Report : tri and quad GPU systems (page 2: Drivers and practical details)

Drivers

Activating SLI or CrossFire X (CrossFire X is now the name given to all CrossFire technology in Windows Vista) in drivers is very simple as you only have to check a box in their specific panel. Both AMD and Nvidia integrate a hardware and software version of their multi-GPU technology. The hardware version uses dedicated components present in GPUs as well as the specific connector to transfer information (or at least the largest possible part of it) from one card to another. This is the highest performance mode. The software version works with the PCI Express bus to transfer all the data between cards and is therefore a little less efficient.

For AMD, activation and deactivation is done very quickly. On the other hand, for Nvidia it takes longer. You must be patient and there are two confirmations not to be missed between turning off the screen and deactivating Aero.

Also for Nvidia, the driver has still not progressed in terms of multi monitor; in multi-GPU mode you can only use a single monitor while AMD has lifted this limitation. Another constraint is that if Quad SLI poses a problem in games while SLI otherwise functions well (and therefore a single 9800 GX2), you have to physically remove the card from the system to go into SLI. Deactivating Quad SLI also deactivates all multi-GPU modes and prevents you from taking advantage of the total power of a single 9800 GX2. In short, it’s all or nothing. For AMD, the Radeon HD 3870 X2’s GPUs function when quad CrossFire isn't activated.

Practical details

Of course, we used the hardware version in all cases, or at least placed the connectors to enable it to function. It’s not always easy to know what the driver is doing with more than 2 GPUs, especially for AMD, which moreover, wasn't too clear on the subject. On the one hand, they told us that the Radeon HD 3870 X2 uses exclusively the PCI Express switch for communication, but also that there is CrossFire cabling on the PCB. Obviously, at least one of these sources seems confused. So how about for triple and quad CrossFire? Either way, a connector is required for the driver to accept activation although we wonder if it is even used.

For CrossFire, a single connector is now required and it’s up to you to choose either one if your card has two. Otherwise, putting the two connectors into place doesn’t pose a problem. For Nvidia, a single connector is required for SLI or Quad SLI (which has always been the case), while a special one is needed for triple SLI which allows interconnecting all the cards (the 1st to the 2nd, the 2nd to the 3rd and the 3rd to the 1st). This is something that doesn’t come with nForce 680i motherboards and has to be acquired. NForce 780i and 790i models include it with their bundles.

CrossFire and 3-way SLI connectors.
In triple SLI, we may wonder why all the GPUs have to be connected the all the other ones while this isn’t the case in Quad SLI. The latter only uses one connector between the two pairs of GPUs instead of the four that would be needed for it to resemble triple SLI communication. This means that either one of the connections in triple SLI is useless (in this case, two classic SLI connectors would have been enough instead of a specific one), or triple and Quad SLI do not use the same communication architecture which means there is dead weight on the driver level.

We encountered several practical problems with SLI connectors when connecting the two GeForce 9800 GX2s. First of all, a flexible SLI connector, which barely covered the distance between the two cards (and therefore laying on top) partially melted and was damaged in hours of testing. The reason was that it was too close to the vent which evacuated hot air. We then exchanged it for a hard SLI connector and a new problem arose; contact was poor and caused the screen to blink. The casings of the 9800 GX2s are a bit higher than the PCB and consequently some of the relatively flat connectors (such as the reference versions found on nForce 680i and 780i motherboards) do not plug in correctly. The solution was to use a long flexible model so that it could make a loop on the top of the cards and not be in direct contact with the hot part of the casing.

The ‘’hard’’ SLI connector is flatter than the soft one and cannot be entirely plugged into the 9800 GX2 due to its casing