GP104’s Architecture

Looking at an architecture diagram for GP104, Pascal ends up looking a lot like Maxwell, and this is not by chance. After making more radical changes to their architecture with Maxwell, for Pascal NVIDIA is taking a bit of a breather. Which is not to say that Pascal is Maxwell on 16nm – this is very much a major feature update – but when it comes to discussing the core SM architecture itself, there is significant common ground with Maxwell.

We’ll start with the GP104 SM. Simply named the SM for this generation – NVIDIA has ditched the generational suffix due to the potential for confusion with the used-elsewhere SMP – the GP104 SM is very similar to the Maxwell SM. We’re still looking at a single SM partially sub-divided into four pieces, each containing a single warp scheduler that’s responsible for feeding 32 CUDA cores, 8 load/store units, and 8 Special Function Units, backed by a 64KB register file. There are two dispatch ports per warp schedule, so when an instruction stream allows it, a warp scheduler can extract a limited amount of ILP with an instruction stream by issuing a second instruction to an unused resource.

Meanwhile shared between every pair of sub-partitions is 4 texture units and the combined L1/texture cache, again unchanged from Maxwell. Finally, we have the resources shared throughout the whole SM: the 96KB shared memory, the instruction cache, and not pictured on NVIDIA’s diagrams, the 4 FP64 CUDA cores and 1 FP16x2 CUDA core.

Overall then at the diagram level the GP104 SM looks almost identical to the Maxwell SM, but with one exception: the PolyMorph Engine. Although the distinction is largely arbitrary for GP104, the PolyMorph Engine has been moved up a level; it’s no longer part of the SM, but rather part of the newly re-introduced TPC, which itself sits between the GPC and the SM.

The TPC exists because although GP104 still has a 1:1 ratio between PolyMorph Engines and SMs, the Pascal architecture itself allows for different SM configurations, which is in turn used on GP100 to allow it to have multiple smaller SMs of 64 CUDA Cores. For GP100 the TPC allows for multiple SMs to share a PolyMorph Engine, but for GP104 there’s no sharing involved. To that end the TPC as an organizational unit technically exists across all Pascal parts, but it has no real significance for GP104. In fact it doesn’t even have a real name; NVIDIA reused the acronym from earlier DX10 architectures, where the TPC was the name assigned to the Texture Processor Cluster.

Looking at the bigger picture of the complete GP104 GPU, the similarities continue between GP104 and GM204. GP104’s SMs are clustered five-a-piece inside of the GPC, with each cluster sharing a single Raster Engine. Overall there are 4 such GPCs, giving us 20 SMs altogether. Compared to GM204 then, we’re looking at the same number of GPCs, with each GPC having gained 1 SM.

Things get more interesting when we look at the back end of the rendering/execution pipeline, which is comprised of the L2 cache, ROPs, and memory controllers. The ROP/L2 count has not changed relative to GM204 – we still have 64 ROPs paired up with a total of 2MB of L2 cache – however the memory controller count has. And with it the logical configuration of the ROP/L2 blocks have changed as well.

Whereas GM204 had 4 64bit GDDR5 memory controllers, each connected to 2 or 4 memory chips, GP104 breaks that down further to 8 32bit GDDR5X memory controllers, each of which is connected to 1 memory chip on GTX 1080. I’ll go into greater detail on GDDR5X a bit later, but the significance of this backend organizational change has to do with the introduction of GDDR5X. Because GDDR5X reads and writes data in 64B amounts (versus 32B amounts on GDDR5), NVIDIA has reorganized the memory controllers to ensure that each memory controller still operates on the same amount of data. With GDDR5 they teamed up two GDDR5 channels to get 64B operations, whereas with GDDR5X this can be accomplished with a single memory channel.

This in turn is where the ROP reorganization comes from. As there’s a 1:1 relationship between ROP partitions and memory controllers, the 64 ROPs are now broken up into 8 partitions for GP104, as opposed to 4 partitions on GM204. There are some performance tradeoffs that come from having more ROP partitions, but to the best of my knowledge these should not be significant.

Meanwhile the new GDDR5X memory controllers are also backwards compatible with traditional GDDR5, which in turn is used to drive the GTX 1070 with its 8Gbps GDDR5. The difference in operation between GDDR5 and GDDR5X does make the ROP situation a bit trickier overall for NVIDIA’s architects – now they need to be able to handle two different memory access patterns – though for NVIDIA this isn’t a wholly new problem. Previous generation architectures have supported both GDDR5 and DDR3, the two of which have their own differences in memory access patterns.

In a by-the-numbers comparison then, Pascal does not bring any notable changes in throughput relative to Maxwell. CUDA cores, texture units, PolyMorph Engines, Raster Engines, and ROPs all have identical theoretical throughput-per-clock as compared to Maxwell. So on a clock-for-clock, unit-for-unit basis, Pascal is not any faster on paper. And while NVIDIA does not disclose the size/speed of most of their internal datapaths, so far I haven’t seen anything to suggest that these have radically changed. This continuity means that outside of its new features, GP104 behaves a lot like GM204. Though it should be noted that real world efficiency isn’t quite as cut and dry, as various factors such as the increased SM count and changes in memory technology can greatly influence this.

GP104: The Heart of GTX 1080 FP16 Throughput on GP104: Good for Compatibility (and Not Much Else)
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  • TestKing123 - Wednesday, July 20, 2016 - link

    Sorry, too little too late. Waited this long, and the first review was Tomb Raider DX11?! Not 12?

    This review is both late AND rushed at the same time.
    Reply
  • Mat3 - Wednesday, July 20, 2016 - link

    Testing Tomb Raider in DX11 is inexcusable.

    http://www.extremetech.com/gaming/231481-rise-of-t...
    Reply
  • TheJian - Friday, July 22, 2016 - link

    Furyx still loses to 980ti until 4K at which point the avg for both cards is under 30fps, and the mins are both below 20fps. IE, neither is playable. Even in AMD's case here we're looking at 7% gain (75.3 to 80.9). Looking at NV's new cards shows dx12 netting NV cards ~6% while AMD gets ~12% (time spy). This is pretty much a sneeze and will as noted here and elsewhere, it will depend on the game and how the gpu works. It won't be a blanket win for either side. Async won't be saving AMD, they'll have to actually make faster stuff. There is no point in even reporting victory at under 30fps...LOL.

    Also note in that link, while they are saying maxwell gained nothing, it's not exactly true. Only avg gained nothing (suggesting maybe limited by something else?), while min fps jumped pretty much exactly what AMD did. IE Nv 980ti min went from 56fps to 65fps. So while avg didn't jump, the min went way up giving a much smoother experience (amd gained 11fps on mins from 51 to 62). I'm more worried about mins than avgs. Tomb on AMD still loses by more than 10% so who cares? Sort of blows a hole in the theory that AMD will be faster in all dx12 stuff...LOL. Well maybe when you force the cards into territory nobody can play at (4k in Tomb Raiders case).

    It would appear NV isn't spending much time yet on dx12, and they shouldn't. Even with 10-20% on windows 10 (I don't believe netmarketshare's numbers as they are a msft partner), most of those are NOT gamers. You can count dx12 games on ONE hand. Most of those OS's are either forced upgrades due to incorrect update settings (waking up to win10...LOL), or FREE on machine's under $200 etc. Even if 1/4 of them are dx12 capable gpus, that would be NV programming for 2.5%-5% of the PC market. Unlike AMD they were not forced to move on to dx12 due to lack of funding. AMD placed a bet that we'd move on, be forced by MSFT or get console help from xbox1 (didn't work, ps4 winning 2-1) so they could ignore dx11. Nvidia will move when needed, until then they're dominating where most of us are, which is 1080p or less, and DX11. It's comic when people point to AMD winning at 4k when it is usually a case where both sides can't hit 30fps even before maxing details. AMD management keeps aiming at stuff we are either not doing at all (4k less than 2%), or won't be doing for ages such as dx12 games being more than dx11 in your OS+your GPU being dx12 capable.

    What is more important? Testing the use case that describes 99.9% of the current games (dx11 or below, win7/8/vista/xp/etc), or games that can be counted on ONE hand and run in an OS most of us hate. No hate isn't a strong word here when the OS has been FREE for a freaking year and still can't hit 20% even by a microsoft partner's likely BS numbers...LOL. Testing dx12 is a waste of time. I'd rather see 3-4 more dx11 games tested for a wider variety although I just read a dozen reviews to see 30+ games tested anyway.
    Reply
  • ajlueke - Friday, July 22, 2016 - link

    That would be fine if it was only dx12. Doesn't look like Nvidia is investing much time in Vulkan either, especially not on older hardware.

    http://www.pcgamer.com/doom-benchmarks-return-vulk...
    Reply
  • Cygni - Wednesday, July 20, 2016 - link

    Cool attention troll. Nobody cares what free reviews you choose to read or why. Reply
  • AndrewJacksonZA - Wednesday, July 20, 2016 - link

    Typo on page 18: "The Test"
    "Core i7-4960X hosed in an NZXT Phantom 630 Windowed Edition" Hosed -> Housed
    Reply
  • Michael Bay - Thursday, July 21, 2016 - link

    I`d sure hose me a Core i7-4960X. Reply
  • AndrewJacksonZA - Wednesday, July 20, 2016 - link

    @Ryan & team: What was your reasoning for not including the new Doom in your 2016 GPU Bench game list? AFAIK it's the first indication of Vulkan performance for graphics cards.

    Thank you! :-)
    Reply
  • Ryan Smith - Wednesday, July 20, 2016 - link

    We cooked up the list and locked in the games before Doom came out. It wasn't out until May 13th. GTX 1080 came out May 14th, by which point we had already started this article (and had published the preview). Reply
  • AndrewJacksonZA - Wednesday, July 20, 2016 - link

    OK, thank you. Any chance of adding it to the list please?

    I'm a Windows gamer, so my personal interest in the cross-platform Vulkan is pretty meh right now (only one title right now, hooray! /s) but there are probably going to be some devs are going to choose it over DX12 for that very reason, plus I'm sure that you have readers who are quite interested in it.
    Reply

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