Multi-GPU Mode Explained

Multi-GPU Mode 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 that are present, the worse the scaling becomes and the more driver issues you will run into. Honestly, two GPUs in most scenarios is ideal in terms of multi-GPU gaming, 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 a 3-way SLI GeForce GTX 980 graphics card configuration.

• A Crossfire compatible motherboard is pretty much ANY motherboard with multiple PCIe x16 slots 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, X79, Z97 and X99 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 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 a supplied Crossfire connector or, in Nvidia's case, an SLI connector. Then install/update the drivers, after which most games can take advantage of the extra horsepower we just added into the system.
 

Screenshot of three cards with SLI enabled on the NVIDIA control panel.

Once you have your hardware setup it's time to install the latest drivers. In the Nvidia control panel, make sure that Maximize 3D Performance is activated. For SLI + Multi monitor setup you need to click 'Span Displays with Surround'.

Multi GPU rendering -- the idea is not new at all. 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 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 a home built DIY (Do It Yourself) X79 based Core i7 system that consists out 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
• 1500 Watt Power Supply
• Three GeForce GTX 980 graphics cards

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 active and 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 is currently a Core i7 processor based on a Z87/Z97/X79/X99 motherboard as it has plenty of PCIe gen 2.0 and 3.0 lanes and thus cross-link bandwidth really is optimal.

What about PCI Expres 2.0 is that enough ?

Yup, even with 3-way SLI at x8 each that's plenty. 

If you look at BUS2 and BUS3 values you'll spot that card 2 and 3 are using less than 15% of the PCI BUS (Gen 2.0 X8). The primary card located at BUS1 peaks out at only 29% usage. So bandwith is just not a problem.