GPU selection and render usage.

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1

Hi folks. A quick question on the selection list of GPU’s in the User Preferences.

In the System tab in the User Pref’s, under Compute Device is as Follows:
GeForce GTX 670 + 780 Ti (x2)
GeForce GTX 670
GeForce GTX 780 Ti
GeForce GTX 780 Ti

So, obviously choosing the top option is going to use all 3 three cards for cycles.
The second option is pretty self explanatory.
But, is choosing one the 780’s going to use just one card, or is it going to use both?
Confusingly there isn’t an entry for 780 Ti (x2)

I’m still trying to get to the bottom of the random crashes I have while rendering and have found the following:
Using the option of all three cards rendering a complex (very) mesh will crash the app (non responsive).
Using one of the 780 entries will fail to render the same mesh, but the app doesn’t crash.

So, does the 780 have more complex memory usage detection, is the 670 running out of memory and crashing the app, while the 780 runs out of memory but handles it better?

I’m further confused as I’m not using any textures at all, just simple colours. 33,674 verts and 32,740 faces. Also interestingly I suffer more crashes with SubSurf render turned up. Now, obviously this is adding to the complexity of the mesh, but the verts and faces count doesn’t increase.

Any thoughts?

2

You do realize that the subsurf modifier has separate settings for viewport and render? So, if you crank up the subsurf levels at rendertime, the vertex count for the viewport will not increase, but the complexity of the geometry might still go through the roof when rendering.

Nine out of ten times a Blender crash while rendering is caused by running out of memory. The maximum amout of VRAM Blender can use for rendering is limited by the card with the lowest amount of VRAM, so, is that a 2 GB or a 4 GB GTX 670?

And one thing to also notice: There is a bug with the current Blender and memory management on GTX 6xx cards. Even on 4 GB cards Blender can not use more than 2GB for geometry. This seems to be fixed with Blender 2.71, so maybe try a RC and see if that also crashes.

3

I presume you’re working in windows…
If you want to use just the two 780s for rendering you need to disable the 670 as a cuda device in the Nvidia contol panel.

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Then you should get an option like 780(x2)

4

Or try a buildbot version (2.70.5), which I did, and now i can render ass much geometry or textures as I want to on my 680 :slight_smile:

5

Imari, most of your advice is known, but I didn’t know about the memory bug, I’ll upgrade today and continue to test. Thank you.

cegaton, I didn’t know about this setting, I’ll get this done today as well, thank you for that.

Wega, this sounds very promising, I’m looking forward to trying this. Thank you.

I think this is the most active, informative and helpful forum I’ve ever joined!

6

My comment was only about the memory bug that Imari mentioned… Just to be clear.