Multi-GPU mode explained

Multi-GPU Crossfire gaming explained

Both Nvidia's SLI and AMD's Crossfire allow you to combine/add a second, third or sometimes even a fourth similar generation graphics card (or add in more GPUs) to the one you already have in your PC. This way, you effectively try to double, triple or even quadruple your raw rendering gaming performance (in theory). The reality is this though; the more GPUs, the worse the scaling becomes and the more driver and CPU bound issues you will run into. Honestly, two GPUs in most scenarios is ideal in terms of multi-GPU gaming performance, always remember that.

You could for example place two or more AMD graphics cards into a Crossfire compatible motherboard, or two or more Nvidia GeForce graphics cards in SLI mode on a compatible motherboard. In today's article we'll use two way SLI GeForce GTX 780 graphics cards and thus two AMD Radeon R9 290X cards.

• A Crossfire compatible motherboard is pretty much ANY motherboard with multiple PCIe x16 slots (Gen 2.0 or higher) that is not an nForce motherboard.
• An SLI certified motherboard is an nForce motherboard with more than two PCIe x16 slots or a certified P55, P67, Z68, X58, Z77,Z87 or a X79 motherboard. Please check with the motherboard manufacturer whether or not it is SLI compatible. Keep that in mind, but most of the latest generations AMD and Intel based motherboards are compatible. A small note, if you are gaming on an AMD processor then on AMD's side the 900 series chipset supports SLI as well.

Once we seat the similar graphics cards on the carefully selected motherboard, we just bridge them together with in Nvidia's case, an SLI connector. This is no longer needed for AMD Radeon with the enw cards though. We'll explain in the next chapter. Then we install/update the drivers, after which most games can take advantage of the extra horsepower we just added into the system.
 

Once you install/update the drivers, the Catalyst control Center will detect the second GPU, after which most games can take advantage of the extra horsepower we just added into the system. Multi GPU rendering -- the idea is not new at all, if you are familiar with the hardware developments over the past couple of years you'll remember that 3dfx had a very familiar concept with the Voodoo 2 graphics cards series. There are multiple ways to manage two cards rendering one frame; think of Super tiling, it's a popular form of rendering. Alternate Frame Rendering, each card will render a frame (even/uneven) or Split Frame Rendering, simply one GPU renders the upper or the lower part of the frame. So you see, there are many methods where two or more GPUs can be utilized to bring you a substantial gain in performance.

The Crossfire connector going AWOL

Allow me to deal with this item straight away, are the Crossfire bridges dissapearing ? Yes and No. Not for the refresh products (260/270x/280x) just yet, but starting with the Radeon R9 290 and 290X the Crossfire bridges will dissapear.
 

That Crossfire bridge we all know and learned to love is slowly being phased out, starting with R9-290/290X. In the near future Crossfire bridges will be thing of the past as AMD is going to run Crossfire over the PCI Express bus. Especially with standards like PCI-E Gen 3.0 there's plenty of bandwidth there, but even at Gen 2.0, it really is not an issue. For the R290 cards setup in Crossfire, PCIE Gen 3.0 is recommended. What if you do not have PCIE 3.0 compatibility ? Well, the bus will revert to Gen 2.0 which will probably not make more then a marginal difference as it is really hard to flood even two x8 Gen 2.0 ports. 

Yup ! - Crossfire bridges - now a thing of the past.

Of course things get a little messy cable management wise when you add cards, you'll need multiple PCIe PEG power headers to feed this beast of a setup, and we'll need a PSU that can provide enough wattage as well. A question that we receive a lot is this, What is the difference in-between Crossfire and CrossfireX mode ? If you use the very same cards in multi-GPU mode, that is Crossfire. However However if you mix models, say the R7970 with a R7950 .. that's CrossfireX mode.

The computer components used

To be able to understand what we are doing today, we need to briefly take you through some of the key components used for our PC. Today we have both a home built DIY (Do It Yourself) X79 based Core i7 system that consists of the following gear:

Benchmark Setup:

• Core i7 3960X with all cores clocked at @ 4.6GHz
• Motherboard -- MSI X79 Big Bang XPower II
• Memory -- 8GB (4 x 2048 MB)
• 128 GB SSD for storage
• 1200 Watt Power Supply
• Two GeForce GTX 770 graphics cards (reference speeds)

These are some pretty nifty parts and bear in mind, when you opt for multi-GPU gaming, always have your gear right. You'll need that quality power supply, you'll need that proper SLI supporting motherboard, a processor and then you'll need a chassis with some very decent airflow to keep the graphics cards nicely chilled down.The tests have been performed on a X79 / Core i7 3960X processor (six-cores) all overclocked to 4.6 GHz. There's no downclocking on long duration CPU load.

If you decide to go for high-end Multi-GPU gaming, our recommendation currently is an Core i7 processor based on a Z77/X79 motherboard as it has plenty of PCIe gen 2.0 and 3.0 lanes and thus cross-link bandwidth really is optimal. For installation, make sure you do not forget to use a proper SLI bridge -- your motherboard should come with them by default. If not, there is a wide variety available. As you can see with three dual slot cards, space is an increasing issue for airflow. Make sure you end up with a well ventilated PC chassis. Luckily the GeForce GTX 770 graphics cards have an exhaust located at the rear of the card.

You'll need a total of four PCIe PEG power connectors (2x 6-pin and 2x 8-pin) headed from your power supply. Purchase a good quality PSU preferably with high efficiency to save on power consumption. And with so much power under the hood, don't mess around with the Molex to PCIe 6/8-pin power converter cables okay? We'll show you a thing about power consumption on the next pages.