The Radeon RX 6800 XT and RX 6800 launched first, followed by the Radeon RX 6900 XT. In March 2021, AMD released the Radeon RX 6700 XT, and more recently the Radeon RX 6600 XT. So far, we haven’t seen a trimmed down Navi 22 variant (except in mobile), and AMD instead opted to create Navi 23, the smallest ‘Big Navi’ GPU to date at less than half the size of Navi 21. We’ve updated this article with revised details, though there are still future RDNA2 products yet to be revealed.

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Based on what we’ve seen, Big Navi has finally put AMD’s high graphics card power consumption behind it. Or at least, Big Navi is no worse than Nvidia’s RTX 30-series cards, considering the 3080 and 3090 have the highest Nvidia TDPs for single GPUs ever. Let’s start at the top, with the new RDNA2 architecture that powers RX 6000 / Big Navi / Navi 2x. Here’s everything we know about AMD Big Navi, including the RDNA 2 architecture, specifications, performance, pricing, and availability.

Every generation of GPUs is built from a core architecture, and each architecture offers improvements over the previous generation. It’s an iterative and additive process that never really ends. AMD’s GCN architecture went from first generation for its HD 7000 cards in 2012 up through fifth gen in the Vega and Radeon VII cards in 2017-2019. The RDNA architecture that powers the RX 5000 series of AMD GPUs arrived in mid 2019, bringing major improvements to efficiency and overall performance. RDNA2 doubled down on those improvements in late 2020.

First, a quick recap of RDNA 1 is in order. The biggest changes with RDNA 1 over GCN involve a redistribution of resources and a change in how instructions are handled. In some ways, RDNA doesn’t appear to be all that different from GCN. The instruction set is the same, but how those instructions are dispatched and executed has been improved. RDNA also added working support for primitive shaders, something present in the Vega GCN architecture that never got turned on due to complications.

Perhaps the most noteworthy update is that the wavefronts—the core unit of work that gets executed—have been changed from being 64 threads wide with four SIMD16 execution units, to being 32 threads wide with a single SIMD32 execution unit. SIMD stands for Single Instruction, Multiple Data; it’s a vector processing element that optimizes workloads where the same instruction needs to be run on large chunks of data, which is common in graphics workloads.

This matching of the wavefront size to the SIMD size helps improve efficiency. GCN issued one instruction per wave every four cycles; RDNA issues an instruction every cycle. GCN used a wavefront of 64 threads (work items); RDNA supports 32- and 64-thread wavefronts. GCN has a Compute Unit (CU) with 64 GPU cores, 4 TMUs (Texture Mapping Units) and memory access logic. RDNA implements a new Workgroup Processor (WGP) that consists of two CUs, with each CU still providing the same 64 GPU cores and 4 TMUs plus memory access logic.

How much do these changes matter when it comes to actual performance and efficiency? It’s perhaps best illustrated by looking at the Radeon VII, AMD’s last GCN GPU, and comparing it with the RX 5700 XT. Radeon VII has 60 CUs, 3840 GPU cores, 16GB of HBM2 memory with 1 TBps of bandwidth, a GPU clock speed of up to 1750 MHz, and a theoretical peak performance rating of 13.8 TFLOPS. The RX 5700 XT has 40 CUs, 2560 GPU cores, 8GB of GDDR6 memory with 448 GBps of bandwidth, and clocks at up to 1905 MHz with peak performance of 9.75 TFLOPS.

On paper, Radeon VII looks like it should come out with an easy victory. In practice, across a dozen games that we’ve tested, the RX 5700 XT is slightly faster at 1080p gaming and slightly slower at 1440p. Only at 4K is the Radeon VII able to manage a 7% lead, helped no doubt by its memory bandwidth. Overall, the Radeon VII only has a 1-2% performance advantage, but it uses 300W compared to the RX 5700 XT’s 225W.

In short, AMD was able to deliver roughly the same performance as the previous generation, with a third fewer cores, less than half the memory bandwidth and using 25% less power. That’s a very impressive showing, and while TSMC’s 7nm FinFET manufacturing process certainly warrants some of the credit (especially in regards to power), the performance uplift is mostly thanks to the RDNA architecture.

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