Can I combine video cards to increase render speed?

jeff_someone

I've looked all over and kind of saw some posts discussing this, but couldn't get a straight answer.  I have nvidia 970 GTX currently but plan to buy a 980T and I'd like to keep both installed in my computer.   I know I can't do SLI, but assuming I have a power supply that supports both at the same time, will Daz Studio utlize the CUDA cores from each card for a render, or no?  Any help or info is appreciated.

Thanks,

Jeff

 

mjc1016

Yes, you can have multiple cards and Iray will use them all...up to the memory of a card.  If the scene won't fit on a particular card, then it won't be used.

 

jeff_someone

Ok, thanks much for the quick response !

jeff_someone

Unfortanately, keep both video cards (gtx 970 and gtx 980ti) didn't reall work as expected.  On a complex scene, I get the following rendet times in iray:

 

1) With just my 970 turned on w/Optix = 111 seconds

2) With just my 980ti turned on w/Optix = 23 seconds.

3 ) With both cards turned on = 24 seconds.

It's as if the 970's cuda cores go to waste when the 980ti is active.  

Any thoughts?

 

Thx, 

Jeff

namffuak

jeffam112368_9a28fbd572 said:

Unfortanately, keep both video cards (gtx 970 and gtx 980ti) didn't reall work as expected.  On a complex scene, I get the following rendet times in iray:

 

1) With just my 970 turned on w/Optix = 111 seconds

2) With just my 980ti turned on w/Optix = 23 seconds.

3 ) With both cards turned on = 24 seconds.

It's as if the 970's cuda cores go to waste when the 980ti is active.  

Any thoughts?

 

Thx, 

Jeff

There is a noticeable amount of time required to write the scene into the card's vram; as near as I could tell the few times I looked, Iray builds a data buffer, writes it to card 1, then card 2. And again. And again, until the entire scene has been populated to both (presumably all, but I've only got the two). I can see this taking several seconds for each card. Try again, only with a render that runs about five minutes and see if there is a difference between 1 card and 2.

nicstt

jeffam112368_9a28fbd572 said:

Unfortanately, keep both video cards (gtx 970 and gtx 980ti) didn't reall work as expected.  On a complex scene, I get the following rendet times in iray:

 

1) With just my 970 turned on w/Optix = 111 seconds

2) With just my 980ti turned on w/Optix = 23 seconds.

3 ) With both cards turned on = 24 seconds.

It's as if the 970's cuda cores go to waste when the 980ti is active.  

Any thoughts?

 

Thx, 

Jeff

Best way to test is to start a render, then stop it; don't close the window.

Start rendering again; the time for rendering won't include uploading to the device on any subsequent tries.

jeff_someone

Thanks all...I'll diagnose a little with these approaches.  My suspicion is that the 980ti overpowers the 970 so much that it just isn't giving cycles to the 970.  But we'll see!

mjc1016

A second or two variation is well within the 'normal' of running several renders. 

A scene that is finishing in around 20 seconds on the 980 isn't really complex enough to be a fair test.

Tobor

jeffam112368_9a28fbd572 said:

It's as if the 970's cuda cores go to waste when the 980ti is active.  

Not uncommon for a short render, especially if your 970 is also used for the monitor, where Iray will preferentially use the other card for the bulk of rendering in order to maintain responsiveness in the display. (You'll see a notice to this effect in the log.)

For some kinds of scenes, users have reported an *increase* in overall rendering time when using multiple cards. It may seem counter-intuitive, but consider there is a certain amount of data transfer plus coordination that must take place when using multiple cards. You don't get a reciprocal curve in performance, and as Mjc noted, unlikely you'll see anything meaningful at all with a short render. Try one that takes your 980 alone a half hour to do.

jeff_someone

All, thanks again for the great responses.  I ran a longer test, with a 10 minute render.  I does seem the 970 is now contributing...I guess the short render time for my earlier tests was inadequate.  Also, I read on the Iray forum at Autodesk site that I should turn off Physx on both cards.  So I did that (I had it set to my 970), and I think that may have also helped (though I'm only guessing).   

Wanderer

I know this is old, but Google brought me to this thread today. For anyone curious about GPU impacts on render times, you really need to check out this thread . Lots of benchmarks, single and multiple GPU stuff. Small scene, but very interesting results.