Problems with Twin Video Cards in Crossfire or SLI?

[Bino]

I've read with interest a few posts in here alluding to problems with dual video cards, either in ATI Crossfire or NVidia SLI.  Anyone care to elaborate, please?

My only hands-on experience with multiple video cards dates all the way back to the "Voodoo" series from the late, lamented 3dfx.  I had no trouble whatsoever with that rig, so I'm curious to learn what the issues might be with modern multi-card setups.

Thanks!   

[Chalenge]

If your OS is XP and you try to use two Geforce cards in SLI you will discover that vsync is forced to off (you cant turn it on). Vsync off in AH is very bad!

With Vista or Win7 either one SLI works fine BUT you will have to trim your processes or there will not be any benefit over using just a single card on an XP system.

PSUs can be easily overburdened by two cards. Ati cards dont seem to need the same power that Geforce cards do. You absolutely must check your PSUs output (in Amps) on the 12V side and make sure it delivers enough to handle both cards simultaneously. For instance a single 8800 GT requires 22 Amps but in SLI you will need 24 Amps. If your PSU only puts out 22Amps then SLI may work initially but eventually your PSU and possibly your graphics cards and anything else being powered could have a real problem (as in dead right there - DRT).

[Bino]

Hi Chalenge,

I ask about this as part of planning a new system.  I intend to run it under Win 7 Pro 64-bit, so the vsync-off problem will not be an issue.  The PSU I have in mind is a single-rail unit that can handle up to 71 (seventy-one) amps on the 12 volt output, so no worries there. 

Thanks! 

[Spikes]

You can go both ways on this...get two cards like 4890's or something, or get a single 5770 or 5870 (just using Radeon as example). I'm sure there are benefits with SLi/Xfire but for what I built my comp for, didn't seem it was necessary...I guess you have to ask yourself why do you need SLI/Xfire? What boost will it give you that a single 'better' card won't?

[Chalenge]

Its not so beneficial in games like AH (maybe 30% fps) but if your fps is dropping from 60fps down to 45fps only near big furballs then adding a single card in SLI will prevent the drop and possibly without spending a lot of money (depending on what card you have).

In games like Lost Planet which has a specific multi-GPU profile the benefit is much greater and probably because it is using the PhysX capabilities much more in SLI but I couldnt be sure.

I am hoping when the new GF100s are finally available that SLI will have a much more dramatic effect because I am really looking forward to using 3D on the newer 120Hz LCD monitors with FSX (to name just one).

[Spikes]

Rgr,
I run a single 5770...was *thinking* about getting a second, but it's worthless since I only play games such as MW2, AH, FS2004. I've got FSX but don't have it installed. I never drop below 59 fps in any of said games.

[Denholm]

Spikes, do you mind if I permanently borrow your computer and AH subscription?

[Spikes]

Spikes, do you mind if I permanently borrow your computer and AH subscription?

Sure, anytime. But I charge a fee of 100 bucks a month to have my PC.

[Denholm]

You're worse than Blockbuster.

[Bino]

You can go both ways on this...get two cards like 4890's or something, or get a single 5770 or 5870 (just using Radeon as example). I'm sure there are benefits with SLi/Xfire but for what I built my comp for, didn't seem it was necessary...I guess you have to ask yourself why do you need SLI/Xfire? What boost will it give you that a single 'better' card won't?

Well, from the reviews I read, this is how selected crossfire pairs and single AMD cards rank, fastest to slowest, from top to bottom

• 2 x 4890 ($400)
• 2 x 5770 ($330)
• 1 x 5870 ($400)
• 1 x 5850 ($300)

I don't think I can bring myself to pay $600 to $800 for a pair of 5850s or 5870s, or for a single 5970.  

[BaldEagl]

I had two Evga 8800 GTS 512's in SLI for a few days.  As someone said, I couldn't enable vsync (XP Pro) and, while I was getting about 280 fps in the game, the screen tearing was so annoying I pulled the second card and RMA'd it.  Now I'm back to my single 8800 GTS running a steady 59 fps (monitor refresh rate) with everything turned up except shadows and soft shadows.  I can hold my fps with shadows on but not with soft shadows.  I can't stand the look without soft shadows so I just turn both off.

If it were me I'd buy the best single card you can get within your budget and forget about Crossfire/SLI.  The best graphics card benchmark charts are at www.tomshardware.com.  They chart both single card and Crossfire/SLI configurations.

[zack1234]

V synch off is bad? What are the effects of V synch being disabled.

[BaldEagl]

V synch off is bad? What are the effects of V synch being disabled.

It's bad on an LCD monitor where frame rates need to be aligned with monitor refresh rates.  On a CRT monitor it doesn't matter.

When frame rates exceed refresh rates on an LCD "screen tearing" can occur.  On mine, when I snapped views around me large blocks of the picture would be offset by several pixels.  Imagine if you took the top 50 pixels of the screen and shifted them all 10 pixels to the left leaving the rest of the screen in place.  Except this happened over and over as you looked down the screen.  While it only lasted less than a second it really got quite annoying.

[Skuzzy]

It does not matter what kind of monitor it is.  The "tearing" effect is due to the video card over-writting the frame buffer while the frame buffer is being sent to the monitor.

Just FYI.

[zack1234]

rgr on V synch.

I have a 4850 with everything turned on stays around 75 frame rate and 59 on burning fields etc.

I was thinking about getting a second card but from what I have read i don't think i will

by the way my Track IR5 is the best thing to have for AH