PCI-e 3.0 Graphic Card on PCI-e 2.0 motherboard driving five 27" monitors at 1920 x 1080 using Eyefinity

Question

I'm planning to upgrade my current pc of 4 years for gaming purpose. I've done quite a bit of research but couldn't find what i was looking for or maybe i'm just too newbie to put everything together to form a bigger picture. Therefore I'm hoping you guys could help with my questions.

Currently I have a PCI-e 2.0 motherboard (Asus M3A32-MVP Deluxe - http://www.asus.com/Motherboards/AMD_AM2Plus/M3A32MVP_DeluxeWiFiAP/) with 8GB of DDR2 RAM and more than enough power (800w psu). I'm contemplating of purchasing a HD7970 graphic card which is a PCI-e 3.0 card to put into my motherboard and will drive five 27" monitors at 1920 x 1080 resolution using Eyefinity (tricking the OS into thinking that I have ONE very big screen instead of 5). Will such system work without the performance suffer.... much?

I understand that PCI-e 2.0 x16 is capable of 8Gbps bandwidth and PCI-e 3.0 x16 has up to 16Gbps bandwidth. A simple arithmetic calculation gives me the number of bits required to store information of ONE monitor at 1920 x 1080 resolution in 24 bits per pixel (bpp) colour depth of ONE frame.

1920 * 1080 = 2,073,600 pixels
2,073,600 * 24bpp = 49,766,400 bits

So for the number of bits for 5 monitors

49,766,400 * 5 = 248,832,000 bits

If I want a smooth 60 frames per second (FPS) it'll be

248,832,000 * 60 = 14,929,920,000 bits/sec

So the data transfer rate is a whopping 15Gbps and only PCI-e 3.0 can support it nicely.

FIRST QUESTION

What happen if I still plug that card into a PCI-e 2.0 slot? Will the system tune the FPS down automatically (which means my FPS will suffer and drop to about 32 FPS) OR the entire system will fail ultimately (or slowed to a crawl which isn't worth investing in)?

SECOND QUESTION

Can I get two identical graphic cards to distribute the burden (3 monitors on the first card and 2 on the second card)? According to my mobo specification, dual card mode can still achieve x16 lanes each which means the total bandwidth can be increased up to 16Gbps. Will Eyefinity still work (5 monitors combined together to make ONE very large display)? Do I need CrossFire (which i prefer not to due to microstuttering)?, which in this case the configuration will be all 5 monitors on the primary card.

I really hope someone is able to shed some light on my dilemma. Can I just buy a high end card and get a maximum gaming experience, or should I save up and change my entire gaming system? Thanks in advance =)

Answer 1

I don't know if you've found an answer to this so I'll give it a shot.

First off, we gotta make sure we know the difference between Gbps and GB/s.

Gbps = Gigabits per second
GB/s = Gigabytes per second

PCIe 2.0 is capable of 5GT/s (Gigatransfers per second) and PCIe 3.0 is capable of 8GT/s. PCIe 2.0 using 8b/10b encoding while PCIe 3.0 uses 128b/130b encoding. This is the transaction overhead that eats away at that raw data rate. GT/s essentially equals Gbps.

So to fix up your assumptions. A PCIe 2.0 x16 link is capable of:

5GT/s per lane * 16 lanes = 80GT/s
80GT/s * 8b/10b encoding = 64Gbps
64Gbps / 8 bits per byte = 8GB/s

Just want to make sure we know the difference on that information so you understand my answers. Also, I believe there is now an 8bit transparency region for pixels so we have 32bits per pixel these days. That changes your calculations.

2,073,600 * 32bpp = 66,355,200

So 5 monitors

66,355,200 * 5 = 331,776,000 bits

At 60fps

331,776,000 * 60 = 19,906,560,000 bits/sec

So we really have 20Gbps of data that needs to be transferred.

NOTE: I did some research after doing the calculations and found that we have 30bit and up to 48bits these days so you can need to transfer up to 30Gbps of data.

FIRST ANSWER

PCIe slots train (connect/link up) to the lowest supported standard. So a PCIe 3.0 card in a PCIe 2.0 slot will train to a PCIe 2.0 link. At x16 that allows for 8GB/s or 64Gbps in each direction. You should theoretically have the PCIe bandwidth to accomplish 5 monitors. I think the graphics card will be the problem there more than the protocol bandwidth.

SECOND ANSWER

From what I've found, all monitors must be connected to the primary card to have eyefinity work. If you use a second card, it can only be set up in crossfire to handle some of the GPU load. The real issue you are going to run into is the GPU overall. You might be able to get some better performance through the use of crossfire but I seriously doubt you can get 5 monitors to run smoothly on a single graphics card using eyefinity. I've never set up this configuration so I recommend asking on an enthusiast forum like this: http://www.widescreengamingforum.com/wiki/Main_Page