SLI / CrossFire FAQs

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I can't remember where I saw it but I heard AMD is having driver problems with Crossfire recently, so that if I add another 7850 it won't add the performance that it should. This will be my first time running Crossfire/SLI and don't know if I should just sell my card and get a better one instead.
 


Can say from experience that HD 7850s scale very nicely when you add a second card, what mobo do you have? if its PCI-Ex 2.0 then you want at least 8x 8x to get decent scaling, if you have 2 PCI Ex 3.0 slots then 8x 4x is fine assuming you have a decent CPU and a decent 600w+ PSU with 50A or higher on the 12v rail. If so then your golden and should be looking at close to double the performance you are getting now in games that utilize crossfire and be comparable to a GTX 770 performance wise. Not sure about the latest ATI drivers as I'm using Nvidia cards currently but even the ATI drivers from early last year were great with crossfire HD7850s.
 


This is my exact build: http://pcpartpicker.com/user/CMac019/saved/20Xe
I'm pretty sure I'll have to upgrade my PSU though
 


Yeah your PSU is border line but you might get away with it if you leave everything at stock , by the looks of your component selection your already aware of the OCing potential wasted unless you upgrade the PSU. ATI are currently trying to sort out frame pacing issues, no doubt these issues have been there all along however I only noticed micro stutter in 1 game at the time, Skyrim. If you like to tinker and problem solve then adding a second 7850 and upgrading your PSU will likely prove rewarding. I would be weighing up "the cost of a new PSU and another 7850" vs "selling your 7850 and buying a 7970 or gtx770" vs "SLI gtx 760+new PSU". On a side note the biggest/easiest upgrade for your overall PC performance is adding an SSD...

 


Ah my bad I did not see you had an SSD already. Have you read about the catalyst 13.8 betas? How's that for good timing!
If I were you the first thing Id do is upgrade the PSU to a decent 800W or better, some might say that's overkill but I call it peace of mind..
 


On pc partpicker it shows estimated wattage as 316W according to link posted by him,so how come a 550W psu not enough?sorry if i am wrong,but would like to know that.
 


Well, adding another 7850 might push it a little bit, but I think 550w should sill be enough as long as no hefty OC occurs.
 
welll im not to sure idk you already have one GTX 650 ti boost then go for 2 way sli but if you dont i would go for a single 760

 


dude my psu is coolermaster thunder 500w and its having 1x6 pin but gtx 760 require 2of them should I take the risk and power up using molex?

 


as far as i know, molex adapters won't harm your hardware. just make sure it's dual molex to 1 pci-e connector. if you have 2 6 pcie connector, then 2 molexes per pcie connector.
single molex to pcie may not be sufficient for the power draw.
 
well ya molex wont harm your hard warere like the other guy said but i would suggest getting a little bit bigger power supply if you are going for the 760 but they sound like really nice cards

 




ya i will buy a new psu but not now after 2months can it work for 2months?
 
um.. idk aslong as you dont overclock anything it mioght be fine

 
Ugh, can't read 20 pages, especially with so "incredible" quoting system. And it's sad that with all the useful information, a very important... the most important for me actually question is not answered(or I couldn't figure it on the first read).
I read:
"Brands don't matter.It's however recommended that the cards have the same amount of memory and speed."
Does that mean that any 78x or 79x AMD family, for instance, can be Crossed, as long as they have relatively similar Mhz models and same 2gb memory?

My idea until now(which means literally having "no idea" about the subject), was that SLI\XF are virtually a way to recycle your GPUs. Like adding more RAM or another HDD. I've already read that some Nvidias can be SLI-ed to work in this pattern: weak GPU does PhysX, strong GPU does the rest. I think it was somewhere on THW.

After reading this article, I am further confused. Does it indeed mean that only "rather similar" models can be SLI-ed?
Does that mean that if I buy some high-mid scale card, like HD7870, which should work it's whole price for at least 2 years, when it finally gets a bit behind and forces me to choose between ultra and 40+ FPS, if I want to add a new GPU, most likely from new gen family, either same high-mid or high-end card - it will NOT Crossfire with HD7870, because of rather apparent difference in Mhz\Memory?

Does this really mean that SLI\XF is merely just an option of spending your money? Like getting 2-3channel for multi-screen films\games, which probably would work better, like multi-core processors for multitask?(well, just a guess)

So basically, does this technology not allow to "upgrade" GPUs, but instead merely pair the same species in the Ark, per say?
 


Any can be crossfired, even with different Mhz and memory, but you'll still be limited by the card with the least memory and frame time variance may be higher if the Mhz is much different. However, you can use the CCC Overdrive software to clock both cards to the same speeds.

SLI is a little more picky, and will require the use of software that will update the registry so the card with the extra memory will not have access to the extra memory (Coolbits is the software).


That is one way you can look at it, but it is often a way to upgrade your system for more performance or even start from scratch with more performance than you can get with a single card. You could even use it as a way to save money.

Often times the more powerful single cards are much more expensive than the performance they gain, so you can buy two weaker cards for less and get more FPS than the faster more expensive single card, but you do have to live with the disadvantages of SLI/CF if you do.



If the cards are the same model, like the HD7870, it will Crossfire with cards of the same name. But becareful about cards like the HD7870 XT, The XT is based on the 79xx models, so I am not sure it will work. The clocks can be altered through software and the cooling methods do not matter.



Yes, it is an option to upgrade, it is an option to make games work better, and it is an way to spend more money.

 
So if I "add" a (theoretically) 1200 mhz\3gb card to 900mhz\2gb card, and the result will be 900mhz\2gb... Then how exactly is the performance improved? I've read\seen charts which show ~70% improvement, which would justify "recycling" old card, if it adds these 70% to the new card. How can it improve a card, when it diminishes the stats? Is it too technical? Like the "architecture of Intel" somehow beating raw numbers of ghz\cache\cores of AMD(for CPUs)?(Just an example of what I do not understand).

So either way, is getting 50%+ improvement to NEW card possible, if it gets stats of the old?
And since I am making a hard choice between two cards, GTX660OC and HD7870OC, being relatively same in price\performance comparison, where AMD seems to be "easier" to crossfire, while Nvidia has all the 3D\PhysX stuff.
Which card can be safe to buy now(since both seem to be the top of price\efficiency rank) and can be more possibly upgraded in theoretical future, with newer families of GPU?
 


I do not know of any card that comes with options of 2Gb and 3Gb. Usually they only offer two versions, one with double vram and I do not know of any cards of the same model that are that drastically different in clocks either.

You still have to get the same model number, as in HD 7870 (XT if the other has XT in the name).

However, if you got two 7870's, one at 900Mhz, and the other at 1000Mhz, you will want to use MSI afterburner or CCC Overdrive and change the clocks of the slower or faster card to match the other (or meet in the middle).

The increase in speed is due to each card rendering every other frame. Two cards can make more frames than 1 card, even if that card is 10% faster than the slower card, but together, they may be 70% faster.
 
Oh, now I get it. So it's just raw "2 heads is better than 1". But then... "Any can be crossfired", followed by "You still have to get the same model number, as in HD 7870", which I remember from article.
So that does indeed mean that even in the following years, if I want to upgrade my performance, my only chance is to buy ~exactly the same card, which would hopefully fall down in price gradually... and expect that it will together outdo something that would be fresh-new and cost much more?

Like, 200$ current card. 3 years later it costs 150$. Put them together, and it will work at least like 300$ card for at least 1-2 more years? Can this be achieved?
 


Yes, it can be achieved, but VRAM limitations can sometimes get in the way, as in the future, more VRAM may be needed to run games well. Effective VRAM does not increase by Crossfire or SLI. You still are limited by what one card has. So there can come a time when Crossfire/SLI may not be effective.
 
After reading some lines from the article and your reply "Yes, it can be achieved, but VRAM limitations can sometimes get in the way, as in the future, more VRAM may be needed to run games well.", I've been thinking quite a lot about what does it mean.
First I took it as: "If you SLI 2gb and 4gb card, 4gb card will become 2gb, and total memory will be 2gb nontheless".
After reading theads like http://www.tomshardware.co.uk/answers/id-1857498/sli-760s-4gb-utilize-4gb-vram.html \ http://www.tomshardware.co.uk/answers/id-1857056/gtx-770-4gb-2gb-vram-sli.html ,
I may think that you meant that SLI 2gb+4gb will not be 6 gb, but 4(2+(4\2))gb. I do feel like 2gb is not really future-proof(those theads say that even modern mods and graphics sometimes favor 2gb+), but I thought that if I can't really push 4gb card now, should I go for decent 2gb card to SLI it later?

Since I would also like to stick with Nvidia, GTX760 for a couple of years, because it alone supports PhysX(althought I've read that later both greens and reds will share some similar technology to relieve customers from this torturous decision), and because benches at THW show GTX being superior to it's counterpart HD7950 in everything, but bitcoin, obviously.

I remember from the article that AMD can XF 2+4gb cards fine, just 4gb will be scaled down, and that Nvidia on the other hand - can't, without specific soft or something.

So the question is: will SLI of GTX760 2gb + GTX760 2gb(possibly different manufacturers, but otherwise same models) make it total of 4gb, or still 2?
 
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