Product review: The AMD Radeon HD 3870 X2 (page 1: Introduction, CrossFire)

Since the release of the GeForce 8800 GTX and even after its small update in the form of the Ultra, AMD has not been able to rival Nvidia for the top performance slot. Indeed Nvidia has a more performance efficient architecture and it’s difficult for AMD to compete with that of its Radeon HD 2000 and 3000. On the other hand, the arrival of a mid-range high performance chip with lower power consumption and a relatively lower cost has enabled AMD to release a card equipped with 2 of these GPUs. Finally, this may give them some hope to take back first place.

CrossFire

The technology used for the functioning of this card is of course based on CrossFire which is now more mature. One small subtlety is that AMD opted for its software version which is already used on the entry level. This is in contrast with the "hardware" version which is generally used for the mid and high end and requires two CrossFire connections to create a bridge between the two cards.
The hardware method’s advantage is that it enables higher performances because the dedicated connection is used for the transfer of images from one card to the other. In Crossfire software this transfer takes place via the PCI Express bus and can therefore limit performances. So is there a lack of efficiency with the multi-GPU Radeon HD 3870 X2? Actually no, it does look promising (at least on paper) and this is something we are going to verify in the following pages.

So why the optimism? First of all, because there is a PCI Express switch on the PCB. Although limited to the 1.1 norm, it manages three 16x ports, one towards the system and one towards each GPU. This authorizes P2P transfer which means avoiding having to go through the chipset and consequently improving efficiency. Another and possibly more important point is related to Radeon HD 2000 and 3000 architecture. The PCI Express bus is a bus through which writing is executed and not reading. This consequently implies action on the part of the GPU which has to send its work to the other one. With an older GPU or a GeForce, the GPU is blocked when it is writing via the PCI Express bus. This means that it is unable to calculate a frame at the same time, signifying that transfers from one GPU to another in multi-GPU mode are a waste of calculation time which can be even greater in high resolution. However, if you recall the presentation of the Radeon HD 2900, AMD mentioned that they have integrated a dedicated processor for PCI Express bus management, which means that these cards do not suffer from this inconvenience.

The two RV670 GPUs and PLX PCI Express switch.
In the end, the main performance related weaknesses of CrossFire software disappear or are at least reduced. At the same time there is added flexibility because a single versatile connection is used instead of a higher performance solution that is dedicated and only limited to certain functions. This is a simplification which should prove beneficial in the long term and finally we don’t find any negatives to the functioning of CrossFire software the way it was implemented on the Radeon HD 3870 X2.