PMC Bottleneck: 2x1080=7x1070? Tweet

[Dizern]

Hi!
I'm working on pretty complex scene in C4D with a lot of different lights and glossy mats. I chose to go with PMC to get rid of hotpixels in PT. I thought that some extra rendering is not a big problem as I have 7x1070 GPU farm(PC2)...but here's the thing..it's slower on PMC algo than 2x1080(PC1).
This problem only appears in PMC algo, so I don't really understand what is the problem.
PC1:

i7-4790k
32GB RAM
2xMSI 1080 gaming x
SSD win8.1

PC2:

Xenon E5 2609v3
64GB RAM
7xMSI 1070 gaming x
HDD win10

Here's few screenshots:

PC1 stats

PC1 GPU usage

PC2 GPU Usage

PS2 Stats

Render Settings

[glimpse]

hi, Dizern

could You give us a bit more details about Your setup, like:

* what motherboards You're using,
* are those 7x GPUs are directly seated into motherboard or are You using some cables to displace those cards from motherboard,
* last, but not least, if You leverage network rendering, what network solution You use. (10G, 1G, WIFI..what's in between)

Sinc.
tom

[Dizern]

Thanks for the reply, I use Asus X99-E WS/3.1 and PCIe raisers(some are x16 to x16, some are x1 to x 16.
I do not use Network rendering.
Works ok with DL/PT algorithm.

[Dizern]

I've tested with and without multipass.

The situation goes better with render without multipass. The results are 7x 1070 are twice as fast as 2x1080....but it should be 3x fast.

It uses all the cards, but I see big gaps on graphic anyway.

[glimpse]

I've tested with and without multipass.

The situation goes better with render without multipass. The results are 7x 1070 are twice as fast as 2x1080....but it should be 3x fast.

It uses all the cards, but I see big gaps on graphic anyway.

Since v3, x1 risers have more impact to performance as information should go to & out of GPU while rendering as image is now assembled in RAM rather than VRAM, so interface mattes =) could You unplug x1 GPUs & see how Your rig would behave?

[anereton]

Tom, Hi,

Can you explain better that concept? I planning to buy more GPU and my Motherboard have 3 slots.

Thank you. : )

[glimpse]

Tom, Hi,

Can you explain better that concept? I planning to buy more GPU and my Motherboard have 3 slots.

Thank you. : )

simply put, if You would be using any kind of risers, better use good one, like x16 ribbon cables, not some cheap x1/usb option..

as for 3x GPUs, it's better to put them straight to Your motherboard. X8 is recommended, but it would work in x4 slots, just speed might be impacted a bit as GPU would have some dips 0% utilization as while rendered information is transferred in/off GPU via slower interface would cause GPU waiting & not rendering.

makes sense?

[Dizern]

Tom, Hi,

Can you explain better that concept? I planning to buy more GPU and my Motherboard have 3 slots.

Thank you. : )

First of all, you need to read your mobo manual and see if you have enough lines for 3 gpu to work on x8. if so, you should have no problem with using ribbon cables.

The problem for me is that they usually 20-25cm, so I can't place cards at least 3-4cm between each other...what will cause overheating.

If you use x1 extenders, you will loose 4-10% of processing speed, I've done those tests while ago on single gpu. Just for the sake of test.



Tom, I've removed x1 extenders and.....the picture is still there
1gpu in mobo, 3 on ribbon risers. I don't understand why it works so bad in PMC_MP only...
Here's the graphs with 4 pgu and 1graph with 1plugged gpu only:

PMC_MP

PMC_noMP

PT_MP

PT_noMP

1GPU_PMC_MP

[anereton]

Got it!

I got my motherboard specs and right now and i have GTX 980.

The GPU prices in Brazil is way too high, and im plannign to get another one.. below my motherboard specs for GPU.

---
2 x PCI Express x16 slots, running at x16 (PCIEX16_1, PCIEX16_2)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16_1 slot; if you are installing two PCI Express graphics cards, it is recommended that you install them in the PCIEX16_1 and PCIEX16_2 slots.
1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16_2 slot. When PCIEX8 is populated with a PCI Express expansion card, the PCIEX16_2 slot will operate at up to x8 mode.
2 x PCI Express x16 slots, running at x4 (PCIEX4_1, PCIEX4_2)
1 x PCI Express x1 slot
(All PCI Express slots conform to the PCI Express 2.0 standard.)
1 x PCI slot
--

I will using that concept to choose a new GPU in near future.. Thank you!
(Sorry about my english kk)

[Dizern]

Got it!

I got my motherboard specs and right now and i have GTX 980.

Also, keep in mind that 9** much hotter than 10**. So think about cooling. If Place them around 8-10cm between (if aircooled)