• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

P55 and Crossfire

yottabit

yottabit

Golden Member
Hi all! I have a (relatively quick) question for everyone regarding crossfire on P55 motherboards. I know that the 1156 CPU's only have 16 lanes of PCI express 2.0 to go around. I've heard that good motherboards split each slot to 8x, and crappy ones only give 4x to the second slot, or something like that. Well I have a pretty low end mobo:

My motherboard is this one (pretty low end):
http://www.msi.com/index.php?func=proddesc&maincat_no=1&cat2_no=&cat3_no=&prod_no=1936

Slots
• 1 PCI Express gen2 x16 slot (PCI_E2)
• 1 PCI Express gen1 x16 slot supports x4 speed (PCI_E5)
- supports ATI® Crossfire™
• 3 PCI Express gen2 x1 slots
• 2 PCI slots, support 3.3V/ 5V PCI bus Interfac

I'm very confused by the specifications. It lists PCIe 2.0 x16 for the first slot and PCIe 1.0 x16 for the second slot, but supports x4 speed. What exactly does this mean? Does this mean it is a PCIe 1.0 x16 slot that is capable of running at x4 speed but would run at x16 in crossfire? Or does it meant that it's a physical x16 slot but can only run at a max of x4 speed?

Sorry, I feel a bit mislead by these specifications. I'm just trying to figure out if its worth it to do crossfire with this motherboard or if I'll get terrible scaling and am better off buying a better single-gpu. I'm running a 5770 right now so the crossfire is very tempting 🙂

Thanks guys!
 
Some useful links:
http://www.hardware-revolution.com/p55-motherboards-crossfire-sli-performance-problem/
http://forums.hardwarezone.com.sg/showthread.php?t=2516025

From the Service Manual
1 PCI Express 2.0 x16 slot (PCI_E2)
1 PCI Express 2.0 x16 slot supports up to PCI Express gen1 x4 speed (PCI_E5)

So it seems my motherboard is one of the ones that doesn't support an 8x/8x electrical split but rather 16x PCIe 2.0/4x PCIe 1.0. That means the second card is operating at about 1/8th bandwith. I don't understand how they can even call it a x16 slot if it can only run at x4 speed (and PCIe 1.0 at that). I guess because the physical connector is x16, what a joke.

Moral of the story: Don't get a cheap P55 board if you want crossfire or SLI 😀

Despite this, the second link I posted seems to show good crossfire scaling with low end cards. But I don't think I'm really willing to take that gamble, I'll just wait until the next generation of ATI cards roll around 🙂
 
You should find a board that supports at least 8x/8x. There are some boards that use nV's N200 chip and support 16x/16x, but they cost a premium.
 
I already have the motherboard as I got it as a package deal. It wouldn't make sense for me to buy a new motherboard AND a 5770 for the sake of running 5770's CF. If I had a 5850 or something I might do that but it seems like it would just make more sense for me to wait and upgrade to a 5850 or 5870 since the 1156 socket will be obsolete anyways.
 
Shmee said:
dont worry about it. This review shows PCIE bandwidth testing, and on a 5870. The performance loss is pretty much negligible at x4 and x8. You should be fine, even if you didnt use a 5770 and used the 5850 crossfire instead.

http://www.techpowerup.com/reviews/AMD/HD_5870_PCI-Express_Scaling/25.html
Click to expand...

I'm not so sure. You have to bear in mind they tested with PCIe 2.0 scaling.

A PCIe 2.0 1x slot has a bandwith of :1x500= 500 MB/s
4x 4x500= 2000 MB/s

However the P55 motherboards (like mine) that don't feature the 8x/8x PCIe 2.0 split use a 4x PCIe 1.0 slot which has a bandwith of
4x= 4x250=1000 MB/s

So that would actually put it square between the 1x and 4x results of those benchmarks. Although the 1x results aren't completely crippling, imagine if EACH card were slowed down by 20% (since one card can't process frames faster than the other, although I'm not sure if that's how it would really work). Then you would have a roughly 40% reduction in the effectiveness of the crossfire rig.

Maybe it wouldn't be a problem with 5770s, but I wouldn't be willing to try with it unless I saw some benchmarks on PCIe 1.0 4x... who knows if it's enough or not without benching it? I've read of some users being disappointed with crossfire on these motherboards
 
Last edited:
The bigger issue is that x4 slot is not connected to the CPU. It's really a no-no for CrossFire.
 
lopri said:
The bigger issue is that x4 slot is not connected to the CPU. It's really a no-no for CrossFire.
Click to expand...

I wasn't so sure about this as I've heard claims about terrible microstutter and such using these. The x4 slot is driven off the P55 which is connected to the 1156 with a 2 GB/s DMI link right?

I mean, X58 setups do great without having their CPU's connected directly to the GPU but they have a QPI interface with their chipset. Does DMI have much more latency than QPI or is it just the bandwidth that is the issue?
 
You must log in or register to reply here.

TRENDING THREADS

Back
Top