A Quick Note on Architecture & Features

With pages upon pages of architectural documents still to get through in only a few hours, for today’s launch news I’m not going to have the time to go in depth on new features or the architecture. So I want to very briefly hit the high points on what the major features are, and also provide some answers to what are likely to be some common questions.

Starting with the architecture itself, one of the biggest changes for RDNA is the width of a wavefront, the fundamental group of work. GCN in all of its iterations was 64 threads wide, meaning 64 threads were bundled together into a single wavefront for execution. RDNA drops this to a native 32 threads wide. At the same time, AMD has expanded the width of their SIMDs from 16 slots to 32 (aka SIMD32), meaning the size of a wavefront now matches the SIMD size. This is one of AMD’s key architectural efficiency changes, as it helps them keep their SIMD slots occupied more often. It also means that a wavefront can be passed through the SIMDs in a single cycle, instead of over 4 cycles on GCN parts.

In terms of compute, there are not any notable feature changes here as far as gaming is concerned. How things work under the hood has changed dramatically at points, but from the perspective of a programmer, there aren’t really any new math operations here that are going to turn things on their head. RDNA of course supports Rapid Packed Math (Fast FP16), so programmers who make use of FP16 will get to enjoy those performance benefits.

With a single exception, there also aren’t any new graphics features. Navi does not include any hardware ray tracing support, nor does it support variable rate pixel shading. AMD is aware of the demands for these, and hardware support for ray tracing is in their roadmap for RDNA 2 (the architecture formally known as “Next Gen”). But none of that is present here.

The one exception to all of this is the primitive shader. Vega’s most infamous feature is back, and better still it’s enabled this time. The primitive shader is compiler controlled, and thanks to some hardware changes to make it more useful, it now makes sense for AMD to turn it on for gaming. Vega’s primitive shader, though fully hardware functional, was difficult to get a real-world performance boost from, and as a result AMD never exposed it on Vega.

Unique in consumer parts for the new 5700 series cards is support for PCI Express 4.0. Designed to go hand-in-hand with AMD’s Ryzen 3000 series CPUs, which are introducing support for the feature as well, PCIe 4.0 doubles the amount of bus bandwidth available to the card, rising from ~16GB/sec to ~32GB/sec. The real world performance implications of this are limited at this time, especially for a card in the 5700 series’ performance segment. But there are situations where it will be useful, particularly on the content creation side of matters.

Finally, AMD has partially updated their display controller. I say “partially” because while it’s technically an update, they aren’t bringing much new to the table. Notably, HDMI 2.1 support isn’t present – nor is more limited support for HDMI 2.1 Variable Rate Refresh. Instead, AMD’s display controller is a lot like Vega’s: DisplayPort 1.4 and HDMI 2.0b, including support for AMD’s proprietary Freesync-over-HDMI standard. So AMD does have variable rate capabilities for TVs, but it isn’t the HDMI standard’s own implementation.

The one notable change here is support for DisplayPort 1.4 Display Stream Compression. DSC, as implied by the name, compresses the image going out to the monitor to reduce the amount of bandwidth needed. This is important going forward for 4K@144Hz displays, as DP1.4 itself doesn’t provide enough bandwidth for them (leading to other workarounds such as NVIDIA’s 4:2:2 chroma subsampling on G-Sync HDR monitors). This is a feature we’ve talked off and on about for a while, and it’s taken some time for the tech to really get standardized and brought to a point where it’s viable in a consumer product.

AMD Announces Radeon RX 5700 XT & RX 5700 Addendum: AMD Slide Decks
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  • Mil0 - Tuesday, June 11, 2019 - link

    Well for the first release of a new arch, this is pretty good. More FineWine potential here, also with arch tweaks - see how much they gained with Zen 2.
    And efficiency wise I think the cooperation with Samsungccould turn out to be very helpful. Targeting mobile/tablets necessitates this.
  • neblogai - Tuesday, June 11, 2019 - link

    It is because it is clocked so. If you look at the 5700, that is rated the same 180W, with performance (judging from the slides) very close to 2070. Of course, that is ~parity when comparing the products- cards, not the technological level (because this is all 12nm nVidia vs 7nm AMD).
  • mode_13h - Wednesday, June 12, 2019 - link

    Looking at AMD's GPU roadmap slides, they say that perf/W will be an area of continued focus.

    That's the right idea, anyhow. Efficiency isn't a one-time feature - it's an ongoing journey.

    Nvidia has gone so far that it's not surprising AMD hasn't yet caught them.
  • evernessince - Wednesday, June 12, 2019 - link

    If you factor in that the 5700 XT is faster, those power numbers seem extremely competitive.
  • CiccioB - Tuesday, June 18, 2019 - link

    When you factor the 5700 XT is a PP ahead of nvidia, those power numbers (and die size) are awful for the performance they get.
  • m16 - Tuesday, June 11, 2019 - link

    I see no problem with no HDMI support. That's a TV standard, and can easily get adapted from DP via an adapter. It also makes the card pricier to have a new controller to support that standard, so there's that. They might release a HTPC version of a card later to support that, but that's not even in my or most of the PC gamers' I know radar.
  • Beaver M. - Tuesday, June 11, 2019 - link

    They are both crap. You should wait for the next gen, which is probably coming next year anyway.
  • evernessince - Wednesday, June 12, 2019 - link

    Ray Tracing and DLSS are worthless on a 2070. At least with AMD you can use their upscaler / sharpener with a less then 1% performance impact with better results then DLSS. I don't see a problem with AMD introducing features that work instead of pipe dreams that suck in implementation like Nvidia.
  • haukionkannel - Thursday, June 13, 2019 - link

    Ray trasing is irrelevant at this point. Only one card 2080ti can run raytrasing at reasonable speed... and 2080ti cost 1200$ so no it is good that Navi don`t have it yet!
    If Navi will have it next year. Expect less speed with more money next year... because raytrasing takes up chip space for pure raw draving speed...
  • powerarmour - Monday, June 10, 2019 - link

    Probably DOA these...

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