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Only One of Two GPUs Being Used - Possible Fix With Owned Hardware?

Posted on 2014-12-26
Medium Priority
Last Modified: 2015-01-15
Hey Guys -

After purchasing a Shield Portable a while back (now have a Shield Tablet), I had to change the Graphics Card in my system from an AMD one to an Nvidia one.  I decided to purchase a GeForce GTX 660 2GB RAM. installed, and used it without issue.

About a month ago, I decided to upgrade by purchasing a 2nd GTX660 and running them in SLI via a hardware bridge.  I purchased another GTX 660 except this time got one with 3GB of RAM which wasn't an option before, but I thought would be ok since they were both still 660s.

After installation of the 2nd card plus connecting the two via hardware bridge; SLI still wasn't working.  After much troubleshooting, I found out that SLI only works if the cards are exactly the same - even if one just has more RAM than the other.  I was still told that even with SLI disabled, I'd still have better performance than just the single one.

My Issue
Fast forward to today.  I was considering overclocking my CPU a bit just to play so opened the app "Open Hardware Monitor" to record a baseline.  Later, I decided to play Watch Dogs for a few minutes so as Open Hardware Monitor was still open moved it to my 2nd monitor while playing the game on the primary.

Once into the game, I noticed that only one of my video cards was being used!  Below is a screenshot of what I saw.  As you can see, it seems only one is being used.  For exmaple, the top GPU's core is around 324Mhz (what it is for both GPUs when at the desktop) and the 2nd one's core is 1045Mhz.  The used RAM seems to be similar which is strange as they have different amounts total yet show the same used %.  
It actually isn't exactly during gameplay, but was taken as fast as I could minimize and take screenshot.  You should still get the idea, though...
Open Hardware Monitor Screenshot (During Gameplay)
Below is a screenshot of the same thing when I had the game paused and minimized:
Open Hardware Monitor Screenshot (Gameplay Paused & Minimized)
So - is this normal?  Does anyone know of a method / hack / modded driver when I could get both cards to work together at all?  I wondered if there was even a way to trick the 3GB card into thinking it was 2GB just so I could use SLI.  Didn't know if at all possible thought driver, firmware, or even Windows mod.

Ohh - also... I'm running Windows 8.1 x64, i7-3770k CPU, 16gb RAM, and OS is installed on SSD.

Any suggestions besides starting an RMA (if still possible)?  Thanks!
Question by:BzowK
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LVL 63

Assisted Solution

by:☠ MASQ ☠
☠ MASQ ☠ earned 668 total points
ID: 40519122
So this is really back to your abandoned question from November when I told you you can't get nVidia cards with different memory sizes connected using SLI even if they have the same core GPU?


Nothing has changed since - this still isn't going to work.

Accepted Solution

Network Zero earned 668 total points
ID: 40519216

It looks like your in a grey area man. MASQ is 100% on this Nvidia it's self does not  the support configuration of two of the same cards using different memory sizes.

Reference : https://forums.geforce.com/default/topic/760860/sli-doesnt-work-/?offset=5

** Official Nvidia Forum **

The reason behind it may have to do with some sort of dual channel configuration
using the memory size.

- It might be best to sell the 2GB card and buy another 3GB one then to  risk damaging the cards by pulling something off that Nvidia it's self wont even stand behind.

LVL 24

Assisted Solution

VB ITS earned 664 total points
ID: 40519219
As MASQ has stated, you can no longer SLI cards with different amounts of memory. It used to be possible using the Coolbits registry tweak a while back but Nvidia have since stopped supporting this method with their more recent drivers. If you try this method now, you will most likely end up with random crashes in games. It'd be a waste of memory anyway even if you did get it working as the Coolbits registry tweak will force both cards to use 2GB of memory.

Your best solution is to either purchase another identical card (I suggest going with another 3GB version for the additional memory) or to invest in a single better high-end card like a GTX 790. I'd personally stick with a single high-end card but that's just me (less heat, less SLI configuration issues, not all games use SLI, easier to overclock a single card, etc.).
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Author Comment

ID: 40519363
Thanks Guys -

OK, so i just searched on NewEgg for the cards I have and found out the following:
- My 2gb is a 2662 Model which they sell New for $209.99 and Refurb for $159.99
- My 3gb is a 2666 Model which they do not currently sell new anymore but sell Refurbs for at $174.99

So basically, a Refurb would set me back about $200 after shipping.  I don't know how much i could get if I sold the pair I have now individually on eBay or whatever, but probably $150 each on the high end.

Should I go the route of getting the refurb to have SLI 3gb 660s or just get a new single card as "VB ITS" suggested?  He suggested the 790 however i don't even see an Nvidia 790 for sell on NewEg at all and the 780 doesn't seem to have great reviews.

If going with a new card over the 3gb SLI (must be Nvidia due to my Shields), what model would you guys suggest that would be better than spending $200 for the 3gb SLI?  I don't mind going to something new, but will spend a touch bit more for a newer model so that if I decided to upgrade it using SLI, I can still find a matching card for sale.

Thanks Guys!

Author Comment

ID: 40519365
One more thing...

Jut looked on GPUBoss and compared a 660 & 970.  i choose the 970 as it was rated as one of the "Best Values" per points scored in benchmarks.

Thoughts?  Alternative suggestions?  How would the 660 SLI compare to the 970?

Thanks again!

Author Comment

ID: 40519366
Whoops  Thought it was $245 but it's $330.
LVL 24

Expert Comment

ID: 40519448
Sorry, forgot that the 700 series cards have been superseded by the 900 series.

Go with the GTX 970 if you can afford it. Performance wise the 970 should beat two 660's in most scenarios. There's also the fact that the 600 series cards are old technology and not all games support SLI anyway.
LVL 63

Expert Comment

by:☠ MASQ ☠
ID: 40519524
If you want a rough comparison on performance then
http://hwbot.org/compare/videocards **
But you'll need to add the GTX660 combinations manually
SLI'd you'll see them being comparable with a single GTX 770

If you still get the supplier to RMA the 3GB simply because you bought it in error then replacing it with a 2GB would be the best option for your wallet.

**YMMV !!

Author Comment

ID: 40524726
Thanks again guys - FINAL Question...

As mentioned before, I have a 2gb 660 (device ID 2662) and a 3gb 660 (device ID 2666)...

I looked on NewEgg and they still carry the 2gb 2662 but no longer carry the 3gb 2666.  However, they do carry the 2664 which is also 3gb.  

So - if I got the 2664 would it work alongside the 2666 since it's also 3gb or not since it's a different device ID?  Or... should I just be safe and go for the extra 2gb.  I don't know how much difference 1gb makes (or 2gb in SLI), but it's only $20 more than the 2gb.

LVL 24

Expert Comment

ID: 40524735
Nvidia's recommendation for SLI is to use the cards that are as closely matched as possible spec-wise so I'd stick with the 2GB 2662 model. Whilst SLI may work with the 3GB 2664 and your 3GB 2666, it's not guaranteed so I'd say it's better to be safe than sorry and stick with the 2GB models.

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