Setting up 7970 crossfire.....

[Briggsy]

Hi All,

I'm due to get my second Sapphire 7970 OC at the end of the month and where I haven't ran a rig with crossfire or SLi before I was wondering if there is anything I need to be aware of?

Is it just as simple as connecting it to the other PCIe slot, attaching the crossfire bridge, plugging the power to it and away you go?

Thanks

 

[Coxey]

So long as you have a sufficient power supply, enough cables & enough pci-e slots / space, your pretty much good to go, from the top of my head anyway.

 

[Master&Puppet]

Yea it's just plug in and go. Make sure you download the amd catalyst application profiles - these have really helped in the past with cross fire performance.

Only other issue you might get is the PCI-e bandwidth. With 2x 2.0 x8 in crossfire you will be limited to 4gb/s to feed each card which can limit performance in some cases. See here z77 with IB would solve that if you find you are too affected.

Your PSU will probably be fine but if you get strange issues then it will probably be a lack of power so you might need to look into an 850W.

M&P

 

[Briggsy]

Yea it's just plug in and go. Make sure you download the amd catalyst application profiles - these have really helped in the past with cross fire performance.

Only other issue you might get is the PCI-e bandwidth. With 2x 2.0 x8 in crossfire you will be limited to 4gb/s to feed each card which can limit performance in some cases. See here z77 with IB would solve that if you find you are too affected.

Your PSU will probably be fine but if you get strange issues then it will probably be a lack of power so you might need to look into an 850W.

M&P

Thanks - I'm getting an AX1200 as well so power will be fine.

So your saying my board may struggle with 2 7970 in crossfire due to bandwidth?

 

[Master&Puppet]

In certain games, yes! Did you take a look at the Anandtech link? It has pretty pictures in there which explain it nicely. Effectively you will be going from using the green bars in the graphs to the purple bars (because pcie2 is half the bandwidth so you need to look at half the lane value, x16 pcie2 = x8 pcie3 and so on)

Ironically I've just written a post about the exact same subject here so I'll just quote it if no one minds:

You've got it right, here's the maths.

Cpu performance wise an i5 2500k would be fine with 2x 680s however SB does not support PCI-e 3.0 and if you need PCI-e 3 you will need an IB or 2011 cpu to support it. This is important to a certain extent. Let me explain PCI-e bandwidth:

The greatest bandwidth is given by PCI-e 3.0 x16 which delievers 16gb/s

After this the relationships are linear so:

PCI-e 3.0 x8 and PCI-e 2.0 x16 = 8gb/s

PCI-e 3.0 x4 and PCI-e 2.0 x8 = 4gb/s

and so on.

The latest generation of cards are not able to saturate an 8 gb/s lane but will fill a 4 gb/s lane resulting in bottlenecking. (I'm unsure about the 7990 and 690 since they are not out yet)

If you take a look at the wiki for the 1155 and 2011 sockets you will see that they have the following lanes available:

x68 = PCI-e 2 x16 (total = 8 gb/s)

z77 = PCI-e 3 x16 (total = 16 gb/s)

x79 = PCI-e 3 x40 (total = 40 gb/s)

This means that if anyone is running a single 680/79xx card then a standard z68 mobo will be fine. However if you sli/xf on a x68 then the bandwidth would drop to 4 gb/s per lane which is potentially limiting the gpus depending on the game.

A z77 board with a IB processor supports PCI-e 3.0 so it's basically two z68s so you can run 2 x8 lanes each feeding 8 gb/s to 2x gpus.

An x79 is therefore only necessary if you are running more than 2 gpus and it will theorectically handle 5 gpus in sli/xf providing plenty of 8gb/s slots.

So there you have it, if you are running the latest gen gpus:

x68/i5 SB for single gpu.

z77/i5 IB for dual gpus.

x79/lga2011 for more than 2 gpus.

More info here

As for quad channel memory - well that allows you to put more RAM on the mobo, simple as that. Gamers only need 8gb though which all these support. RAM speed is irrelevent to gaming as well.

M&P

 

[Briggsy]

I'm playing wow at 2560x1440 res so I'm adding a second GPU to help improve on fps during raiding.....

Hopefully the bandwidth reduction won't hamper performance.....?

 

[Master&Puppet]

Yea at that high a resolution you are more likely to max out the gpus before the bandwidth I would think and in any case I would expect the networking to have a greater effect than both of those.

 

[Briggsy]

Yea at that high a resolution you are more likely to max out the gpus before the bandwidth I would think and in any case I would expect the networking to have a greater effect than both of those.

So you think I will be alright?

Thanks

 

[Master&Puppet]

http://www.techpower...ossFire/19.html

Yea your FPS will sky rocket in situations where you aren't affected by other users.

If Rastalovich were to respond to this he would say something like: (but much more eloquently)

In places like ironforge/orgrimmar or 25 man raids your fps still has to respond to the data of other users coming down the network because every user has to be in sync. Therefore fps can be network dependent.

 

[Briggsy]

Well my fps when aren't affected by others is good (60fps, with v-synce enabled) but when I'm raiding it is 20-25 at lowest.

With my current board etc will adding a second gpu actually improve that?

Thanks

 

[Master&Puppet]

Yea I noticed that in the graph, I think a 7970 should be able to handle wow above 30 fps in demanding situations.

With your motherboard you might not see as much scaling with 2 gpus as you would with a pcie 3 board - I can't tell you how much you would loose in wow specifically but I think that is avoiding the actual problem anyway.

Your fps is probably limited by the nature of online gaming and in large 25 man raids I don't believe it is your pc that is causing the fps droop but other factors.

I believe that the fps is limited by the "weakest link in the chain". I'm not an expert on this though since I'm not a mmo player anymore.

There are probably a few people who have poorer internet connections

Or are running on too higher settings for their pc.

Or even the server is running slow.

Your pc has to wait for the input from the server before it can generate the frames so everyone is in sync, so if someone, or something is slowing that then you fps will drop. In single player your pc doesn't have to wait since it gets what it needs in real time from the HDD.

Before you shell out for a second card you might want to try something in your next raid - set the games graphical settings to lowest. If your fps doesn't improve much (or is very spikey) then you can be sure it's the online environment.

 

[Briggsy]

OK thanks

 

[Briggsy]

With your motherboard you might not see as much scaling with 2 gpus as you would with a pcie 3 board

My current board is PCIe 3 but this is only active with an IB cpu?

 

[Master&Puppet]

Yea, the PCI-e controller is a function of the CPU.

1155 SB only has a pcie2 controller so any gen 3.0 slots will only run at gen 2.0 if you put a 2500k in it.

1155 IB has a gen 3 controller so the lanes will fully be used.

This only matters if you xf/sli with the latest gen cards. pcie2 x16 will handle any card currently released. pcie2 x8 will bottleneck in certain games.

Strangely enough SB-e supports pcie3

z77 will support 2 cards since pcie3 x8 is the same bandwidth as pcie2 x16. Any more than two cards and you need x79.

http://www.anandtech...ocking-and-msaa

 

[Master&Puppet]

Sorry for the double post but i should add that in the worst case scenario you would see only 60% scaling when the latest gen cards seem to average about 90%. That equates to a 15% loss in fps compared to the unhindered setup. However with the fps that a latest gen dual gpu setup produces you probably wouldn't be able tell the difference anyway.

It depends on which game you play because each scales differently and is affected by pcie bandwidth differently.

For example:

BF3 is a very demanding game. Say your gpu can run it at 45fps. With a 90% xf scaling you would see 85 fps. However BF3 takes a minor 3% hit on 4gb/s lanes so you would see 83fps which works out at 84% scaling.

Shogun 2 is a less demanding game but suffers a 10% hit with a 4gb/s lane. So a single card might bench at 60 fps, again scaling by 90% in xf giving 114fps but the pci issue reduces that to 103 fps. So you would see an effective scale of 71%.

In summary - the fps gain of xf/sli easily outweighs the pci handicap. It's just useful to be aware of where the system limitation is when you think about upgrading. I would keep your z68 but replace it with haswell.

 

[Briggsy]

Sorry for the double post but i should add that in the worst case scenario you would see only 60% scaling when the latest gen cards seem to average about 90%. That equates to a 15% loss in fps compared to the unhindered setup. However with the fps that a latest gen dual gpu setup produces you probably wouldn't be able tell the difference anyway.

It depends on which game you play because each scales differently and is affected by pcie bandwidth differently.

For example:

BF3 is a very demanding game. Say your gpu can run it at 45fps. With a 90% xf scaling you would see 85 fps. However BF3 takes a minor 3% hit on 4gb/s lanes so you would see 83fps which works out at 84% scaling.

Shogun 2 is a less demanding game but suffers a 10% hit with a 4gb/s lane. So a single card might bench at 60 fps, again scaling by 90% in xf giving 114fps but the pci issue reduces that to 103 fps. So you would see an effective scale of 71%.

In summary - the fps gain of xf/sli easily outweighs the pci handicap. It's just useful to be aware of where the system limitation is when you think about upgrading. I would keep your z68 but replace it with haswell.

Thanks dude, great stuff.....

 

[Edgeyforsure]

alot of MMO's are CPU dependant, i would try turning down shadows (a MAJOR cpu hog in wow) not saying your CPU is bad, quite the opposite.

 

[Briggsy]

alot of MMO's are CPU dependant, i would try turning down shadows (a MAJOR cpu hog in wow) not saying your CPU is bad, quite the opposite.

Yep tried that....thanks anyway.

Let me just give a brief history regarding my issue.

I brought a new monitor Samsung S27850D, beautiful monitor which runs at 2560x1440, originally I was using a Gigabyte GTX 580 soc to run it but it was obvious I needed more v-ram due to the higher res, so I got myself an 7970 which did improve things.

But there are still issues with frame drops in certain areas of the game world and when raiding, so I was advice to get a second 7970 as this would give it the horse power to improve frame rates.

 

[Rastalovich]

alot of MMO's are CPU dependant, i would try turning down shadows (a MAJOR cpu hog in wow) not saying your CPU is bad, quite the opposite.

hmmm what's the cpu doing then.... why's it doing shadows.. I'd use a gpu to do shadows..