Vega 20 Under The Hood: GCN on 7nm

Though we’ve known of Vega 20 for some time with the launch of server-class Radeon Instinct MI50 and MI60, its arrival to the consumer space does mark the first 7nm gaming card. Rapidly moving down to a lower node – this time from GlobalFoundries 14nm LPP to TSMC 7nm (CLN7FF) – used to be an AMD/ATi hallmark, and once again AMD is pushing the envelope by bringing it so early to consumers. That being the case, all the attributes of Vega 20 look squarely to be for the professional/server crowd, though not devoid of benefits for gaming.

Of the many threads in the Radeon VII story, the fate of Vega as a whole has been nothing short of enigmatic. To start things off, the 7nm Vega 20 GPU making its way to consumers was a surprise to most, at least arriving this early. And while AMD only made mention of Vega 20 in reference to Radeon Instinct products – referring to the forthcoming Navi when speaking about 7nm GPUs for gamers – AMD now maintains that the plan was always to bring Radeon VII to market. Perhaps the same might have been said regarding Vega on 14nm+/12LP and the Vega 11 GPU (not to be confused with Ryzen 2400G’s 11 Vega compute units), though to be clear this is hardly unusual given the nature of semiconductor development.

To be fair, AMD has only been coy at best about Vega since the RX Vega launch, which didn’t quite land where AMD wanted it to. But even as a bit of Schrodinger’s silicon, the existence of Radeon VII does raise some interesting questions about 7nm. For one, AMD had already moved up their Vega 20 sampling and launch windows previously. So Radeon VII’s launch timing is realistically the earliest it could be for a consumer derivative of the Radeon Instinct. Even more so is that with a die size of 331mm2, these aren’t small mobile SoCs or comparable ‘pipecleaner’ silicon that we’ve seen so far on TSMC 7nm. Designed with compute/ML-oriented enhancements, equipped with 4 HBM2 stacks, and fabbed on a maturing and cutting-edge 7nm node, Vega 20 has nothing on paper to push for its viability at consumer prices. And yet thanks to a fortunate confluence of factors, here we are.

At a high level. Vega 20 combines an updated GCN 5 architecture with a 7nm process, coming out to 13.2B transistors on 331mm2 (versus 12.5B transistors on 496mm2 for Vega 10). Typically with die shrinks, these space-savings are often reinvested into more transistors – for a gaming card, that can mean anything from more CUs and functional blocks, to layout redesigns and hardened data paths for improved frequency tolerance. The latter, of course, is to enable higher clockspeeds, and this design choice was a big part of Vega 10, where a significant number of transistors were invested to meeting the requisite timing targets. In conjunction with power-savings of a smaller node, a chip can then get to those higher clocks without additional power.

For Vega 20 however, much of the saved space was left simply as that: more space. There’s several reasons of this, some obvious, and some less so. To start things off, as a relatively large high-performance GPU on a leading-edge 7nm node early in its life, development and production is already costly and likely lower-yielding, where going any larger would cost substantially more and yield less. And though TSMC’s 7nm process has only publicly been seen in mobile SoCs thus far, Vega 20 presumably makes good use of the HPC-oriented 7.5T libraries as needed, as opposed to using the 6T libraries intended for mobile SoCs.

But more importantly, the saved space allows room for two more stacks of HBM2 on a similarly-sized interposer. For current-generation HBM2 densities and capabilities, the limit for a two stack chip is 16GB of memory, using a pair of "8-Hi" stacks. But for a server-grade GPU – especially one targeting machine learning – a four stack configuration needed to happen in order to allow for 32GB of memory and a wider 4096-bit bus. For Vega 20, AMD has delivered on just this, and furthermore is producing both 32GB (8-Hi) and 16GB (4-Hi) versions of the chip.

Radeon VII, in turn, is tapping one of these 16GB chips for its core design. It should be noted that this isn't AMD's first 16GB Vega card – they also produced one early on with their early-adopted focused Vega Frontier Edition card – but since the Frontier Edition's retirement, this is the first (re)introduction of a 16GB card to AMD's Vega lineup.

Going with a 16GB for a consumer(ish) card is a bit of a gamble for AMD. And, I suspect, this is part of the reason we're also seeing AMD chase part of the professional visualization market with the Radeon VII. When it comes to workstation use and content creation tasks, more VRAM is an easy sell, as there are already datasets that can use all of that VRAM and more. But for gaming this is a harder sell, as games have more fixed VRAM requirements, and absent such a large card until now, developers haven't yet started targeting 16GB cards. On the flip side, however, the statement "this card has more than enough VRAM" has proven to be famous last words, and in 2019 a flagship enthusiast-grade gaming card ought to have that much anyway.

Getting back to Vega 20's design then, the other step AMD has taken to reduce the complications and cost  of 7nm is by sticking to a die-shrink of a known architecture. Here AMD has added optimizations over Vega 10, but they did not risk a large redesign. Basically it's the logic behind the ‘tick’ of Intel’s old ‘tick-tock’ strategy.

In fact Vega 20 is such a straightforward shrink of Vega 10 in this manner that, outside of the number of memory controllers, all of the other functional unit counts at the same. The GPU packs 64 CUs and 256 texture units segmented into 4 Shader Engines, which in turn are paired with 64 ROPs and AMD's multi-queue command processor.

(I should add that by going this route, AMD also neatly sidesteps the question of shader engine scaling. The nature of that 4 SE limitation has been left vague in recent years, but with Vega there were hints of a path beyond with improved load-balancing via the intelligent workgroup distributors (IWD). Regardless, it would be a complex task in itself to tweak and redesign a balanced 4+ SE configuration, which might be unnecessary effort if AMD has fundamental changes to GCN in the pipeline.)

So on an architectural level, Vega 20 is very much an evolutionary design. But with that said, there is a bit more evolution to it than just the die-shrink, the combination of which means that Vega 20 should in practice be a bit faster than Vega 10 on a clock-for-clock basis.

The big improvement here is all of that extra memory bandwidth; there's now over twice as much bandwidth per ROP, texture unit, and ALU as there was on Vega 10. The bodes particularly well for the ROPs, which have traditionally always been big bandwidth consumers. Not stopping there, AMD has also made some improvements to the Core Fabric, which is what connects the memory to the ROPs (among other things). Unfortunately AMD isn't willing to divulge just what these improvements are, but they have confirmed that there aren't any cache changes among them.

Another piece of the puzzle is that AMD has added some new instructions and data types that will speed up machine learning in certain cases. AMD hasn't given us the complete details here, but at a high level we know that they've added support for INT8 and INT4 data types, which are useful for some low-precision inference scenarios. AMD has also added a new FP16 dot product that accumulates as an FP32 result, a rather specific scenario that is helpful for some machine learning algorithms, as it produces a higher precision result than a FP16-in/FP16-out dot product.

Speaking of data types, AMD has also significantly ramped up their FP64 performance for Vega 20. As a core architecture, GCN allows a GPU to be built with a rate ranging from 1/2 to 1/16 the FP32 rate. For pure consumer GPUs this has always been 1/16, however for GPUs that have pulled double-duty as server-focused chips, they have enabled 1/4 and 1/2 rates in the past. Vega 20, in turn, is the first 1/2 rate FP64 GPU from AMD since Hawaii in 2013. This means that while its general FP32 performance gains over Vega 10 cards are somewhat limited, its FP64 gains are nothing short of massive – better than 8x over the RX Vega 64, on paper. Of course as a consumer card the Radeon VII doesn't quite get to enjoy these benefits – it's limited to 1/4 rate – but more on that later.

Meanwhile, for AMD's video and display controller blocks, there have only been minor, incremental updates. Officially, the display controller (DCE) is up to version 12.1, while we're on unified video decoder (UVD) 7.2, and video coding engine (VCE) 4.1. No additional encoding or decoding feature support have been added compared to Vega 10. For what it’s worth, we’ve already seen the successor blocks with Raven Ridge’s Display Core Next and Video Core Next 1.0, so this may be the last architecture that those ASICs grace us with their presence.

Wrapping up the functional blocks is a new style of SMU, discussed in recent Linux kernel patches. Orthogonal, but closely related, is improved thermal monitoring, where the number of temperature diodes has been doubled to 64 sensors. As a consequence, AMD is now fully utilizing junction temperature monitoring instead of edge temperature monitoring. Junction temperature measurements were used in Vega 10 (showing up as ‘hotspot’ temperature) but Vega 20 has made the full jump to junction temperature for the full suite of fanspeeds, clockspeeds, and such. The result is more accurate reporting, as well as minor 1-2% performance gains (via reduced throttling) that AMD cites from internal testing.

The updated SMU also brings with it a knock-on effect: temperature, clockspeed, and related GPU metrics are no longer read through registers, but instead grabbed straight from the SMU. Naturally, this breaks compatibility with third-party utilities (i.e. GPU-Z), and while AMD has already notified some developers of these changes, applications will still need to be updated to use AMD's new API calls.

Finally, as this is AMD's first new high-end Vega silicon since the original Vega 10 in 2017, there have been a few questions swirling around Vega’s forward-looking hardware features. AMD's communication hasn't always been clear here, and as a result these features have become sort of a perpetual source of consumer confusion.

To settle matters for Vega 20 then, AMD doesn’t seem to be changing the situation here. Which is to say that there have been no further developments as far as AMD's primitive shaders are concerned. Primitive shaders will still require explicit developer support – something AMD has not enabled – and so the full capabilities of Vega 20's Next-Gen Geometry path aren't being utilized (though we should note that the Intelligent Workgroup Distributor portion has always been enabled).

Meanwhile, AMD's Draw Stream Binning Rasterizer (DSBR) was already up and working in Vega 10, so this hasn't changed; the feature is enabled for an unspecified list of games. And checking in quickly on Rapid Packed Math (fast FP16), this is in use for two known games: Far Cry 5 and Wolfenstein II.

The AMD Radeon VII Review FP64 Perf and Separating Radeon VII from MI50
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  • eva02langley - Thursday, February 7, 2019 - link

    I think you are the one in denial over this.

    This is a Radeon Instinct M150. This is a compute card that was never intended to be a gaming card. The biggest integration AMD had to do were drivers. Drivers will indeed be better in the next 3 months.
    Reply
  • yasamoka - Thursday, February 7, 2019 - link

    So please explain why AMD are designating this as a gaming card. I explained this in my previous post. Their lack of product differentiation is exhausting them and the apologetic acrobatics pulled by their die-hard fanboys is appallingly misleading. This is the same Vega architecture. Why isn't AMD releasing their cards with the drivers optimized beforehand? They have been doing this since the 7970. Remember that card getting beat by the GTX 680 just because they had unoptimized drivers? It took AMD almost a year to release drivers that thoroughly bested that series. CrossFire performance on these was also way ahead of SLi. It took them around another year to solve microstuttering. I had 2x 7970's back then. These delays need to stop, they're literally murdering AMD's product launches. Reply
  • Bp_968 - Thursday, February 7, 2019 - link

    Nvidia is also guilty of the same thing. The entire Turing lineup is a die designed and R&Ded for the Enterprise customers. The "flagship features" they keep raving about for Turing are slapped together ways to use all the ASIC cores on the dies designed for AI and content creation.

    Its why when you compare a 1080 to a 2070, or a 1080ti to a 2080 (the same price bracket) you get almost zero rasterization improvements. Its a huge and expensive die reused from the enterprise department because no one else has anything competitive in the same space.

    Nvidia is likely holding back their actual 12nm/7nm gamer design for 2020 out of concern for what Intel might have and possible concern over Navi. I also think Nvidia vastly underestimated how poorly the repackaged cards would sell. I expect Turing to be a very short generation with the next series being announced in late 2019 early 2020 (depending on what intel and AMD end up fielding).
    Reply
  • Alistair - Thursday, February 7, 2019 - link

    exactly! Reply
  • ToyzRUsKid - Friday, February 8, 2019 - link

    The updated NVENC encoder chip may become a major selling point for the RTX cards for streamers/content creators. I'm actually disappointed Nvidia is not emphasizing this feature more. Once OBS Studio releases their new build that will further increase NVENC encoding efficiency it will create an even more compelling argument to switch to NVENC.

    I have been testing the new encoder and it's rivaling and beating medium preset x264 at 1080p60 using 8k bitrate. Single pc streamers will see steam quality improvements along with massive cpu resource savings. I'm of the opinion the dark horse selling point of these cards will be the new NVENC encoder. It appears the Turning generation is more of an advancement for content creators than the average gamer. Ray tracing is superfluous at this point for sure.

    I'm ok with this. I still run a 1080Ti in my gaming rig and I'm comfortable waiting another generation. But the RTX 2070 in my streaming rig is delivering the best quality stream to date. That is comparing against x264 medium running on an i9 9900k@5GHz. This flies in the face of conventional wisdom and people with more credibility than me will need to help change the winds here. But this is my anecdotal experience.
    Reply
  • rahvin - Friday, February 8, 2019 - link

    If and I mean IF nvidia is holding back it's a purely financial move due to the huge overstock on GPU's caused by crypto currencies. Both AMD and nvidia massively underestimate how much demand crypto was creating. (IIRC AMD said during the earnings call now that crypto has dropped off that monthly GPU sales are less than half what they were) Supposedly there are more than 100K of nvidia cards (again the stuff I saw said that was somewhere between 3-6months normal gamer sales) sitting out there on store shelves rotting because of it, and it's so bad nvidia is having to take stock back from retailers that want the shelf space freed up.

    That's prime incentive to sit on the designs until the existing stock is used up. For both nvidia and AMD. Sure they might push out some high end high price product but they aren't doing anything in the middle of the market until that stock is cleared out.
    Reply
  • Korguz - Thursday, February 7, 2019 - link

    yasamoka...

    at the least.. this could force, what seems to be your saint nvida.. to drop the price of their cards.. as it stands before today.. ALL of their 20 series cards.. are out of the price range i would pay for a video card, or are pushing it/hard to justify the cost over my current 1060, as the 1070/80 were way out of my price range..
    Reply
  • D. Lister - Thursday, February 7, 2019 - link

    @Korguz

    The RVII performs 5%-6% below the 2080 (as per this review) and yet is priced the same. How is that going to force Nvidia to cut prices?
    Reply
  • Korguz - Friday, February 8, 2019 - link

    well.. where i am at least.. the radeon 7 starts at 949 ( preorder ), only 2 cards listed, Asus and xfx. the gtx 2080 ( which the radeon 7 is aimed at ) starts at 1130, almost 200 more... IMO.. 200 is not worth the premium for a 5-6% faster card.. the top of the line 2080 is the GeForce RTX 2080 SEA HAWK X which is priced at $1275... for my cash... id be looking at the radeon 7.. and saving 200+ bucks to use some where else in my comp... Reply
  • TheinsanegamerN - Thursday, February 7, 2019 - link

    The Instinct M150 is a VEGA class card, the architecture is incredibly similar to Vega 56/64. There are not massive gains to be made here.

    If there are, then AMD must be completely incompetent at driver management.
    Reply

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