PCIe X1 Slot Performance Question

[jujubee]

If a video card was bridged to a x1 slot, will that impact GPU performance?
My understanding is that you wouldn't want to run a display off of it, but the calculations do not suffer the lower bandwidth.
Is that correct?

[savage309]

I haven't test it myself, but it can slowdown the rendering (not the calculations themselves), because we do a lot input/output operations.
The feedback will be worse for sure (the rendered images will come slower, so in the viewport there will have higher lag).
If you don't care much about it (if you need production and not preview), you may try increasing bundle size and rays per pixel in order to feed the GPU will data better and have less IO (probably something like 256/24).

[jujubee]

I'm wondering if the extra slots on a motherboard can be used for just RT calculations - not as a main display card.
Is there any way you can run a test or ask around?
I noticed that there are a lot of bitcoin miners and expansion boxes that essentially plug into the X1 slot and that they say it doesn't impact GPU calculations negatively.
Granted, it's not as fast as the X16 lanes but if you've exhausted all the existing slots, I'm wondering if this is a way to expand beyond a full motherboard already.

[tashko.zashev]

Hi,

Unfortunately we don't have a PCI to PCI-E Card Adapter right now to run few tests. We will see if we can do something about it.
But like my colleague mentioned this approach will be more suitable for production renderings, because the adapter allows approximately 2.5Gbps transfer bandwidth.

[tashko.zashev]

Hi again,

I made a test using one PCI to PCI-E Card Adapter and here are the results:

Build : V-Ray RT 3.00.05 /2014
Graphic card : GTX 560
Settings : 256/24 (ray bundle size /ray per pixel)

Render time PCI-E :



Render time PCI X1:



It seems to be ~8% slower.

[jujubee]

Interesting. Is it a PCI-E x1 to X16 Adapter?
What are your thoughts?
My question: is it better to use them than to not to if you have additional GPUs?
I'm trying to be cost-efficient yet being able to expand without having to buy lots of new hardware.

As of now, I'm able to use 3 cards. One X8 and two X1 slots gets covered by them.
I've seen X16 4 way splitters however they usually render them X4 or X1 lanes I believe if they're all used.
It would eliminate one X16 but enable four more cards on the X1s at a reasonable cost.

Bandwidth of X1 is 250 mb/s.
I've seen Fiber Optic expanders which have a transfer rate of 2.5 to 5 gb/s.
Thunderbolt is 10-20 GBps but the enclosures are currently expensive.
Cubix is reporting up to 128 GBps connectivity and from a marketing standpoint seem most reliable/advanced.

I hear the theoretical bandwidth of X16 for GPUs is close to 1 gb/s anyways, so anything over that might be overkill unless you're talking multiple cards.

Just to note: Cubix is reporting that you guys are using a Cubix Expander. Which system are you using and what are your thoughts?

[super gnu]

looking at that cubix expander website, the prices seem very high. from my brief look it seems you could easily build another workstation with 4x dual width pcie slots for the price of one of their expanders..

[jujubee]

Yeah. Well you're definitely paying for convenience.
There might be a speed advantage over a gigabit network.
Less maintenance.
Also portability in case you travel or juggle comps.
Some RT licensing might also have limitations so this is a potential way around it.
Some RT packages do not currently support network rendering as well.
Definitely is pricey.

Building a cheapo workstation has a lot of advantages.
In a new system:
1) SSd drive - $100
2) Case - $150
3) OS - $100
4) Processor: $250-$500
5) Beefy PSU: $300-500
6) 4 slot mobo: $500+
7) 32 gig RAM: $300

So roughly $2,000.

However I've been investigating less expensive solutions even if it means taking a small performance hit as long as there's significant overall gains. If my idea works out, I might be able to set up my own expansion kit for under $200-$300.
I'm trying to move away from adding computers and servers - at one point I had 15. I like the idea of taking up less space and having to run less maintenance.

[tashko.zashev]

Interesting. Is it a PCI-E x1 to X16 Adapter?
What are your thoughts?

In my opinion the speed is good, most probably using higher ray bundle size /ray per pixel settings will add more speed, but i have to test this.

My question: is it better to use them than to not to if you have additional GPUs?

The most effective approach is to install all GPU cards on the same machine (PCIE), all other variants will lead to some slow down.
When the GPU card used for rendering is on another machine, there will be some slow down as well, because the speed depends on other things (Network bandwidth for an example).

Just to note: Cubix is reporting that you guys are using a Cubix Expander. Which system are you using and what are your thoughts?

Actually I'm not aware of this, I have to contact my colleagues for more information about the Xpander.

[jujubee]

LOL. Says "Chaos Group" under Technology Partners unless they're full of BS:
http://www.cubix.com/sites/default/f...Oct%202012.pdf

[jujubee]

So what is the deal with them listing you as "Technology Partners" then?
It makes me skeptical to even consider purchasing their product or something similar.