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Explain to me how memory width (128/192/256 bit, etc) is related to memory amount.

Discussion in 'Graphics Cards' started by MaxAwesome, Aug 16, 2012.

  1. MaxAwesome

    MaxAwesome

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    Hello guys!

    I'm asking this out of curiosity, because I've been reading GTX 660 TI reviews, and in some forums people are saying that it's ridiculous to have 192bit memory bus paired with 2GB of VRAM, because it can't use it all.

    Why is this the case? How does the bus width affect amount of memory on a GPU?
     
  2. mlee49

    mlee49

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    Memory bus can be thought of literally as lanes of traffic. More lanes dedicated for traffic, the greater the flow. The graphics processor is connected to the RAM on the card via a memory bus.

    As far as affects on gpu memory bus, I've seen mixed results. A wider bus doesn't simply mean the card is faster or will perform better. From my experience, I purchased an eVGA GTX 275 FTW with a wider memory bus and my gpu memory overclocked better than stock.
     
  3. MaxAwesome

    MaxAwesome

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    This much I know.

    You actually didn't mention anything about memory size and how it's related to bus width :D

    I'll give the example above again: how does 192bit bus interfere with the 2GB of VRAM available on the GTX 660 Ti? Can all the VRAM be adressed or not? Is that the problem?
     
  4. Zubasa

    Zubasa

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    The way it works is quite simply the bus width pretty much controls the number of memory chips that can be used on the card.
    A GPU with 256-bit bus has 8 memory chips minimum, since each memory chip have a 32-bit wide bus.
    Some cards with double the memory amount will have 16 memory chips.
    The 8 extra chips usually on the back of the card shares the bus with the 8 chips soldered to the opposite (front) side of the card.

    The 660TI is a 192-bit bus which normally will have 6 memory chips resulting in 1.5GB of memory.
    But in this case nVidia added 2 memory chips to the back of the 660TI 2GB to achieve this.
    The 2 extra chips shares the bus with 2 memory chips on the front side of the card.
    [​IMG][​IMG]

    The first card that I know to employ this technique was the 550Ti.
    All the vram can be addressed yes, but it is quite an unorthodox way to have such a set up.
     
    Last edited: Aug 16, 2012
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  5. Frag Maniac

    Frag Maniac

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    Yep, as said it's bandwidth, or 256bit being like 4 lanes of traffic (data), 192 like 3 lanes, 128, like 2, etc.

    That said, many have speculated the Ti 660 with 192bit interface will be crippled, but others have pointed out that it appears Nvidia has found a way to make better use of less bandwidth.

    So it's not just specs, a lot of it has to do with improvements in architecture and how well they can make use of a given GPU's specs.
     
  6. TRWOV

    TRWOV

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    The only drawback I see from the 192bit bus is that the last 512MB of those 2GB are going to deliver less bandwith as it uses a shared 64bit link, so if your GPU memory usage goes >1.5GB you could get an fps hit... at least that's how I understand it, please correct me if I'm wrong.
     
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  7. hat

    hat Maximum Overclocker

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    They're still doing that goofy shit? I haven't seen that in a long time.
     
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  8. Hilux SSRG

    Hilux SSRG

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    Taking from all these comments, it seems to me that 2gb on a 660Ti is more marketing than usefulness. Now I wonder about those 3gb 660Ti?
     
  9. mlee49

    mlee49

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    I would only tell you what I know.

    So, back to traffic: Larger bus handles more memory, but higher memory(ie 2GB) thats alot of bits(cars) on the bus(road/lanes).

    So just widening a highway and adding a lane does not mean traffic jams won't happen. Opening lanes for more cars is fine, but adding more cars(ie vram) will just fill up the lanes just as fast.

    In theory, all the VRAM can be addressed. In reality, there is a buffer that will limit the amount of VRAM addressing before performing some other action(like offloading it to the cpu).

    If Nvidia was a city planner and hoped to add this ultra fast highway with 4 lanes of traffic, but said "This highway will handle the amount of traffic 2 other highways" then they might have just created a bigger traffic jam than before.

    Nvidia is smart though and certainly will use smart techniques to ensure this larger vram is warranted. Think Physx :cool:
     
  10. qubit

    qubit Overclocked quantum bit

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    That's some weird sht and not something I'd like to see on my card. It should always be kept to a power of 2 for optimal design.

    Good explanation. :)
     
  11. MSI650ti New Member

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    Yes a lower bit card can be crippled if the memory is set at a low speed like a GT 610 or a ATI 5450.
    And yes the theory of the 128bit 192 bit 256 bit being lanes on a highway is correct, But there is one thing not mentioned about the smaller bit memory buses "or more narrow lanes" Is this, The wider highway (256 bus) could have a slower speed limit than the 2 lane (128 bit bus or 192 bit bus)
    So this means that some newer cards even though they have a smaller memory bus does not mean they are slower than a card with a wider 256 bit bus. There are actually some 128 bit memory bus cards that are faster than 256 bit memory bus cards. This is because memory clock speed can be different and so on. The actual important thing to look at is the memory bandwidth of the card, How many (GB/sec) it is. The basic 660 has 144.2 GB a second in memory bandwidth, Which is very fast, And more than enough to handle even 3-4GB of video memory. I hope this helps.
     
  12. Vario

    Vario

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  13. Aquinus

    Aquinus Resident Wat-man

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    That's perfectly fine if you don't really plan to play video games on it very often. DDR3 is cheap enough to produce anyways so I suspect that it was the best cheap option nVidia had for the price range. You want GDDR5? Pay for it. It's that simple.
     
  14. Pill Monster

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    You missed his point. The card has 4GB of DDR3......completely stupid amount it could never use it.
     
  15. Aquinus

    Aquinus Resident Wat-man

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    Like I said, they went with the cheapest ICs they could get their hands on. DDR3 is fairly cheap in comparison. Of course it's rediculous but from a cost perspective, it very well could have costed less to produce the video card than putting 2GB or even 1GB of GDDR5 on it instead. Not to mention you need a controller on the GPU to handle it.

    It's all about cost to produce the product.

    Also, if someone is doing CUDA, OpenCL, or DirectCompute work, you may not need a ton of GPU horse power to get a lot done. This card clearly doesn't target gamers, that's my point.
     
  16. Pill Monster

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    Yeah that's true.....even so they prob could've made the card cheaper by using less memory.....but I digress. Hell with DDR3 discrete card u may as well get an A10 APU anyway. lol
     
  17. Vario

    Vario

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    Its not that. Its arrangement is completely suboptimal for performance. It does not have enough bandwith to even use that ram. its a total gimmick. http://techreport.com/news/21536/point-of-view-insults-intelligence-with-geforce-550-ti-4gb

     
  18. Pill Monster

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    LOL I just looked at the specs on POV homepage, as if DDR3 wasn't bad enough by itself it's only 1066mhz. What were they thinking.....0_o

    actually speed prob doesn't matter they could make it 2133mhz it would still take a week for data to get to the GPU just like being on a Greyhound bus. :D (get it?) lol
     
    Last edited: Jan 6, 2014

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