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I have a dual monitor machine at work that I use to write code, run scripts, and plot results. The amount of plotting can be quite demanding. I have ran out of video memory a couple of times (its either 1 or 2 gb). I wan't to extend this setup to have quad monitors by adding a another graphics card, but I am unsure of a few things. The more I think about it the more I realize I have no idea how this is supported at the lower level. I know that I won't be using SLI, (these questions only apply to the case where you have two separate models of graphics card plugged into the pci-e and not connected via SLI).

Can the two graphics cards run different drivers (ie nvidia vs amd or quadro vs geforce)?

Which card's video memory is used for things like Matlab plots? Does it matter what screen the plot is on? If I move it to another screen does it switch which card has it in memory?

If my 2nd card is a lower end card with a very small amount of video memory, how does this effect my performance? Will I still be able to have a large amount of plot data shown at once? Will this data be restricted to only show on the monitors connected to the high end card? Will having two cards allow more data to be displayed (since there is more total video memory?)

An understanding of whats happening at the lower level would really help me out.

The machine has 2 pci-e slots available. Currently there is a single nvidia quadro graphics card in there. (I'm not sure which one, but its fairly high end ~$1500). It is running windows XP 64bit with 24gb of ram.

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Windows XP does not support multiple video cards that require different drivers. Windows 7 does. Also, can you be more precise about exactly what happened when you "ran out of video memory". –  David Schwartz Jan 18 '12 at 1:07
    
As I recall, I got a pop up that warned of low/no video memory. I wasn't able to create any additional plots after that point. I'm not concerned with running out again, I did have a lot open. I just wanted to point out that I am plotting enough data that memory is a legitimate concern. I think my card has 1 or 2 gb on it, if I installed a card with 512mb of memory it might not be enough (if in fact the memory on the 2nd card effects how much can be open on the monitors it drives, which is one of my main questions). –  MajorApus Jan 18 '12 at 1:43
    
You sure it was low video memory it was warning about? I can't see why plotting would require lots of video memory. –  David Schwartz Jan 18 '12 at 3:15
    
I suppose it could have been some other memory and I am recalling it correctly, but the machine has 24 gb of system memory, so I doubt it ran out of that. –  Miebster Jan 18 '12 at 6:54

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Windows XP does not support multiple graphics drivers. However, quadro and geforce are both made by Nvidia, so you can probably get them both working together. AMD and Nvidia together are a no-go.

As far as video memory, there seems to be some confusion as to its use. Primarily, video memory is used to store the data so the GPU can convert the raw display data into the pixels that will be displayed on your screen. However, in recent years, the GPU has been used for more and more applications. In your case, you run no applications that require advanced 3D calculations, so you should be able to run off of no more than 64MB for all four monitors (though this obviously depends on the resolution that you are running). Given that your plotting application, however, relies on video memory to perform its plotting and calculations, you should be sensitive to the total amount of video memory you have. This (should be) independent of the monitor that the plotting application is on, but is rather a function of the specific GPU used for the calculations, and how much memory is availabilyt to that GPU.

If you give us more information on the plotting application, we can give you a more specific answer.

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All of my plotting is done in Matlab. Some plots will be using the openGL features in Matlab, but most don't. How does the system decide which GPU will perform any calculations it needs a GPU for? I'm also starting to wonder if the plotting tools use the GPU at all. –  Miebster Jan 18 '12 at 2:38
    
Given that you run out of video memory, I guarantee you that the plots are using VRAM to assist in the calculations. Though I'm not exactly sure how MATLAB decides which GPU to use. You might be better off asking your question in the MATLAB community. –  Nick2253 Jan 18 '12 at 3:38

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