Feeding Pascal, Cont: 4th Gen Delta Color Compression

Now that we’ve seen GDDR5X in depth, let’s talk about the other half of the equation when it comes to feeding Pascal: delta color compression.

NVIDIA has utilized delta color compression for a number of years now. However the technology only came into greater prominence in the previous Maxwell 2 generation, when NVIDIA disclosed delta color compression’s existence and offered a basic overview of how it worked. As a reminder, delta color compression is a per-buffer/per-frame compression method that breaks down a frame into tiles, and then looks at the differences between neighboring pixels – their deltas. By utilizing a large pattern library, NVIDIA is able to try different patterns to describe these deltas in as few pixels as possible, ultimately conserving bandwidth throughout the GPU, not only reducing DRAM bandwidth needs, but also L2 bandwidth needs and texture unit bandwidth needs (in the case of reading back a compressed render target).

Since its inception NVIDIA has continued to tweak and push the technology for greater compression and to catch patterns they missed on prior generations, and Pascal in that respect is no different. With Pascal we get the 4th generation of the technology, and while there’s nothing radical here compared to the 3rd generation, it’s another element of Pascal where there has been an iterative improvement on the technology.

New to Pascal is a mix of improved compression modes and new compression modes. 2:1 compression mode, the only delta compression mode available up through the 3rd generation, has been enhanced with the addition of more patterns to cover more scenarios, meaning NVIDIA is able to 2:1 compress blocks more often.

Meanwhile, new to delta color compression with Pascal is 4:1 and 8:1 compression modes, joining the aforementioned 2:1 mode. Unlike 2:1 mode, the higher compression modes are a little less straightforward, as there’s a bit more involved than simply the pattern of the pixels. 4:1 compression is in essence a special case of 2:1 compression, where NVIDIA can achieve better compression when the deltas between pixels are very small, allowing those differences to be described in fewer bits. 8:1 is more radical still; rather an operating on individual pixels, it operates on multiple 2x2 blocks. Specifically, after NVIDIA’s constant color compressor does its job – finding 2x2 blocks of identical pixels and compressing them to a single sample – the 8:1 delta mode then applies 2:1 delta compression to the already compressed blocks, achieving the titular 8:1 effective compression ratio.

Overall, delta color compression represents one of the interesting tradeoffs NVIDIA has to make in the GPU design process. The number of patterns is essentially a function of die space, so NVIDIA could always add more patterns, but would the memory bandwidth improvements be worth the real cost of die space and the power cost of those transistors? Especially since NVIDIA has already implemented the especially common patterns, which means new patterns likely won’t occur as frequently. NVIDIA of course pushed ahead here, thanks in part to the die and power savings of 16nm FinFET, but it gives us an idea of where they might (or might not) go in future generations in order to balance the costs and benefits of the technology, with less of an emphasis on patterns and instead making more novel use of those patterns.

To put all of this in numbers, NVIDIA pegs the effective increase in memory bandwidth from delta color compression alone at 20%. The difference is of course per-game, as the effectiveness of the tech depends on how well a game sticks to patterns (and if you ever create a game with random noise, you may drive an engineer or two insane), but 20% is a baseline number for the average. Meanwhile for anyone keeping track of the numbers over Maxwell 2, this is a bit less than the gains with NVIDIA’s last generation architecture, where the company claimed the average gain was 25%.

The net impact then, as NVIDIA likes to promote it, is a 70% increase in the total effective memory bandwidth. This comes from the earlier 40% (technically 42.9%) actual memory bandwidth gains in the move from 7Gbps GDDR5 to 10Gbps GDDR5X, coupled with the 20% effective memory bandwidth increase from delta compression. Keep those values in mind, as we’re going to get back to them in a little bit.

Synthetic: Beyond3D Suite - INT8 Buffer Compression

Meanwhile from a graphical perspective, to showcase the impact of delta color compression, NVIDIA sent over a pair of screenshots for Project Cars, colored to show what pixels had been compressed. Shown in pink, even Maxwell can compress most of the frame, really only struggling with finer details such as the trees, the grass, and edges of buildings. Pascal, by comparison, gets most of this. Trees and buildings are all but eliminated as visually distinct uncompressed items, leaving only patches of grass and indistinct fringe elements. It should be noted that these screenshots have most likely been picked because they’re especially impressive – seeing as how not all games compress this well – but it’s none the less a potent example of how much of a frame Pascal can compress.

Finally, while we’re on the subject of compress, I want to talk a bit about memory bandwidth relative to other aspects of the GPU. While Pascal (in the form of GTX 1080) offers 43% more raw memory bandwidth than GTX 980 thanks to GDDR5X, it’s important to note just how quickly this memory bandwidth is consumed. Thanks to GTX 1080’s high clockspeeds, the raw throughput of the ROPs is coincidentally also 43% higher. Or we have the case of the CUDA cores, whose total throughput is 78% higher, shooting well past the raw memory bandwidth gains.

While it’s not a precise metric, the amount of bandwidth available per FLOP has continued to drop over the years with NVIDIA’s video cards. GTX 580 offered just short of 1 bit of memory bandwidth per FLOP, and by GTX 980 this was down to 0.36 bits/FLOP. GTX 1080 is lower still, now down to 0.29bits/FLOP thanks to the increase in both CUDA core count and frequency as afforded by the 16nm process.

NVIDIA Memory Bandwidth per FLOP (In Bits)
GPU Bandwidth/FLOP Total FLOPs Total Bandwidth
GTX 1080 0.29 bits 8.87 TFLOPs 320GB/sec
GTX 980 0.36 bits 4.98 TFLOPs 224GB/sec
GTX 680 0.47 bits 3.25 TFLOPs 192GB/sec
GTX 580 0.97 bits 1.58 TFLOPs 192GB/sec

The good news here is that at least for graphical tasks, the CUDA cores generally aren’t the biggest consumer of DRAM bandwidth. That would fall to the ROPs, which are packed alongside the L2 cache and memory controllers for this very reason. In that case GTX 1080’s bandwidth gains keep up with the ROP performance increase, but only by just enough.

The overall memory bandwidth needs of GP104 still outpace the memory bandwidth gains from GDDR5X, and this is why features such as delta color compression are so important to GP104’s performance. GP104 is perpetually memory bandwidth starved – adding more memory bandwidth will improve performance, as we’ll see in our overclocking results – and that means that NVIDIA will continue to try to conserve memory bandwidth usage as much as possible through compression and other means. How long they can fight this battle remains to be seen – they already encounter diminishing returns in some cases – but in the meantime this allows NVIDIA to utilize smaller memory buses, keeping down the die size and power costs of their GPUs, making PCB costs cheaper, and of course boosting profit margins at the same time.

Feeding Pascal: GDDR5X Asynchronous Concurrent Compute: Pascal Gets More Flexible
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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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