Hi,
I was planning in near future build a new pc and i was wondering if blender’s cycles would support 7 GPU’s like in this octane pc build https://www.youtube.com/watch?v=9hsQmcSwGv0&t=113s
I would like to know that before i buy so many GPU gards?
And for example if 1GPU take 12hours to render will it mean that with 2GPU it is 6hours and with 3GPU 3 hours and so on?
Hi, in my last benchmark there was users with 4 GPU, nothing against more for Cycles.
The performance is not 100% linear but nearly, so 4 GPU is ~25% render time.
Cheers, mib
https://blenderartists.org/forum/attachment.php?attachmentid=370114&d=1427653408
Hmmm…just wondering why rendering speed is not linear ? I think in octane render engine the gpu scaling is almost linear.
Also, if you look at the specs for that motherboard, only four of those PCIe slots run at full x16 speed (the blue ones). The other three slots (white) run at x8 (half speed).
So at best, you’ll get ~5.5x speed up from a single GPU, but likely less than that, probably closer to 4.5x - 5x.
And you’ll want to carefully research power supply requirements for your seven video cards. A single GTX 1080 needs almost 300 watts with another 300-500 required for everything else in the system. So if you were to use seven 1080s, you’d be looking at a PS output between 2300 and 2600 watts.
that is totally incorrect when it comes to 8x slots…
Rendering is done fully within the GPU and its memory, so outside of the initial scene upload to the framebuffer, there is no penelty for rendering on 8x slot over a 16x slot.
Additionally, though gaming wise. many test show that in PCIe Gen3, there is nealry no visible performance difference between 8x and 16x slots.
I won’t say you will get full 7x boost, but at least in the 6.5+ range
If you watch that video link for 7GPU build in my first post, you can see that he is using 7x1080 with 1600w PSU.
I also found 2000w PSU http://www.super-flower.com.tw/products_detail.php?class=2&sn=16&ID=119&lang=
Also there are plenty of solutions to link 2 PSU’s together, so there won’t be any issues running that.
That wasn’t my experience running multiple GPUs.
That’s fine when the PS is new(ish) but after a couple of years, any PS will decrease in output. Also, he’d be sending that 1600w to the wall every time he rendered and it wouldn’t provide optimal performance for very long. I suppose if you plan to replace the PS every 12-18 months, it’s no big deal.
a good psu, like super flower, should be able to work past its wattage. but for proper efficiency, you need at least 1.5x the total wattage at full load. 2x is preferred.
its not always a matter of replacing a worn out psu, they might go up in smoke LITERALLY, taking down your whole pile of gpus with it.
psu is NOT to be taken lightly.
Can you elaborate a bit more on this?
If adding second same gpu gives 50% speed increase, 3 gpu gives 33% and 4 gpu 25% is it even worth it to use 7 GPU in one pc if the cycles scaling is not so good? 
It’s not like that. Every GPU renders the same scene separately. If 1 GPU renders a scene in 7 hours, 7 GPUs will render the same scene in 1 hour(and maybe a few minutes more). If 1 GPU renders a scene in 7 minutes however, it does not mean that 7 GPUs will render the scene in a minute because it takes time to manage the rendering process with 7 GPUs like to load the scene to all of them. If you are rendering a lot of fast rendering frames it might not be logical to divide the rendering of each individual frame to 7 GPUs.
I would like to believe you that it works like that, i need some evidence that it works like you say it should work. I can’t just by 7 1080Ti 5600€ and notice later that the the benefit of so many cards is not so great.
Do you know any benchmark test that has been done with more than 4 same GPU cards?
This thread is on the benefits of multiple GPUs. It shows some of the reasons for linear and non-linear speedups.
I added a second GPU to my system several years ago and render time increased by only 17% because with both x16 slots in use, they slowed to x8, half the speed expected.
The conclusion I reached from this was that the speed at which any GPU crunches data is dependent on the limitations of the motherboard. In the case of the MB you’re looking at, only four of those slots will run at full x16 while the other three run at x8. Even then, communication with the MB is going to create at least a bit of a bottleneck meaning you can’t simply take the speed of one GPU and multiply it to get an accurate speed at which seven cards will do the same job. Hard drive access, motherboard limitations, and BUSS speed will always play a part.
The only way to find out—with any accuracy—how much extra speed you’d get with seven cards in that MB is to build the system and test it… or find someone else who already has and ask them.
Not necessarily. I don’t know what went wrong for your particular build, but PCIe bandwidth doesn’t affect Cycles renders that strongly (you should find numbers on that if you look hard enough).
Also, a 1080 doesn’t need anywhere near 300W (TDP is 180W) even under peak usage (which you will probably not reach with most Cycles scenes).
You’re misreading what mib2berlin said: 4 GPUs = ~25% of render time, i.e. about 4x speedup. I’ve seen people reporting worse scaling than that, but there’s always overhead that depends on the CPU (startup time, BVH builds, post effects) and how long you’re stuck on the “last tile” (only one GPU can work on a single tile at the moment).
I wouldn’t go with a 7x GPU system either way, that’s really pushing things and you’ll pay a premium for that.
I believe it depends on the situation. Say for some reason you have a large complex scene with simple direct lighting and few materials that renders really fast, but you need to render an animation. In that case loading the scene to memory starts to be important. I am just saying one should always consider the situation.
It is a bit extreme to buy 7 GPUs at once. What’s up with the rush? Why don’t you try 2 first, see how that goes, then add 2 more, and then 2 more and then the last one? If it doesn’t go well with 6 of them, just get another pc for the price of the 7th one and put 3 in each.
Finished building my workstation with 3rd RX 480 in. the 4th not working as there isnt’ sufficient amount of PCIE resources… waiting for ASROCk to verify what the issue ist… still
1x RX 480 - 8m 59s (539s)
2x RX 480 - 4m 35s (275s) - 1.96 (2 perfect) scaling
3x RX 480 - 3m 05s (185s) - 2.91 (3 perfect) scaling
4x RX 480 - … (…) - … (4 perfect) scaling
So very very close to linear improvement …