how does 3 GPU scale in terms of cycles performence? (memory problem)

i searched up on the topic, and it came up with some worrying info
personally, i have the budget to go for GTX 980 (X2) or GTX 970 (X3, perferred)
but some people say that you can only access the first card’s memory when rendering, which is worrying, because GTX 970 only has 3 GB of VRAM, and i don’t want it to crash…

and if it doesn’t only use 3 GB out of 12GB of VRAM, how does the performance scale in 3 cards?
i thought it would just go 1/3 the time because the separate cards can just render different light source, and then combined them after, but some say it doesn’t, and they inter communicate when rendering…

please gimme answers (T^T) thx

thisis the system with triple 970
and thisis the double 980

I have 3x 750Ti and it breaks down basically like this for me:

1x 4 min 00 sec
2x 2 min 00 sec
3x 1 min 20 sec

Look closer at the 780s before you drop your cash on the 900’s.
The 780 has a faster memory interface and more cores than the 900’s

Bench numbers between the 780’s and the 980’s can be seen here, lines 10 and 11.

how about the memory amount?
cuz 780 has 3GB

Hi, there are GTX 780 with 6 GB, may you can find a used one for a good prize.

Cheers, mib

no, the problem(actually, more like happy)
i found official retails that are super cheap, so i would like to buy 3 780, but
PLEASE answer this T^T, they are going to run out of stock soon

should i go 970 X3? 980X2? 780X3? 780ti X2?

please answer… T^T all of them are running out of stock

780’s all the way. If you can afford it go for the ti versions. Don’t know what the future holds for the 980, with driver updates and improvements in Blender/Cycles they may come good, but at the moment they can’t touch the 780’s.

The only benefit I know of to running the ATI up front is it frees the CUDA cards of any memory responsibility as far as you desktop is concerned. I have considered that direction myself but have no first hand knowledge if it works well. If it is in your budget I would think that trip 780’s would be the better option for all out render performance. However, if vram is a concern then the ATI primary should save a few hundred meg depending on desktop size.

what do you mean by ATI? u mean amd GPU?
or do you mean APU? amd’s CPU

My bad, I totally read 290x in that list for some reason.

If 3 GB VRAM is enough for your needs go for the 780´s.
Look at the benchmark spreadsheet > http://www.blenderartists.org/forum/showthread.php?350975-The-new-Cycles-GPU-2-72-Benchmark&p=2746601&viewfull=1#post2746601

3 x 780 is faster then 2 x 780Ti.

Btw. the 970 has 4 GB and is the best card prize/performance with much lower power consumption.
How much cost the GTX 780/80Ti?

Cheers, mib

yeah, the 970 that is in my choices are 4GB
but can’t blender use the other VRAM of the other GPU?
if not why?

The whole scene has to fit in the memory of each card, so memory is not added.
If you have two 4 GB cards you can use 4 GB.
If you have one 2 GB and one 4 GB card you can use only 2 GB.
I have a 1.28 GB card and a 4 GB card.
For small scenes I use both cards, for bigger scenes I use only the 4 GB card.

Cheers, mib

i get it now :slight_smile:
thx!
may i ask, how are the memory scaled?
do they directly correspond? (.blend file size)
or does it differ?

Your main card will be short however much memory is used by your desktop and UI.
At idle with nothing but my two monitors running I am showing 156MB usage on my primary.
If I launch Blender and the current scene I am working on that jumps to 990MB.
You can of course minimize Blender at render time to get a nice chunk of that back but some of it will always be unavailable.

With Blender open at render I show 1860MB used, if I minimize it I show 1703MB used.
Blender itself shows that the scene used 1655.09MB.

Render a big scene and watch the peek memory it correspond roughly with VRAM usage.
Texture size is also very important.

Cheers, mib

ANTHONYC

If you don’t mind me asking, why are you using 750ti? I hear that 2 750ti area good render combo but seeing two of them are not much cheaper than a 970 (or other 700 cards) was a little turned off about buying a bunch of them.

Also do you only render still image or do you do some animation as well?

I render both stills and animation. (See my sig for a link to my current animation project and detail of render sizes and times)

I have the 750’s because the 900’s were not out when I decided to make the jump to GPU rendering.
At the time I got all three on sale for not much more than what the 770’s were running.

Now that the 900’s are out I would likely make a different choice, probably not a 900 series card.
The numbers I am seeing just do not sell me on the 900’s yet, not for the cost to performance gain I am seeing.
That may change in the future but I tend to buy based on what I know at the moment not what may be later on.

I think the 770/780’s are a better bang for your buck going right now. I’ve seen 4GB 770’s for $200 after rebate.
And while the 780 is closer in price to the 900’s it has more cores, faster memory interface.

AnthonyC

Thanks for the replay.

From your signature, and the details in some of your postings, you clearly know what you are talking about.

Guess I am off to buy some 770s

Good luck, let me know how the build turns out.