2X Graphics cards = 2X memory?

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Yes it does, but not quite in the way that you might think.

When multiple GPUs are used to render a game each GPU renders alternating frames. This technique is known as Alternate Frame Rendering, or AFR.

For example, when two GPUs are used, one may render frames 0, 2, 4, 6, 8, and so on, while the other would render frames 1, 3, 5, 7, 9 and so on. The same logic can be extended to three, four, or more GPUs. Each subsequent GPU increases both the frame rate and the frame latency, so using more than 2 GPUs can often result in undesirable input lag.

As far as the memory is concerned, each GPU operates in a fully...
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Pinhedd

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Aug 7, 2007
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Yes it does, but not quite in the way that you might think.

When multiple GPUs are used to render a game each GPU renders alternating frames. This technique is known as Alternate Frame Rendering, or AFR.

For example, when two GPUs are used, one may render frames 0, 2, 4, 6, 8, and so on, while the other would render frames 1, 3, 5, 7, 9 and so on. The same logic can be extended to three, four, or more GPUs. Each subsequent GPU increases both the frame rate and the frame latency, so using more than 2 GPUs can often result in undesirable input lag.

As far as the memory is concerned, each GPU operates in a fully independent fashion. Since they are both rendering frames from the same scene, they will both require the same source data. This includes the same textures, the same models, the same geometry, the same shaders, and more. A huge amount of the data is duplicated by necessity because more often than not the only difference between two subsequent frames is the position of the camera and the timing of animations. In theory it would be possible for both GPUs to share the same data via a shared memory buffer, but current memory architectures do not provide enough bandwidth to do this.

This duplication of data is rather unique to gaming. When multiple GPUs are used for other purposes, such as scientific modelling via OpenCL, they operate in a fully independent fashion. Data will be duplicated only if necessary.
 
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