CPU limited or not ?

[bigbossfr]

Hi,

Quick question for ChaosGroup : we are building a GPU render node with 4 x GTX 1080.
We are not sure about the CPU. We can add a i7 6800k (6 cores/12 threads) on it. But we are not sure if it will work with 1 cpu (the i7) or if we have to add 2 cpu (xeon).
Question : 1 cpu is ok with 4 gpu or with 4 gpu we HAVE to use 2 cpu ?

Thank you,
Damien

[eyepiz]

I've used 4 GPU's in a single i7 I't was a 3930k on an ASUS board if I recall correctly. I had 4 x GTX 680's at the time.

[savage309]

As far as RT GPU is concerned, 1 CPU (especially that powerful) will be more than enough for 4 GPUs.
Best,
Blago.

[benb32]

Hi Damien, your setup will be motherboard specific. If it is a dual CPU motherboard, the PCIe lanes from each CPU will go to their own PCIe slots, just like the dual sets of RAM slots - one set for each CPU. The QPI channels on the Xeon processor are used when one CPU needs to talk to a GPU that is attached the other CPU PCIe lanes.

If it's a single CPU socket motherboard, and it has more than 2 PCIe x16 slots there will be another controller chip that will provide more PCIe lanes - the top end xeons and i7 chips only have a total of 40 lanes.

There are some workstation boards out there that can support 7 x16 PCIe cards - that type of setup is hard to keep stable due to the physical size and weight, and providing enough power to all the GPUs and CPU.

All the best Ben

[benb32]

It's also helpful having as powerful (read cores and clockspeed) as you can manage - before the CPU starts keeping the GPUs utilised and busy, it will be generating the displacement and light cache. Having a bit more RAM than you normally use should also be a consideration.

[savage309]

It's also helpful having as powerful (read cores and clockspeed) as you can manage - before the CPU starts keeping the GPUs utilised and busy, it will be generating the displacement and light cache. Having a bit more RAM than you normally use should also be a consideration.

Yes, this is a very good note .

[bigbossfr]

Thanks all.
The new i7 6850k has 40 lanes. We received 4 GTX 1080. We will start to build the computer soon.