Architecting Fermi: More Than 2x GT200

NVIDIA keeps referring to Fermi as a brand new architecture, while calling GT200 (and RV870) bigger versions of their predecessors with a few added features. Marginalizing the efforts required to build any multi-billion transistor chip is just silly, to an extent all of these GPUs have been significantly redesigned.

At a high level, Fermi doesn't look much different than a bigger GT200. NVIDIA is committed to its scalar architecture for the foreseeable future. In fact, its one op per clock per core philosophy comes from a basic desire to execute single threaded programs as quickly as possible. Remember, these are compute and graphics chips. NVIDIA sees no benefit in building a 16-wide or 5-wide core as the basis of its architectures, although we may see a bit more flexibility at the core level in the future.

Despite the similarities, large parts of the architecture have evolved. The redesign happened at low as the core level. NVIDIA used to call these SPs (Streaming Processors), now they call them CUDA Cores, I’m going to call them cores.

All of the processing done at the core level is now to IEEE spec. That’s IEEE-754 2008 for floating point math (same as RV870/5870) and full 32-bit for integers. In the past 32-bit integer multiplies had to be emulated, the hardware could only do 24-bit integer muls. That silliness is now gone. Fused Multiply Add is also included. The goal was to avoid doing any cheesy tricks to implement math. Everything should be industry standards compliant and give you the results that you’d expect.

Double precision floating point (FP64) performance is improved tremendously. Peak 64-bit FP execution rate is now 1/2 of 32-bit FP, it used to be 1/8 (AMD's is 1/5). Wow.

NVIDIA isn’t disclosing clock speeds yet, so we don’t know exactly what that rate is yet.

In G80 and GT200 NVIDIA grouped eight cores into what it called an SM. With Fermi, you get 32 cores per SM.

The high end single-GPU Fermi configuration will have 16 SMs. That’s fewer SMs than GT200, but more cores. 512 to be exact. Fermi has more than twice the core count of the GeForce GTX 285.

  Fermi GT200 G80
Cores 512 240 128
Memory Interface 384-bit GDDR5 512-bit GDDR3 384-bit GDDR3

 

In addition to the cores, each SM has a Special Function Unit (SFU) used for transcendental math and interpolation. In GT200 this SFU had two pipelines, in Fermi it has four. While NVIDIA increased general math horsepower by 4x per SM, SFU resources only doubled.

The infamous missing MUL has been pulled out of the SFU, we shouldn’t have to quote peak single and dual-issue arithmetic rates any longer for NVIDIA GPUs.

NVIDIA organizes these SMs into TPCs, but the exact hierarchy isn’t being disclosed today. With the launch's Tesla focus we also don't know specific on ROPs, texture filtering or anything else related to 3D graphics. Boo.

A Real Cache Hierarchy

Each SM in GT200 had 16KB of shared memory that could be used by all of the cores. This wasn’t a cache, but rather software managed memory. The application would have to knowingly move data in and out of it. The benefit here is predictability, you always know if something is in shared memory because you put it there. The downside is it doesn’t work so well if the application isn’t very predictable.

Branch heavy applications and many of the general purpose compute applications that NVIDIA is going after need a real cache. So with Fermi at 40nm, NVIDIA gave them a real cache.

Attached to each SM is 64KB of configurable memory. It can be partitioned as 16KB/48KB or 48KB/16KB; one partition is shared memory, the other partition is an L1 cache. The 16KB minimum partition means that applications written for GT200 that require 16KB of shared memory will still work just fine on Fermi. If your app prefers shared memory, it gets 3x the space in Fermi. If your application could really benefit from a cache, Fermi now delivers that as well. GT200 did have an L1 texture cache (one per TPC), but the cache was mostly useless when the GPU ran in compute mode.

The entire chip shares a 768KB L2 cache. The result is a reduced penalty for doing an atomic memory op, Fermi is 5 - 20x faster here than GT200.

A Different Sort of Launch A More Efficient Architecture
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  • ioannis - Friday, October 02, 2009 - link

    Although the Star Trek TNG reference is ok, 'Nexus' should have been accompanied by a Blade Runner reference instead, Nexus-6 :)
    Reply
  • - Friday, October 02, 2009 - link

    You are all talking too much about technologies. Who cares about this? DX11 from ATI is already available in Japan and they are selling like sex dolls. And why didnt NVDIA provided any benchmarks? Perhaps the drivers aren ready or Nvidia doesnt even know at what clockspeed this monster can run without exhausting your pcs power supply. Fermi is not here yet, it is a concept but not a product. ATI will cash in and Nvidia can only look. And when the Fermi-Monster will finally arrive, ATI will enroll with 5890 and X2 in the luxury class and some other products in the 100 Dollar class. Nvidia will always be a few months late and ATI will get the business. It is that easy. Who wants all this Cuda stuff? Some number crunching in the science field, ok. But if it were for physix an add-on board would do. But in reality there was never any run for physix. Why should this boom come now? I think Nvdia bet on the wrong card and they will suffer heavily for this wrong decision. They had better bought VIA or its CPU-division instead of Physix. Physix is no standard architecture and never will. In contrast, ATI is doing just what gamers want and this is were the money is. Were are the Gaming-benchmarks for FERMI? Nvidia is over! Reply
  • - Friday, October 02, 2009 - link

    With all this Cuda and Physix stuff Nvidia will have 20-30% more power consumption at any pricepoint and up to 50% higher production costs because of their much bigger die size. ATI will lower the price whenever necessary in order to beat Nvidia in the market place! And when will Nvida arrive? Yesterday we didnt see even a paperlaunch! It was the announcement of a paperlaunch maybe in late december but the cards wont be available until late q12010 I guess. They are so much out of the business but most people do not realise this. Reply
  • Ahmed0 - Friday, October 02, 2009 - link

    I know for sure SD is from Illinois (his online profiles which are related to his rants [which in turn are related to each other] point to it).

    So, Im going to go out on a limb here and suggest that SiliconDoc was/is this guy:

    http://www.automotiveforums.com/vbulletin/member.p...">http://www.automotiveforums.com/vbulletin/member.p...

    A little googling might (or might not) support the fact that he is a loony. Just type "site:forums.sohc4.net silicondoc" and youll find he has quite a reputation there (different site but seems to be the same profile, "handwriting" and same bike)

    And that MIGHT lead us to the fact that he MIGHT actually be (currently) 45 and not a young raging teenage nerd called Brian.

    Of course... this is just some fun guesswork I did (its all just oh so entertaining).
    Reply
  • Ahmed0 - Friday, October 02, 2009 - link

    Well... either that or all users called SiliconDoc are arsholes. Reply
  • k1ckass - Friday, October 02, 2009 - link

    I guess silicondoc would eat **** if nvidia says that it tastes good, LOL.

    btw, fermi cards shown appears to be fake...
    http://www.semiaccurate.com/2009/10/01/nvidia-fake...">http://www.semiaccurate.com/2009/10/01/nvidia-fake...

    and btw, I use an nvidia gtx, propable would get an hd5870 next week because of all this crap nvidia throws at its consumers.
    Reply
  • Pastuch - Friday, October 02, 2009 - link

    Below is an email I got from Anand. Thanks so much for this wonderful site.

    -------------------------------------------------------------------
    Thank you for your email. SiliconDoc has been banned and we're accelerating the rollout of our new comments rating/reporting system as a result of him and a few other bad apples lately.

    A-

    Reply
  • tamalero - Saturday, October 03, 2009 - link

    about time, was getting boring with the constant "bubba, red roosters, morons..etc.." Reply
  • sigmatau - Friday, October 02, 2009 - link

    ....
    ...
    SiliconDoc getting banned.... PRICELESS.
    Reply
  • PorscheRacer - Friday, October 02, 2009 - link

    So it's safe now to post again? Much thanks has to go to Anand to cleaning up the virus that has infected these comments. I mean, it's new tech. Aren't we free to postulate about what we think is going on, discuss our thoughts and feelings without fear of some person trolling us down till we can't breathe? It feels better in here now, so thanks again. Reply

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