Quick Memory Bandwith Question

[SerCleopas]

Hello all,

After reading through the forums, I learned that vRam does not stack in SLI or Crossfire setups. However, no one has really addressed the question as to whether memory bandwith stacks in SLI. If I have two 256 bit cards hooked up in SLI, does that increase my effective bandwith to 512 bits?

 

[ko888]

Hello all,

After reading through the forums, I learned that vRam does not stack in SLI or Crossfire setups. However, no one has really addressed the question as to whether memory bandwith stacks in SLI. If I have two 256 bit cards hooked up in SLI, does that increase my effective bandwith to 512 bits?

No.

 

[SerCleopas]

No.

Can you elaborate at all? I can't seem to find a good source on how memory bandwith ties in with SLI configurations, if at all. I guess the point I'm driving at is that, let's say theoretically a game uses 3.2 gb of vRam during an intensive scene and you have a card with 4gb vRam card but the bus is only 256 bit. Will there be a bottleneck? At what point does the 256 bit bus get bottlenecked by vRam usage? I understand that games right now do not require that much vRam.

 

[briansklein]

No. The answer is in SLI and in Crossfire the memory is mirrored. In other words both cards memory is loaded with the same information. So if you SLI two 1.5GB cards the frame buffer size is 1.5GB is is not additive. The performance increase is due to each GPU rendering alternate frames one after the other. In a game that uses SLI effectively, this can give you in excess of a 90% increase in performance. Generally performance scaling decreases with each additional GPU added. That's the short version.

 

[briansklein]

256 bit bus is usually more than enough for game's. I know the larger the bit the more info can be shared per thingamobobber however and why card company's don't find a solution the bit and memory of the first card in line on your mobo is the max the computer will ever use. Hopefully someday we will be able to see both cards used at their full potential.

 

[briansklein]

One more thing. The data is shared through the first video card and uses that interface for transfer. If I'm correct the computer then uses the sli bridge to take performance from the second GPU and then back through the first cards bus. I'm not a guru but that's just how I understand it works in layman's terms.

 

[SerCleopas]

One more thing. The data is shared through the first video card and uses that interface for transfer. If I'm correct the computer then uses the sli bridge to take performance from the second GPU and then back through the first cards bus. I'm not a guru but that's just how I understand it works in layman's terms.

Thanks briansklein. I think I understand.

 

[j2j663]

Horribly basic overview of SLI/Crossfire:

If you imagine that the frames have a predefined order (frame 0,1,2,3,4,5,6,7,...) Then it goes like this:
GPU0 renders frame 0
GPU1 renders frame 1
GPU0 renders frame 2
GPU1 renders frame 3
GPU0 renders frame 4 ...

So each GPU renders every other frame. So there is no stacking of any resources, that is why when you try and use 2 different GPUs (can be done in rare cases) the more powerful one will only work as hard as the less powerful one can.