Random Access   chris, kp & rob
In The Lab
Dualing Video Cards
by chris

In a previous article (In The Lab, Random Access, May 2005), I tested the relative performance of a Dual Core CPU compared to a Hyperthreading and standard Pentium 4 processor. While the results were not exceptional, they did serve to show that two full featured processing cores could out perform earlier versions. Similar techniques have been applied to Dual Channel (DDR and DDR2) memory to increase its effective performance. Video cards are also undergoing changes and being engineered for higher and higher performance.

As with Dual Core processors and Dual Channel memory, using two video cards in parallel can result in a performance boost in a system. NVIDIA has introduced a series of SLI (Scalable Link Interface) video cards to take advantage of system boards that have two PCI Express video slots available and use an nForce4 SLI chipset. Both ATI and 3dfx have similar two-card offerings. As with the ATI Crossfire series of paired video cards, special motherboard support is required to take advantage of either technology. You cannot just install two PCI video cards to get a boost in performance, and a system designed for two ATI Crossfire cards will not support two NVIDIA SLI cards. Gigabyte produces a 3D1 series of PCI Express video cards that have dual GPUs; when installed on a compatible dual slot SLI-motherboard, your two SLI-linked video cards have a total of four GPUs sharing the load.

The method for joining and synchronizing two different SLI video cards is with the use of an NVIDIA SLI connector bridging the two cards. The NVIDIA SLI connector is a proprietary link between the two video cards, and is used to transmit synchronization, display and pixel data at speeds of up to 1GB/s. If no connector is present on the card, but SLI is supported, the two video adapters must use part of the PCI Express bandwidth to communicate and synchronize their activity; the SLI connector link transfers data externally to the PCI Express bus, consuming none of the PCIe bandwidth. As a rule, this does not present a problem, especially on the mainstream level cards, because even together the two cards cannot process the entire PCIe bandwidth of information that could be transmitted.

None of this means that you will automatically see a dramatic performance boost in games or other applications. The CPU of the system still determines how fast you can send instructions to the graphics card to draw the images, while the slot type and bus speed determine how fast the data or instructions are transferred to the card. Other factors include the clock speed and performance of the GPU (Graphics Processing Unit - this is the video card's CPU), how much video memory is available, the ability of the software application to scale, and the system performance in general. With two SLI video cards installed in a compatible system board and running certain hot games, you could see up to twice the performance of a single PCI Express video card. In terms of video performance, you should see a marked improvement in the smooth rendering of motion, graphical detail and image effects; but don't expect to outlive the monsters any longer than you have been, though.

On many video cards that have two outputs, such as those that have both a VGA and DVI connector, you can have two displays attached at the same time. These displays can show the same desktop or expand the desktop to cover both displays. With system boards supporting two PCIe video slots and with two compatible (SLI or Crossfire) cards installed, you can support up to four monitors attached to your system at one time. However, when the two cards are linked in a multi-GPU mode, SLI currently only supports one monitor.

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