GF110: Fermi Learns Some New Tricks

We’ll start our in-depth look at the GTX 580 with a look at GF110, the new GPU at the heart of the card.

There have been rumors about GF110 for some time now, and while they ultimately weren’t very clear it was obvious NVIDIA would have to follow up GF100 with something else similar to it on 40nm to carry them through the rest of the processes’ lifecycle. So for some time now we’ve been speculating on what we might see with GF100’s follow-up part – an outright bigger chip was unlikely given GF100’s already large die size, but NVIDIA has a number of tricks they can use to optimize things.

Many of those tricks we’ve already seen in GF104, and had you asked us a month ago what we thought GF110 would be, we were expecting some kind of fusion of GF104 and GF100. Primarily our bet was on the 48 CUDA Core SM making its way over to a high-end part, bringing with it GF104’s higher theoretical performance and enhancements such as superscalar execution and additional special function and texture units for each SM. What we got wasn’t quite what we were imagining – GF110 is much more heavily rooted in GF100 than GF104, but that doesn’t mean NVIDIA hasn’t learned a trick or two.



GF100/GF110 Architecture

Fundamentally GF110 is the same architecture as GF100, especially when it comes to compute. 512 CUDA Cores are divided up among 4 GPCs, and in turn each GPC contains 1 raster engine and 4 SMs. At the SM level each SM contains 32 CUDA cores, 16 load/store units, 4 special function units, 4 texture units, 2 warp schedulers with 1 dispatch unit each, 1 Polymorph unit (containing NVIDIA’s tessellator) and then the 48KB+16KB L1 cache, registers, and other glue that brought an SM together. At this level NVIDIA relies on TLP to keep a GF110 SM occupied with work. Attached to this are the ROPs and L2 cache, with 768KB of L2 cache serving as the guardian between the SMs and the 6 64bit memory controllers. Ultimately GF110’s compute performance per clock remains unchanged from GF100 – at least if we had a GF100 part with all of its SMs enabled.

On the graphics side however, NVIDIA has been hard at work. They did not port over GF104’s shader design, but they did port over GF104’s texture hardware. Previously with GF100, each unit could compute 1 texture address and fetch 4 32bit/INT8 texture samples per clock, 2 64bit/FP16 texture samples per clock, or 1 128bit/FP32 texture sample per clock. GF104’s texture units improved this to 4 samples/clock for 32bit and 64bit, and it’s these texture units that have been brought over for GF110. GF110 can now do 64bit/FP16 filtering at full speed versus half-speed on GF100, and this is the first of the two major steps NVIDIA took to increase GF110’s performance over GF100’s performance on a clock-for-clock basis.

NVIDIA Texture Filtering Speed (Per Texture Unit)
  GF110 GF104 GF100
32bit (INT8) 4 Texels/Clock 4 Texels/Clock 4 Texels/Clock
64bit (FP16) 4 Texels/Clock 4 Texels/Clock 2 Texels/Clock
128bit (FP32) 1 Texel/Clock 1 Texel/Clock 1 Texel/Clock

Like most optimizations, the impact of this one is going to be felt more on newer games than older games. Games that make heavy use of 64bit/FP16 texturing stand to gain the most, while older games that rarely (if at all) used 64bit texturing will gain the least. Also note that while 64bit/FP16 texturing has been sped up, 64bit/FP16 rendering has not – the ROPs still need 2 cycles to digest 64bit/FP16 pixels, and 4 cycles to digest 128bit/FP32 pixels.

It’s also worth noting that this means that NVIDIA’s texture:compute ratio schism remains. Compared to GF100, GF104 doubled up on texture units while only increasing the shader count by 50%; the final result was that per SM 32 texels were processed to 96 instructions computed (seeing as how the shader clock is 2x the base clock), giving us 1:3 ratio. GF100 and GF110 on the other hand retain the 1:4 (16:64) ratio. Ultimately at equal clocks GF104 and GF110 widely differ in shading, but with 64 texture units total in both designs, both have equal texturing performance.

Moving on, GF110’s second trick is brand-new to GF110, and it goes hand-in-hand with NVIDIA’s focus on tessellation: improved Z-culling. As a quick refresher, Z-culling is a method of improving GPU performance by throwing out pixels that will never be seen early in the rendering process. By comparing the depth and transparency of a new pixel to existing pixels in the Z-buffer, it’s possible to determine whether that pixel will be seen or not; pixels that fall behind other opaque objects are discarded rather than rendered any further, saving on compute and memory resources. GPUs have had this feature for ages, and after a spurt of development early last decade under branded names such as HyperZ (AMD) and Lightspeed Memory Architecture (NVIDIA), Z-culling hasn’t been promoted in great detail since then.


Z-Culling In Action: Not Rendering What You Can't See

For GF110 this is changing somewhat as Z-culling is once again being brought back to the surface, although not with the zeal of past efforts. NVIDIA has improved the efficiency of the Z-cull units in their raster engine, allowing them to retire additional pixels that were not caught in the previous iteration of their Z-cull unit. Without getting too deep into details, internal rasterizing and Z-culling take place in groups of pixels called tiles; we don’t believe NVIDIA has reduced the size of their tiles (which Beyond3D estimates at 4x2); instead we believe NVIDIA has done something to better reject individual pixels within a tile. NVIDIA hasn’t come forth with too many details beyond the fact that their new Z-cull unit supports “finer resolution occluder tracking”, so this will have to remain a mystery for another day.

In any case, the importance of this improvement is that it’s particularly weighted towards small triangles, which are fairly rare in traditional rendering setups but can be extremely common with heavily tessellated images. Or in other words, improving their Z-cull unit primarily serves to improve their tessellation performance by allowing NVIDIA to better reject pixels on small triangles. This should offer some benefit even in games with fewer, larger triangles, but as framed by NVIDIA the benefit is likely less pronounced.

In the end these are probably the most aggressive changes NVIDIA could make in such a short period of time. Considering the GF110 project really only kicked off in earnest in February, NVIDIA only had around half a year to tinker with the design before it had to be taped out. As GPUs get larger and more complex, the amount of tweaking that can get done inside such a short window is going to continue to shrink – and this is a far cry from the days where we used to get major GPU refreshes inside of a year.

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  • Haydyn323 - Tuesday, November 09, 2010 - link

    Nobody seems to be taking into account the fact that the 580 is a PREMIUM level card. It is not meant to be compared to a 6870. Sure 2x 6870s can do more. This card is not, however, geared for that category of buyer.

    It is geared for the enthusiast who intends to buy 2 or 3 580s and completely dominate benchmarks and get 100+ fps in every situation. Your typical gamer will not likely buy a 580, but your insane gamer will likely buy 2 or 3 to play their 2560x1600 monitor at 60fps all the time.

    I fail to see how AMD is destroying anything here. Cost per speed AMD wins, but speed possible, Nvidia clearly wins for the time being. If anyone can come up with something faster than 3x 580s in the AMD camp feel free to post it in response here.
    Reply
  • TemplarGR - Tuesday, November 09, 2010 - link

    Do you own NVIDIA stock, or are you a fanboy? Because really, only one of the two could not see how AMD destroys NVIDIA. AMD's architecture is much more efficient.

    How many "insane gamers" exist, that would pay 1200 or 1800 dollars just for gpus, and adding to that an insanely expensive PSU, tower and mainboard needed to support such a thing? And play what? Console ports? On what screens? Maximum resolution is still 2560x1600 and even a single 6870 could do fine in most games in it...

    And just because there may be about 100 rich kids in the whole world with no lives who could create such a machine, does it make 580 a success?

    Do YOU intent to create such a beast? Or would you buy a mainstream NVIDIA card, just because the posibility of 3x 580s exists?Come on...
    Reply
  • Haydyn323 - Tuesday, November 09, 2010 - link

    So, the answer is no; you cannot come up with something faster. Also, as shown right here on Anandtech:

    http://www.anandtech.com/show/3987/amds-radeon-687...

    A single 6870 cannot play most modern games at anywhere near 60fps at 2560x1600. Even the 580 needs to be SLI'd to guarantee it.

    That is all.
    Reply
  • Haydyn323 - Tuesday, November 09, 2010 - link

    Oh and yes I do intend to buy a couple of them in a few months. One at first and add another later. I also love when fanboys call other fanboys, "fanboys." It doesn't get anyone anywhere. Reply
  • smookyolo - Tuesday, November 09, 2010 - link

    PC games are not simply console ports, the fact that you need a top of the line PC to get even close to 60 FPS in most cases at not even maximum graphics settings is proof of this.

    PC "ports" of console games have been tweaked and souped up to have much better graphics, and can take advantage of current gen hardware, instead of the ancient hardware in consoles.

    The "next gen" consoles will, of course, be worse than PCs of the time.

    And game companies will continue to alter their games so that they look better on PCs.

    It's a fact, live with it.
    Reply
  • mapesdhs - Tuesday, November 09, 2010 - link


    'How many "insane gamers" exist, that would pay 1200 or 1800 dollars just for gpus, ...'

    Actually the market for this is surprisingly strong in some areas, especially
    CA I was told. I suspect it's a bit like other components such as top-spec
    hard drives and high-end CPUs: the volumes are smaller but the margins
    are significantly higher for the seller.

    Some sellers even take a loss on low-end items just to retain the custom,
    making their money on more expensive models.

    Ian.
    Reply
  • QuagmireLXIX - Sunday, November 14, 2010 - link

    "Maximum resolution is still 2560x1600 and even a single 6870 could do fine in most games in it..."

    Multiple monitors (surround, eyefinity) resolutions get much larger.
    Reply
  • 7Enigma - Tuesday, November 09, 2010 - link

    Just to clarify your incorrect (or misleading) statement 2 6870's in CF use significantly more power than a single 580, but also perform significantly better in most games (minimum frame rate issue noted however). Reply
  • TemplarGR - Tuesday, November 09, 2010 - link

    True. I made a mistake on this one. Only in idle power it consumes slightly less. My bad. Reply
  • cjb110 - Tuesday, November 09, 2010 - link

    "The thermal pads connecting the memory to the shroud have once again wiped out the chip markets", wow powerful adhesive that! Bet Intel's pissed. Reply

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