Barts: The Next Evolution of Cypress

At the heart of today’s new cards is Barts, the first member of AMD’s Northern Island GPUs. As we quickly hinted at earlier, Barts is a very direct descendant of Cypress. This is both a product of design, and a product of consequences.

It should come as no surprise that AMD was originally looking to produce what would be the Northern Islands family on TSMC’s 32nm process; as originally scheduled this would line up with the launch window AMD wanted, and half-node shrinks are easier for them than trying to do a full-node shrink. Unfortunately the 32nm process quickly became doomed for a number of reasons.

Economically, per-transistor it was going to be more expensive than the 40nm process, which is a big problem when you’re trying to make an economical chip like Barts. Technologically, 32nm was following TSMC’s troubled 40nm process; TSMC’s troubles ended up being AMD’s troubles when they launched the 5800 series last year, as yields were low and wafers were few, right at a time where AMD needed every chip they could get to capitalize on their lead over NVIDIA. 32nm never reached completion so we can’t really talk about yields or such, but it’s sufficient to say that TSMC had their hands full fixing 40nm and bringing up 28nm without also worrying about 32nm.

Ultimately 32nm was canceled around November of last year. But even before that AMD made the hard choice to take a hard turn to the left and move what would become Barts to 40nm. As a result AMD had to make some sacrifices and design choices to make Barts possible on 40nm, and to make it to market in a short period of time.

For these reasons, architecturally Barts is very much a rebalanced Cypress, and with the exception of a few key changes we could talk about Barts in the same way we talked about Juniper (the 5700 series) last year.


Click to enlarge

Barts continues AMD’s DirectX 11 legacy, building upon what they’ve already achieved with Cypress. At the SPU level, like Cypress and every DX10 AMD design before it continues to use AMD’s VLIW5 design. 5 stream processors – the w, x, y, z, and t units – work together with a branch unit and a set of GPRs to process instructions. The 4 simple SPs can work together to process 4 FP32 MADs per clock, while the t unit can either do FP32 math like the other units or handle special functions such as a transcendental. Here is a breakdown of what a single Barts SPU can do in a single clock cycle:

  • 4 32-bit FP MAD per clock
  • 4 24-bit Int MUL or ADD per clock
  • SFU : 1 32-bit FP MAD per clock

Compared to Cypress, you’ll note that FP64 performance is not quoted, and this isn’t a mistake. Barts isn’t meant to be a high-end product (that would be the 6900 series) so FP64 has been shown the door in order to bring the size of the GPU down. AMD is still a very gaming-centric company versus NVIDIA’s philosophy of GPU computing everywhere, so this makes sense for AMD’s position, while NVIDIA’s comparable products still offer FP64 if only for development purposes.

Above the SPs and SPUs, we have the SIMD. This remains unchanged from Cypress, with 80 SPs making up a SIMD. The L1 cache and number of texture units per SIMD remains at 16KB L1 texture, 8KB L1 compute, and 4 texture units per SIMD.

At the macro level AMD maintains the same 32 ROP design (which combined with Barts’ higher clocks, actually gives it an advantage over Cypress). Attached to the ROPs are AMD’s L2 cache and memory controllers; there are 4 128KB blocks of L2 cache (for a total of 512KB L2) and 4 64bit memory controllers that give Barts a 256bit memory bus.

Barts is not just a simple Cypress derivative however. For non-gaming/compute uses, UVD and the display controller have both been overhauled. Meanwhile for gaming Barts did receive one important upgrade: an enhanced tessellation unit. AMD has responded to NVIDIA’s prodding about tessellation at least in part, equipping Barts with a tessellation unit that in the best-case scenario can double their tessellation performance compared to Cypress. AMD has a whole manifesto on tessellation that we’ll get in to, but for now we’ll work with the following chart:

AMD has chosen to focus on tessellation performance at lower tessellation factors, as they believe these are the most important factors for gaming purposes. From their own testing the advantage over Cypress approaches 2x between factors 6 and 10, while being closer to a 1.5x increase before that and after that up to factor 13 or so. At the highest tessellation factors Barts’ tessellation unit falls to performance roughly in line with Cypress’, squeezing out a small advantage due to the 6870’s higher clockspeed. Ultimately this means tessellation performance is improved on AMD products at lower tessellation factors, but AMD’s tessellation performance is still going to more-or-less collapse at high factors when they’re doing an extreme amount of triangle subdivision.

So with all of this said, Barts ends up being 25% smaller than Cypress, but in terms of performance we’ve found it to only be 7% slower when comparing the 6870 to the 5870. How AMD accomplished this is the rebalancing we mentioned earlier.

Based on AMD’s design decisions and our performance data, it would appear that Cypress has more computing/shading power than it necessarily needs. True, Barts is slower, but it’s a bit slower and a lot smaller. AMD’s various compute ratios, such as compute:geometry and compute:rasterization would appear to be less than ideal on Cypress. So Barts changes the ratios.

Compared to Cypress and factoring in 6870/5870 clockspeeds, Barts has about 75% of the compute/shader/texture power of Cypress. However it has more rasterization, tessellation, and ROP power than Cypress; or in other words Barts is less of a compute/shader GPU and a bit more of a traditional rasterizing GPU with a dash of tessellation thrown in. Even in the worst case scenarios from our testing the drop-off at 1920x1200 is only 13% compared to Cypress/5870, so while Cypress had a great deal of compute capabilities, it’s clearly difficult to make extremely effective use of it even on the most shader-heavy games of today.

However it’s worth noting that internally AMD was throwing around 2 designs for Barts: a 16 SIMD (1280 SP) 16 ROP design, and a 14 SIMD (1120 SP) 32 ROP design that they ultimately went with. The 14/32 design was faster, but only by 2%. This along with the ease of porting the design from Cypress made it the right choice for AMD, but it also means that Cypress/Barts is not exclusively bound on the shader/texture side or the ROP/raster side.

Along with selectively reducing functional blocks from Cypress and removing FP64 support, AMD made one other major change to improve efficiency for Barts: they’re using Redwood’s memory controller. In the past we’ve talked about the inherent complexities of driving GDDR5 at high speeds, but until now we’ve never known just how complex it is. It turns out that Cypress’s memory controller is nearly twice as big as Redwood’s! By reducing their desired memory speeds from 4.8GHz to 4.2GHz, AMD was able to reduce the size of their memory controller by nearly 50%. Admittedly we don’t know just how much space this design choice saved AMD, but from our discussions with them it’s clearly significant. And it also perfectly highlights just how hard it is to drive GDDR5 at 5GHz and beyond, and why both AMD and NVIDIA cited their memory controllers as some of their biggest issues when bringing up Cypress and GF100 respectively.

Ultimately all of these efficiency changes are necessary for AMD to continue to compete in the GPU market, particularly in the face of NVIDIA and the GF104 GPU powering the GTX 460. Case in point, in the previous quarter AMD’s graphics division only made $1mil in profit. While Barts was in design years before that quarter, the situation still succinctly showcases why it’s important to target each market segment with an appropriate GPU; harvested GPUs are only a stop-gap solution, in the end purposely crippling good GPUs is a good way to cripple a company’ s gross margin.

Index Seeing the Future: DisplayPort 1.2
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  • GullLars - Saturday, October 23, 2010 - link

    One sollution would be to to move away from pure number based naming, and do something like:
    AMD/nVidia AG#S# ([Maker]_[Architecture][Generation][# generation of architecture][Market Segment][# of relative performance within segment 1-9]
    Or possibly AMD/Nvidia Architecture Gen# S#
    Example:
    AMD EG1E9 or Evergreen Gen1 E9 = 5970 (Enthusiast)
    nVidia FG1E9 = 480
    AMD Evergreen Gen2 G5(?) = 6850 (Gamer)
    AMD Evergreen Gen1 V7 = 5770 (Value)
    AMD Evergreen Gen1 M5 = 5350 (Media)

    These are just early floating thoughts, which could be refined by marketing monkeys.
    Reply
  • Exelius - Saturday, October 23, 2010 - link

    Marketing monkeys have no intent on making it simple to understand; if you don't know exactly what you're buying, it's easier to sell it to you for more than they would be able to otherwise.

    It's not an accident that the numbering is confusing; if you don't know what you're looking at then a 6870 at a lower price than a 5870 looks like a great deal.
    Reply
  • MonkeyPaw - Friday, October 22, 2010 - link

    Big deal, I say. The card is a few % slower, but is more efficient and is cheaper. People who will actually notice the drop off will probably read reviews first. Besides, if the x900 series is for dual GPU designs, then the naming might not be far off.

    Also, if I had to pick between the 5800 or the 6800, I'd probably get a 6800.
    Reply
  • therealnickdanger - Friday, October 22, 2010 - link

    Don't forget improved image quality!

    It's only disappointing because with a new moniker, I expect new tech, but then again, how long did NVIDIA push G92? 3 generations as different products? LOL
    Reply
  • Rafterman - Friday, October 22, 2010 - link

    What exactly have NVidia got to do with this, no fanboyism please. Reply
  • morphologia - Friday, October 22, 2010 - link

    They are a comparable company with comparably ridiculous naming conventions. How do you go from 9000 to 200?

    Talk about fanboyism...claiming irrelevancy when it's totally relevant reveals your fanboy decoder ring quite clearly.
    Reply
  • Alilsneaky - Friday, October 22, 2010 - link

    I hated nvidia for doing it, why should amd now be forgiven for resorting to doing the same bullshit just because nvidia did it before them?

    I had someone tell me earlier 'that's business'.

    WHAT? No it's bloody not, a scam is a scam, when people start equalling questionable practices like these to business then something is really wrong with today's society.
    Reply
  • Mr Perfect - Friday, October 22, 2010 - link

    What Nvidia did was simply rename the 8800 cards to 9800 card. Same exact chip, same exact clocks, same exact board(at least initially). There where owners of 8800GTs who simply edited the name in the BIOS of their card and had a 9800GT!

    The reason AMD is getting a pass from most people is because this isn't a purely renamed card. It's a redesigned chip on a new PCB with a poor name. If, on the other hand, AMD renames the 5750 and 5770 to the 6750 and 6770 you can expect them to get nailed to the wall right next to Nvidia.
    Reply
  • pcfxer - Saturday, October 23, 2010 - link

    It was very clear why he mentioned NVIDIA. You should read his post... Reply
  • snarfbot - Friday, October 22, 2010 - link

    at least all the iterations of g92 improved performance over their predecessor.

    compare this launch to the x1xxx series of ati products, the x1800 was replaced by the x1900 which was replaced by the x1950. all of which improved performance over their predecessor. all the while on the same process 90nm.(save for the 1950pro and gt, which were mainstream parts.)

    imagine if they named the x1900 the x2900, and somehow it actually performed worse than the x1800.

    thats what they did here, and thats why it fails imo.

    if they just called it hd5790 and kept it at the same price people wouldve gobbled it up anyway, without sacrificing their integrity.

    just a bunch of numbers, but what it means in mindshare is important, and all most people will remember about this generation, is that it was worse than the 5 series and worse than nvidias.

    all aboard the fail boat. honk honk.
    Reply

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