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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  • Threska - Tuesday, June 11, 2019 - link

    Why do that when others do it for them?

  • Oxford Guy - Tuesday, June 11, 2019 - link

    And Vega was Fiji recycled with a smaller node and somewhat upgraded VRAM. As I recall, the IPC of both was basically identical.

    "Reissue, repackage, repackage." — Morrissey
  • Korguz - Wednesday, June 12, 2019 - link

    Oxford Guy nvidia has done the same " rebranding " of their cards as well...
  • Phynaz - Wednesday, June 12, 2019 - link

    “Both sides” lol
  • eva02langley - Thursday, June 13, 2019 - link

    WCCF post quality...
  • peevee - Wednesday, June 12, 2019 - link

    So, no competitor for Gforce 1660 then?
  • cmdrmonkey - Wednesday, June 12, 2019 - link

    These cards are dead on arrival at these prices. AMD just doesn't have the kind of brand recognition that nvidia does these days. These cards aren't cheap enough that anyone is going to go with AMD again.
  • Qasar - Thursday, June 13, 2019 - link

    and how cheap should they be ?? my guess.. they would never be cheap enough... so far.. a know a few people that are interested in these cards.. and are waiting till next month to see how they perform.. why ?? cause nvidia priced them out of the market...
  • Hixbot - Friday, June 14, 2019 - link

    Nvidia RTX were overpriced due to lack of competition at the time of their launch. Performance per dollar no better than the 3 year old GTX 1000s. AMD finally release their new gen and their prices do not significantly undercut Nvidia RTX, especially considering the lack of hardware ray tracing. This is disappointing to see as a consumer. We expected to see AMD drive the GPU performance per dollar upwards in the ways they have done to CPU.
  • Korguz - Friday, June 14, 2019 - link

    Hixbot, and what if amd did have ray tracing, was just as slow as turing, had the same features, but was priced less then what nvidia charges ?? then what? would those that are harping on amd now, still be harping on them ?? of course.. but now they would be complaining that amd had a year more development time for ray tracing, and the performance is still the same ?? lets face it.. ray tracing isnt really viable performance wise unless you are using a 2080 or higher.. the hit is just to great to make it usable... most of the people i know.. dont care about ray tracing right now.. its adds to much to the cost, and is barely used... none of them even asked if navi had ray tracing... again.. cause with nvidia's pricing, they are priced out of the ray tracing market

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