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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  • eva02langley - Thursday, June 13, 2019 - link

    Just go back to WCCF man...
  • R3MF - Tuesday, June 11, 2019 - link

    Re: display controller remaining a little ~2018

    Do you think the display controller might get an update for hdmi 2.1 / VRR for low end Navi later this year?

    Later release, so more time to introduce improvements, and more necessary for a part that will be attractive to the home theatre market...
  • TheUnhandledException - Tuesday, June 11, 2019 - link

    HDMI 2.1 hardware is still really expensive and power hungry. Given the dearth of HDMI 2.1 displays and the fact that these cards wouldn't even do 30 fps at 8K I don't see the lack fo HDMI 2.1 being a big deal. DP 1.4 is far more useful for these mid range cards.
  • R3MF - Wednesday, June 12, 2019 - link

    Sure, but it would be nice to get the bandwidth necessary to do:
    4:4:4 chroma / 10bit colour / /HDR / 4k at 60Hz.

    Or even use VRR to let the framerate roll between 40 and 80 fps.

    hdmi 2.1 will be normal on midrange 4k (+fald) TV's next year, shame for small Navi to miss the marketing boat.
  • ET - Tuesday, June 11, 2019 - link

    If AMD is going back to suffixes, it should have at least gone for something more interesting like a palindrome. RX 5775 XR would have been quite cool.
  • Oxford Guy - Tuesday, June 11, 2019 - link

    You're more creative than they are.
  • MDD1963 - Tuesday, June 11, 2019 - link

    That GPU looks like someone took it to the gym in a gym bag, and accidentally dropped a 45 LB plate on it...; the dented look is horrible, IMO... Fire the entire aesthetics team!
  • Psycho_McCrazy - Tuesday, June 11, 2019 - link

    Ryan, can I request you to include UW(3440.1440, 3840.1600) resolutions in the review of GPUs, starting with Navi reviews??
  • gijames1225 - Tuesday, June 11, 2019 - link

    Well, I was letdown by a lot of this, given the pricing. I suspect after launch though we'll see a pricing way between AMD and Nvidia like the days of old, and these may wind up at a better price.
  • andrewaggb - Tuesday, June 11, 2019 - link

    Unfortunately, I think nvidia has the better graphics offerings by a wide margin and got there first.

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