SLI vs CrossFire: which is better?

11th Aug 2010 | 09:30

SLI vs CrossFire: which is better?

What's better than one graphics card? Two, but CrossFire or SLI?

SLI vs CrossFire: Nvidia

For many a year even the thought of setting up a multi-GPU system has caused the most tech-savvy of PC enthusiasts to eat their own heads in complete and utter frustration.

It wasn't just the fact that the mere action of setting a CrossFire or SLI system up was a tiring process of exacting component matching, BIOS tweaking and software hell, promptly followed by mental breakdown.

The fact remained that if you were one of the few with that magical golden ticket, a functional multi-GPU system, you didn't actually get that much of a performance return for the extra silicon you'd painstakingly laid in your rig.

Originally the big sell was the idea of picking up a mid-range graphics card now, then once the thirst for upgrading became too strong to countenance you could pick up another of those cards, drop it in your rig and hey-presto a million times better performance.

Many an SLI or CrossFire motherboards got sold on the back of this idea, to people wanting the ephemeral security of perceived future-proofing. Obviously it wasn't that simple and never quite turned out to give the sort of returns you really desired or deserved.

And the fact was that by the time you came to upgrade to another of those mid-range cards they had become so passé that you would be just as well spending the same again on a superior single-GPU card. And then you wouldn't have to cope with the vagaries of multi-GPU driver support either…

Software-wise though things have been steadily improving. No longer do we have the headaches we once did trying to get our drivers to recognise the extra GPUs, and hot-fixes come thick and fast to solve the inevitable problems that accompany new games releases. Yes, DiRT2, we're looking straight at you.

The rise of the multi-GPU single card has helped this; specifically AMD's change in approach leaving the high-end of its graphics options to those monstrous, dual-wielding pixel pushers. So now the multi-GPU setup is all about the high-end performance systems not adding in extra boards somewhere down the line.

That still left the fact that, at best, you were looking at a 60 per cent increase for adding in a second card, with that increase diminishing as you moved to triple SLI or quad CrossFire. But with both Nvidia and AMD releasing its new generation of DX11 compatible graphics cards it's time to see if you're still better off buying the fastest card you can or whether two cards are finally better than one.

Nvidia GeForce GTS 250

Representing the last generation of graphics we've got Nvidia's lower-end cards offering a multi-GPU setup for less than £200 - namely the GTS 250.

In modern DirectX 10 gaming you're looking at an average percentage increase that tops out just under the 70 per cent mark when you factor in a second card. With the extra graphics memory and processing power it's not a bad boost for the cash.

Obviously, when you're looking at the tougher benchmarks of STALKER: Call of Pripyat and Metro 2033 the results tail off considerably given the GPU's a couple generations out of date now.

Still, if you've got a GTS 250 sitting in your rig and a 1,680 x 1,050 monitor strapped to it, for less than £100 you can be hitting over 30fps with everything turned on in Just Cause 2.

GTS 250 benchmarks

Nvidia GeForce GTX 470

The Heaven benchmark scales brilliantly with multiple GPUs, and the GTX 470 actually manages to post a 101 per cent increase.

The results in-game aren't quite so compelling though, but the key thing to note is the disparity between the increase in the higher resolutions compared to the lower. The average increase at 2,560 x 1,600 is a remarkably impressive 80 per cent, giving a comparatively huge boost to the lucky few out there with such large-screen gaming habits.

On the flip side if you're looking to max out the frame rates at the lower res of 1,680 x 1,050 you'll be disappointed with an average increase of only 32 per cent. Still, this shows where the emphasis for modern SLI is going – the big screens.

GTX 470 benchmarks

Nvidia GeForce GTX 480

As you'd expect from essentially the exact same architecture the performance increase from adding in a second GTX 480 is almost identical to that gained by the GTX 470.

That said though, looking at the raw figures proves that if you want the fastest dual-GPU setup going, then it's time to stump up the cash and drop two of Nvidia's most powerful Fermi cards in the same machine.

That is then a graphics setup totalling almost £1,000 but when you're picking up over 100fps in both DiRT2 and Far Cry 2 at 2,560 x 1,600 on the highest settings your eyes might tell you it's worth it.

High-res gaming is where the Fermi card's SLI emphasis lies, and with these figures it's quite hard to disagree with Nvidia on this one.

GTX 480 benchmarks

SLI vs CrossFire: ATI

ATI Radeon HD 5770

It's only the relatively poor performance increases in Just Cause 2 that stops the low-end cards of this CrossFire test posting a more impressive average increase with a second card.

The 80 per cent and 85 per cent in Far Cry 2 at 2,560 x 1,600 and 1,680 x 1,050 respectively boasts better scaling than both NVIDIA's Fermi cards.

At £260 for a pair of HD 5770s though you're getting significantly better performance than a single HD 5850, which is practically the same price. With the improvements, by AMD's driver team especially, in multi-GPU gaming you actually are better off with twin HD 5770s than a single HD 5850.

The most surprising thing though is the fact that twin HD 5770s actually beat a single GTX 470. Impressive stuff.

HD 5770 benchmarks

ATI Radeon HD 5850

If you're rocking an existing HD 5850 then the news is good. Dropping in a second card will give you some serious performance gains, especially at the high-end resolutions.

In terms of all round speed gains the HD 5850 is the best that AMD has to offer with an average increase of just over 70 per cent at 2,560 x 1,600. That's behind the impressive increases of around 80 per cent that you get with twin Nvidia cards though, but CrossFire HD 5850s still represent a conundrum.

They're almost on par with the HD 5970, but significantly faster almost across the board compared to the GTX 480, a card that's only £30 cheaper than two of these at best.

AMD's multi-GPU strategy seems to be paying off.

HD 5850 benchmarks

AMD Radeon HD 5870

These are the cards that have been propping up the fastest gaming PCs for over half a year. Put together two HD 5870's cost almost £100 more expensive than the HD 5970, but offer much more in the way of gaming performance.

That said though we're into the realms of diminishing returns as the relatively moderate gains you pick up over the HD 5970 hardly justify the extra outlay.

The interesting thing though is that with the latest Catalyst drivers improving AMD's gaming performance so much it has propelled the HD 5870 ahead of the GTX 470 in single and, despite lower percentage performance gains, in dual-card configurations.

AMD's focus on the multi-GPU to help prop up it's high-end components means it works across the board.

HD 5870 benchmarks

SLI vs CrossFire: which is better?

So SLI and CrossFire are really starting to make a name for themselves then. Ease of installation and the burgeoning returns you get for adding in the extra card now makes it a viable proposition.

As we can see though, it's Nvidia that has made the most gains in its multi-GPU technology in the performance sector. Both of its latest cards are hitting an average percentage frame rate increase of 80 per cent at the eye-bleeding resolution of 2,560 x 1,600.

It has conceded the lower end of the scale, garnering a pretty poor showing of 32 per cent at 1,680 x 1,050. But you're only going to be forking out for an extra £300 (or so) for a graphics card, if you're powering a monitor with a screen large enough to do it justice.

Despite putting much of its high-end emphasis on the value of its multi-GPU solution, the HD 5970, AMD hasn't quite got the same performance boost when it comes to adding in that extra card. Sure it's 5 series cards are better than the Fermis at the lower end, but only the HD 5850 manages to post percentage gains of over 70 per cent.

Where AMD can compete though is in the price/performance metrics. Since it released the 10.4 version of its Catalyst drivers the GTX 470 has taken a bit of a pounding. The HD 5770 in CrossFire is easily comparable in performance terms with the slower Fermi card, and a pair of them are also significantly cheaper. They likewise put the equivalently priced HD 5850 to shame and also manage to hit over 60 per cent average percentage gains too.

Final benchmarks

Click here to see the high-res version

Top performance though has to go to the insane numbers you get out of the GTX 480 when it's got a twin sat next to it. Still, that's nearly £1,000-worth of graphical componentry and unless you're powering three 30-inch panels they're hardly worth the cash.

But this whole multi-GPU thing is now relevant, rather than being simply a technical demo for GPU makers to show what their graphical gems are capable of, cash notwithstanding. And it's easy too.

At no point in the testing of these six different setups did I encounter any frustrating problems born of multi-GPU drivers. A simple tick-box in the relevant driver panels in Windows was all it took to get them playing nice.

On the AMD side too you're not looking at spending more cash just for the possibility of adding in a second card later on; most full size mobos these days are rocking multiple PCI-e lanes with CrossFire support.

On the Nvidia side things are a mite stickier; tracking down SLI compatible boards is a tougher ask and can be pricer too. But the future's bright and in the GTX 470's Heaven 2.0 benchmark we've actually seen the first time where adding a second card more than doubles performance.

Nvidia's dominance in the percentage increase stakes means that suddenly SLI is going to be a lot more interesting this summer when the mid-range Fermi cards tip up.

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First published in PC Format Issue 241

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