Multi Video Cards

[rodders1947]

I have an i7-8700K / 32 gb ddr4 3000 Ram / 2 x RTX 2070 8 gb each Video Cards. I have 2 screens attached to each video card to spread the load.
When I do Shift + Z to show FPS etc it shows GPU usage of 3.2/7.1 gb. That is the amount of Vram of ONE of the cards. Why does it not SHOW / USE the TOTAL amount of VRAM of 16 gb ?
Is it because P3D only runs on the performance of the ONE video Card? and not capable of using TWO video cards ?
NOTE : when I remove the second Video card the stability of P3D improves dramatically as does FPS with all 4 screens on the ONE card !!!!!
This is the same for 5.1 and 5.2

[Beau Hollis]

GPU memory budgets are tracked per GPU. Internally we do have the metrics for each card. The infogen window will pull budget info from the GPU driving the display it is showing on, so you should be able to see the budget of your other GPUs by moving the main app window to the other display. In general though, most textures and buffers must be copied to both cards, so as long as your primary GPU has a 3D view on it, the VRAM usages stats should be a good indicator of your usage and should be sufficient for tuning settings to prevent overages. Once exception to this general rule are resources used for rendering output. (render targets, depth-stencil buffers, and swap chains). So if one GPU has more views or is using higher resolution displays, then it's VRAM usage will be higher.

If you're seeing better performance using a single GPU, it's possible your settings and content are such that you are CPU bound. There is some CPU overhead when using multiple GPUs because resources must be copied to both cards. We have seen good scaling of per-view work across multiple GPUs in our testing, but there is work such particle simulation that both GPUs must do.

[Beau Hollis]

If you want to test the scaling of work while GPU bound, max out the volume cloud slider. That should get your very GPU bound.

Also, if you have two GPUs installed without an empty PCI slot between them to allow for air flow, your GPUs may overheat and clock down leading to a big FPS drop. You can use tools like GPU-Z to monitor GPU temperature to see if this is happening.

One last thing to note is that, on many motherboards, not all PCIE slots have the same number of lanes (16x, 8x, 4x, etc). If your performance bottleneck is coming from resource copies to the GPU then you might be better off with one powerful card in the fastest slot. You could try putting all 4 displays on your second GPU and compare the performance results to that of your first.

[rodders1947]

Many thanks for that in depth explanation. I will pursue the change of CPU to a higher spec ...

Rod