The Rest of GF104

Besides adding superscalar dispatch abilities to GF104, NVIDIA has also made a number of other tweaks to the Fermi architecture for this GPU.

As a mid-range product, GF104 does not need to do 2 jobs at once. GF100 had to be usable as a desktop/professional graphics GPU, but also as a compute GPU for NVIDIA’s Tesla line of cards. GF104 will not be a Tesla product, so those compute abilities are not as critical. Specifically, NVIDIA has taken a chisel to Tesla’s flagship compute abilities of FP64 and ECC, which in GF100 desktop GPUs were artificially throttled and disabled respectively.

For GF104, ECC is completely gone. Barring the errant burst of solar radiation, the odds of a flipped bit or other error in the operation of a GPU is extremely slim. NVIDIA only added the feature for Tesla customers who demanded increased reliability as they could not accept a silent error in their work. For graphics however this is unnecessary, so the feature has been dropped.

Double-precision floating-point (FP64) on the other hand hasn’t been entirely dropped. Like ECC, FP64 is primarily a Tesla feature, but at the same time NVIDIA believes it to not be in their best interests to remove the feature. From NVIDIA’s perspective without FP64 on their consumer cards developers could not test and debug FP64 code on their desktops and laptops, which in turn would impede development for Tesla and hurt their efforts to expand in to the professional compute space. As a result GF104 has an interesting compromise on FP64.

For GF104, NVIDIA removed FP64 from only 2 of the 3 blocks of CUDA cores. As a result 1 block of 16 CUDA cores is FP64 capable, while the other 2 are not. This gives NVIDIA the advantage of being able to employ smaller CUDA cores for 32 of the 48 CUDA cores in each SM while not removing FP64 entirely. Because only 1 block of CUDA cores has FP64 capabilities and in turn executes FP64 instructions at 1/4  FP32 performance (handicapped from a native 1/2), GF104 will not be a FP64 monster. But the effective execution rate of 1/12th FP32 performance will be enough to effectively program in FP64 and debug as necessary.

Moving on, we have GF104’s texture units. GF100 was an interesting beast when it came to texturing, as it had texture units more efficient than GT200, but fewer of them overall.  We don’t have any data that points to GF100 being absolutely deficient on texturing speeds, but at the same time it’s hard to imagine that GF100 was overbuilt to the point that losing 32 texture units wouldn’t hurt.

So for GF104, NVIDIA has doubled up on the number of texture units. A “full” GF104 has the same number of texture units at GF100 (64) in half as many SMs. NVIDIA tells us that this change is largely because texture units are small enough that they can be added without consuming too much additional die space, as opposed to requiring additional texture units such as a specific case of lacking texture performance or having too little texture performance relative to shading performance. But this isn’t something we can prove or disprove. High-detail settings optimized for high-end cards often go heavy on anti-aliasing or shading as opposed to textures, so ultimately we’re not surprised that NVIDIA kept the texture unit count constant while reducing the shader count in moving from GF100 to GF104. The shaders will be missed much less than the texture units would have been.

 

Finally, we have the ROPs. There haven’t been any significant changes here, but the ROP count does affect compute performance by impacting memory bandwidth and L2 cache. Even though NVIDIA keeps the same number of SMs on both the 1GB and 768MB of the GTX 460, the latter will have less L2 cache which may impact compute performance. Compute performance on the GTX 460 may also be impacted by pressure on the registers and L1 cache: NVIDIA increased the number of CUDA cores per SM, but not the size of the Register File or the amount of L1 cache/shared memory, so there are now additional CUDA cores fighting for the same resources. In the worst case scenarios, this can hurt the efficiency of GF104 compared to GF100.

For those of you who are curious, with all of these SM changes between GF100 and GF104 the size of a SM did increase, but by nearly as much as one would think: after adding the additional functional units, infusing the warp schedulers with superscalar dispatch capabilities, and removing unnecessary ECC and FP64 hardware, the size of an SM only increased by 25%. This is a tradeoff NVIDIA could not afford on the already massive GF100, but made sense on GF104 where the performance increase could justify the extra die space.

GF104: NVIDIA Goes Superscalar Meet the GTX 460
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  • Alroys - Monday, July 12, 2010 - link

    Nice review, but i would have liked to see how well they overclock. Reply
  • beginner99 - Monday, July 12, 2010 - link

    Not a bad card. I ordered a 5850 for my new build. 460 is a little less performing but more quite. The ordered 5850 is out of stock and no due date. However till the 460's arrive it will probably also be a few weeks...Need to wait on price. usually quite a bit higher here. Reply
  • KITH - Monday, July 12, 2010 - link

    Quiet and Quite are different words...

    460 is a little less performing but more *quiet*

    Usually *quite* a bit higher here.

    See the difference? You even used both in your own post.
    Reply
  • chizow - Monday, July 12, 2010 - link

    Looks like a great part from Nvidia that seems to hit the same target price and performance markets as the wildly 8800GT before it. Much as the G92 and its derivatives dominated the gaming market while bringing DX10 to the mainstream, GTX 460 may be poised to do the same. Reply
  • Griswold - Monday, July 12, 2010 - link

    Its not going to dominate anything but nvidias own lineup. AMD will just - finally - drop prices, and thats that. Reply
  • james.jwb - Monday, July 12, 2010 - link

    I agree. AMD will either drop the price on the 5850 to make this new card redundant, or not do it and make a major mistake. Reply
  • chizow - Monday, July 12, 2010 - link

    I doubt they're willing to drop the price on the 5850 enough to truly compete with the GTX 460, especially the 768MB version.. Maybe match the 1GB version's $230 price point by going to $250 but then what does that do to the 5870? Who's going to buy a 5870 at $400 when a 5850 only costs $250? Either way it looks like Nvidia has that $200-$250 market locked tight and in a few months with MIRs that'll shift to the $160-$220 range. Reply
  • Lonyo - Wednesday, July 14, 2010 - link

    Considering the launch prices were $260/$380, there's no reason to imagine a drop to $250 would leave the 5870 at $400.
    Maybe we would see something like $250/$350. Finally a drop from launch prices.
    Reply
  • papapapapapapapababy - Monday, July 12, 2010 - link

    i mean why bother? to play ports like "singularity" with bump mapped ( and terrible low res textures?) i mean the pos3 game only uses about 140MB of my video ram! ati, nvidia, intel, amd, no real pc games? > no sale. adeus suckers!

    Reply
  • mindbomb - Monday, July 12, 2010 - link

    What does the presence of hdmi v1.4 ports mean?
    Does this card have 3d bluray capabilities not seen on other cards?
    Reply

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