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How much data do GPUs transfer through PCI?

post #1 of 7
Thread Starter 
I understand PCI2.0 x16 and PCI3.0 x8 slots allow 8GB/s data transfer each direction, and PCI 3.0 x16 slots allow 16GB/s each way, and that current gen (and presumably next-gen) GPUs aren't close to attaining those transfer speeds. What I don't understand is why a GTX 680, for instance, is rated at 192GB/s memory bandwidth, and how that relates to PCI data transfer. It has 1000+MHz processor and 1500+MHz memory clock and 192GB/s bandwidth on the card, but how do you tell how much data it actually has going back and forth through the PCI lanes?

I get this: if, for example, that 680 has 5GB/s going through PCI, and with a second 680 in SLI in another PCI2.0 x16 or PCI3.0 x8 slot, they both have plenty of headroom. But what if you have a 690 instead, capable of sending/receiving 10GB/s but getting bottlenecked at 8GB/s... put it in a PCI3.0 x16 and problem solved. I just used those numbers for sake of the easiest example I could come up with, and they are not based on anything; I'm just trying to figure out what that number actually is. I've looked all over for any sort of spec or explanation on this, and am at a loss. Assuming I get an answer, I would also like to know: does overclocking a card have any effect on its PCI data transfer? I would assume yes, but really not sure why... thanks in advance!
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post #2 of 7
The cards internal clocks and buses and the PCIe bus are two separate things. The PCIe slot bandwidth has nothing to do with the cards internal clock speeds or buses. The slot is the link to the outside world allowing exchange of data between between the card and the rest of the system.

The cards memory bandwidth, CPU clock speeds and such are all internal to the card. Think of it as a self contained box, more or less. 192GB/s memory bandwidth is how fast the data travels into and out of the cards memory between GPU and shader cores and such inside the card, not across the PCIe bus.

Yes, if the card is able to process and render data faster than it can receive it you have a bottleneck at the PCIe bus. But as you mentioned we are not in any danger of that right now.


Overclocking the card does not effect the PCIe slot speeds. Again, overdclocking the card is only internal to the card. But most boards do have the option to overclock the bus itself.

Hope that helped.
Edited by the grinch - 9/9/12 at 7:27pm
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post #3 of 7
Thread Starter 
Well, that certainly clarifies a lot of things for me, thank you! But it doesn't answer my root question of how to determine what exactly is going through the PCI bus. However, I'm starting to think that it may be an indeterminable thing for my level of inadequacy/skill wink.gif
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post #4 of 7
Quote:
how to determine what exactly is going through the PCI bus.
I don't know of any way to monitor the data going through the PCIe bus. But it really is not important since we are far from saturating it.
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post #5 of 7
Download this to measure bandwidth speeds http://www.kegetys.fi/forum/index.php?topic=427.0
Multi-monitor users use that to determine which card is bottlenecking when using different videocards.


you can see my old geforce FX5200 bottlenecking my HD6850
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post #6 of 7
Nice little tool. thumb.gif I am not too surprised to find something that can monitor bandwidth.
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post #7 of 7
Thread Starter 
Fantastic, thanks!
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