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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  • rarson - Thursday, June 20, 2019 - link

    "No kidding, rewarding a company for producing mediocre products, so they will continue to produce mediocre products. It takes some special thinking to justify that."

    I tend to support some companies rather than others for various reasons. Unfair business practices means quite a lot to me, as does competition in the market. So if one particular company that I prefer not to endorse has a superior product than the other company that I would prefer to endorse, I heavily compare value for dollar and the actual performance difference, and weigh whether or not it truly matters for my particular application.

    The end result is one of three things. Either I end up buying the product from the company I would rather endorse, because the value is still there and the performance deficit is minimal, or I wait to see what that same company produces at a later point (because the value is not there and my need to buy is not urgent), or I end up buying from the other company because I have a need and the value is too good to ignore. At the end of the day, I'm not going to spend decent money on a crappy product just because of the company that is selling it.

    And I do think even if you're a "fan" of certain companies, it's dumb not to criticize their products when they are bad and urge them to do better, and it's dumb to throw money at products that aren't worth buying. In fact, I think that's the most salient point of the entire conversation. Like you said, if you reward the company for producing mediocre products, they will continue to produce mediocre products.

    Having said all that, I don't think Navi is mediocre. I think it's decent. It still lags behind Nvidia on performance-per-watt (although we really need to see some actual testing and real performance numbers to draw any conclusions). IMO it's a much better showing than Vega was, at least in terms of being a consumer card, but AMD obviously still has some work to do to make it better. What's the most disappointing with these cards, IMO, is price, and I think Nvidia has a lot to do with that. I think the XT should be around $399 (maybe a touch more) and the 5700 no more than $349. I think AMD feels that current 20-series pricing justifies these prices, but they're already a year late, Nvidia has "Super" coming, and likely the ability to adjust the entire price stack down a bit. So it'll be very interesting to see how Nvidia responds and what AMD does to correct the situation, which will likely tilt back towards Nvidia's favor as soon as they respond. AMD may be forced to price these cards more competitively because of that.

    There's a saying in business, there are no bad products, only bad prices.
  • Korguz - Tuesday, June 11, 2019 - link

    phynaz, and members of the green team have been shelling out WAY to much for those cards.. whats your point, nvidia fanboy ?
  • RaV[666] - Monday, June 10, 2019 - link

    Well, i really like the cards, and the extra features, but at these prices and with the retail blower cooling, they dont make sense.
  • Meteor2 - Tuesday, June 11, 2019 - link

    What’s wrong with blowers?
  • Phynaz - Tuesday, June 11, 2019 - link

  • Korguz - Tuesday, June 11, 2019 - link

    heh.. nothing?? usually louder then open air. most i think wait for the custom cooler releases... better cooling.. and quietier
  • Phynaz - Tuesday, June 11, 2019 - link

    Must be a problem with AMD’s blowers
  • Korguz - Tuesday, June 11, 2019 - link

    sorry phynaz.. but its ALSO nvidia's blowers.. but being a blind arrogant nvidia fanboy.. you dont see it
  • evernessince - Wednesday, June 12, 2019 - link

    RX Vega's blower is pretty impressive for how much heat it has to dissipate. It was a more effective design then the one Nvidia used pre-turing.
  • evernessince - Wednesday, June 12, 2019 - link

    Blowers are also more consistent and don't dump heat into your case. If you have a case with poor airflow, a blower will outperform. Blowers are a good choice for a reference card because they will perform equally as well in every PC.

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