In order to talk generally about SPs and their capabilities, all the vertices, primitives, pixel components, etc. to be processed are referred to as threads. This way we can look at each SP as handling its own thread no matter what type of data is being processed. G80 is able to sustain "thousands" of threads at a time, but the actual number of threads that can be active at any given time is not disclosed. While all SPs can handle any type of thread, SPs that share resources must be running the same type of thread at any given time. In this way, each block of 16 SPs can be running one type of shader program on 16 threads. This indicates something about branch granularity as well. For vertex shaders, branch granularity is 16 vertices. For pixel shaders, branch granularity is 32 pixels (arranged in pairs of blocks of 4x4 pixels).

Branch granularity defines how many threads must follow the same path through data. When a group of 32 pixel threads all take the same branch, we don't have a problem. If even one thread must take a path that is different from the others, all 32 threads must be evaluated with both paths following the branch. The branch then defines what result each individual thread will keep and which it will discard. It's easy to see that optimum granularity is 1 thread, as no unnecessary work would be done. The way resources are allocated and the way instructions are run on SPs grouped together currently doesn't allow any more fine-grained branching. Here's a chart that address branch granularity:

GPU Branch Granularity
NVIDIA NV4x ~1K pixels
NVIDIA G70 ~256 pixels
ATI R580 48 pixels
NVIDIA G80 16 vertex
32 pixels

Clearly G80 has the advantage here, as it's less likely that smaller groups of pixels will take different directions through a branch. This gives programmers the ability to more easily integrate branching into their code without getting a massive performance hit. If programmers are able to incorporate more branches, shader code can become more general purpose and we will see many more effects make their way into games. Now that G80 has caught up to ATI in terms of potential branch performance, we hope developers will take the reality of more complex code seriously.

Early-Z, Memory Interface

NVIDIA has added hardware for Early-Z to G80, after their current Z-Cull hardware which removes regions of pixels completely occluded by other geometry. Early-Z is a more fine-grained occlusion culling method that looks at a calculated Z value of a fragment before it hits the pixel pipeline. Z-Cull doesn't look at per fragment Z values, but uses a Z value based on geometry. While Z-Cull can get rid of large blocks of data it has issues handling surfaces that are only partially occluded or intersecting surfaces. Looking at individual depth values per pixel can help remove unnecessary fragments from heading down the pipeline only to be thrown out when the ROPs get to them.

The memory interface has been dramatically redesigned to support the access patterns of all of G80's independent stream processors. Given the theme of increasing granularity within G80 it's no surprise that we are now seeing 5 and 6 channels of GDDR rather than the 2 or 4 channels we have been used to for the past few years. 8800 GTX will have a 384 bit bus (6 x 64-bit channels), while the 8800 GTS will have a 320 bit wide connection to DRAM (5 x 64-bit channels). We would love to delve further into the details of G80's new memory interface, but NVIDIA isn't discussing the details of this aspect of their hardware.

Digging deeper into the shader core General Purpose Processing with G80


View All Comments

  • Sharky974 - Thursday, November 09, 2006 - link

    The new features of DX10 stuff was captivating at first, but quickly grew tiresome and needlessly complex. The IQ comparisons the same thing, some simplicity is needed here. Tell us in a nutshell what looks better and why. The mouse over pictures are well nigh useless as well, and all look like crap. Whatever needs to be changed to get the IQ point across, needs to be changed already, I'm guessing 200 zoom is a problem for starters.

    Then who's bright idea was it to only test one resolution, through the whole article?

    Then who's bright idea was it to dedicate just as many graphs as performance, one per game, to not only power draw, but the even more useless performance per watt? Meaning 66% of your data graphs, in an article about a paradign changin, long-anticipated, brand new GPU, are related to the power usage of the card. Are you electric workers now?

    I am very surprised more of the comments weren't negative, this review was a total failure.

    And yeah, what's with all the non-standard resolution testing? All the big sites like H, Anand, and FS go round and round talking about the incredible depths they go to get the bottom of real world performance as it relates to the real world, average user, and then you guys use stupid resolution likes 1280X960 (FS uses that particular one), that nobody on earth uses, regularly! It's really, really stupid. Hell for that matter, nobody uses 1600X1200 or any non-LCD native res anymore either, yet those are all staples of any review, and so these "real world" articles aren't very real world at all. But that's somewhat of a tangent issue, and I actually dont mind a lot of different resolutions tested, just as long as the big common ones are hit (which is not always the case)
  • DerekWilson - Friday, November 10, 2006 - link

    I'm always working on bringing down the complexity of my explainations. It's one of my weak points as a writer. It's difficult for me to take something and present it at a high level that doesn't reflect exactly what the thing is. Analogies are great -- I like them -- but I have a hard time using them because I can't ever think of analogies that are accurate enough.

    Any suggestions you have for helping me explain things completely, accurately, effectively, and (especially) in the most straight forward manner possible are very welcome.

    As for the IQ comparisons -- these were much more simplified than I had intended (because Anand told me we couldn't do rollovers with 40 images on one page -- it would load too slow). This is our version of putting things in a nutshell. I could get to the point faster though --


    gamma correct aa is great for edges, but it causes problems with thin lines and transparency/adaptive AA making textures look mushy. transparency/adaptive aa are great but have a large performance hit -- except in 8800 which keeps these features playable and offers higher IQ. CSAA is great at brining higher AA levels to edges, but the loss of Z data at the sub-pixel level makes it less effective at solving the thin line problem than equivalent MSAA modes. The roll overs illustrate all this.

    Thats as simple as I can make it -- I hope it helps.

    We did not only test at one resolution -- In every game we tested at 1600x1200, 1920x1440, and 2560x1600. In oblivion we tested at 1280x1024 as well.

    All our resolution data was in the last graph on each page -- resolution scaling. There are two graphs per page on performance. As you can see, at resolutions below 2560x1600, the 8800 GTX is almost over kill.

    1600x1200 is a standard LCD panel resolution and has been for quite some time. It's actually quite affordable now as well. 1280x1024 (while popular) is often too low to matter in a high end performance analysis piece (and where it did matter we tested it). 1920x1440 is a 4:3 resolution that will give 1920x1200 panel owners a very good idea of performance (differnce is usually under 5% in many games). 2560x1600 is a standard resolution for 30" LCD panels.

    I can understand being upset if you missed the performance data at other resolutions, but it seems like the rest of your complaints are that we put too much data in the article. I doubt this will change in the future, but is there anything else we could have done to make this article better? We are very willing to listen to feedback, especially on articles as big as this.

    Derek Wilson
  • flexy - Friday, November 10, 2006 - link

    complaints are that we put too much data in the article. I doubt this will change in the future,

    i doubt you can make it RIGHT for everyone...however i share the opinion w/ MOST that it is an excellent review. TOO much data is seldom bad, NOT on a site where you can expect geeks and nerds digging every bit of information :)

    I remember times when reviews where FAR less detailed...and what can be better than going in-depth into AA/AF modi, showing their differnce in detail ? I think this was right on and i value such in-depth coverage !

    The DX10 coverage MAYBE was "too much info" for some...but then legitimate IMHO. We're talking about totally new h/w architecture, totally new and revamped DX API and the first hardware supporting it was defintly a good place to cover this. always have the option to skip parts of a review...and the MORE detailed it is...the more it is a helpful resource (also later) to come back and read up. You dont need to comprehend any bit of information at once, but it's good to know it's there.

    my $0.2
  • jiulemoigt - Thursday, November 09, 2006 - link

    The first really big issue is that a poly can have more than one color on it, due textures, subsurface scattering, displacements, bump maps, normal maps, occulion passes, specular highlight, transparency, and a few others I can not think of off the top of my head, you could probaly find out just by asking in any cg forum like cgtalk or any dev who has worked with a profesional 3d package. That being said it may have confused people to try and explain how it really works.
    The other issue is to deal with gamma correct AA, maybe my moniter is showing a way different image but I'm not really sure how you can even compare">">
    as the light is highlighting the building from two different direction in the images, the nvidia image is coming from the left and behind the buildings and the ati image is coming from the right and about midway down the image in front of the little building,
    though a question that should be asked what time of day is it supposed to be the nvidia looks like dusk, and the ati looks blown out even for high noon, though the one above seems to be the same time of day and the nvidia is blown and the ati is shadowing correctly... really odd for the images, which suggests that some other filter is causing the issue on both cards like hdr, or something else.
  • DerekWilson - Thursday, November 09, 2006 - link

    Yes a poly can have more than one color on it, and I agree our explaination could have been better ... but it is a difficult topic to talk about.

    The whole basis of multisample AA relies on the assumption that the color of a poly *within one pixel* will not vary significantly. Of course, this is not always true. This is, in fact, the reason supersample AA does make a difference -- it takes into account the actual color of the pixel at the position of the sub-pixel. This is also why its so much more expensive.

    I didn't mean to imply that an entire poly must have only one color. But it's hard to talk about MSAA without pointing out the fact that the algorithm assumes one color per pixel per poly (calculated at the pixel center in most cases).

    We did enable HDR, but we tried our hardest to take the screenshots at exactly the same ammount of time after loading the scene (Valve's HDR uses dynamic exposure which does change saturation over time and with light level coming into the camera).

    While this would impact general image comparison, it doesn't impact the effect of gamma correct AA on thin lines (which is what we were trying to show).

    Thanks for the feedback -- if there's anything you can add to help us be more specific in our description, we would certainly appreciate it. We would like to avoid simply leaving details out -- we'd like to learn how to better impart knowledge.
  • Nimbo - Thursday, November 09, 2006 - link

    This must be the first GPU article that does not derive in a flame war between ATI and Nvidia fanboys... Reply
  • flexy - Thursday, November 09, 2006 - link

    i actually dont care. I look at performance and comparisons, and then chose what card to get :) Although w/ ATI for years already.

    If one card, however, has some substantial advantage over another, i'll gladly point that out and also gladly debate with others why i'd prefer card X over Y.

    Thats the difference between a fanboy and a enthusiast, i think. As long as i can back up statements w/ facts instead of just defeinding a "brand".

    the other "problem" is really that same gen cards USUALLY are pretty much on par prformance debating/defeninf brand X over Y does make as much sense as defending ferrari over lamborghini :)

    But then..if we wouldn't do that and even discuss about the "littlest" details and have lengthy conversations on forums eg. WHICH AA methods is better and why...and why 5 FPS there are better...and/or why this AF method is better than the would be pretty boring.

    I mean we're hardware-enthusiasts, and gfx-cards are (IMHO) the most interesting component in a PC :)
  • DigitalFreak - Thursday, November 09, 2006 - link

    I thought we were done with the days of >$499 single GPU cards after the 7900GTX launch. Guess not. Reply
  • VooDooAddict - Thursday, November 09, 2006 - link

    Great article.

    Now I just need to figure out if a 8800GTX will fit in a mATX UltraFly Case.
  • Araemo - Thursday, November 09, 2006 - link

    Everyone is repeating microsoft's claim that dx10 will be Vista-only.

    the inq (I know, I know....) reported">here that there will be a directx '9.0L' for XP that supports the new rendering features of DirectX10, but without the new virtualization/driver model improvements.

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