X-bit labs

There Will Be Speed: The Clash of Modern Multi-GPU Technologies (page 3)

Category: Video

by Alexey Stepin , Yaroslav Lyssenko, Anton Shilov

[ 03/24/2008 | 05:01 PM ]

Nvidia GeForce 9800 GX2

Work Principles and Technical Specifications

We can’t expect anything particularly special from the new card as uses the well-known G92 core. The specs of the GeForce 9800 GX2 are as follows:

It’s clear that Nvidia’s new card is just a SLI tandem made out of two GeForce 8800 GTS 512MB with different clock rates and with a PCI Express switch to ensure that SLI technology will work on any mainboard. The GeForce 9800 GX2 is no different from the ATI Radeon HD 3870 X2 from this aspect. Interestingly, the clock rates of the GPUs are lowered relative to the 8800 GTS 512MB: from 650/1650MHz to 600/1500MHz. The GPU clock rates of the ATI Radeon HD 3870 X2 are, on the contrary, increased in comparison with the single card. It should also be noted that ATI’s card has a single-PCB design but the 55nm RV670 is more economical and cool than the G92 in full configuration.

The main advantage of the GeForce 9800 GX2 over the ATI Radeon HD 3870 X2 is that is has two times the amount of TMUs and features a scalar architecture of the ALU array, which is more effective in today’s games due to support on the game developers’ part. We can also note the card’s support of PCI Express 2.0 and the lack of support for DirectX 10.1: AMD’s solution is just the opposite. Both standards are not yet widespread, so the availability or lack of such support doesn’t matter much for today’s games. Theoretically, Nvidia’s card should be faster, especially as it is positioned in a more expensive price category than the ATI Radeon HD 3870 X2 ($599-649 against $449) but we should first see how perfectly SLI is implemented. If the technology is not supported properly, the user will only get the performance of a reduced-frequency GeForce 8800 GTS 512MB our of his GeForce 9800 GX2 at a much higher price. We’ll check this out in our gaming tests, of course.

The Nvidia GeForce 9800 GX2 works like any other SLI tandem but the GPUs are located within the same graphics card and share one PCI Express x16 slot. That’s the way the GeForce 7950 GX2 used to be designed:

GeForce 9800 GX2 has only one MIO ports laid outside that is why the maximum configuration is a 4-way SLI system with two 9800 GX2 cards. ATI CrossFireX has the same limitations these days.

As for the way the multi-GPU technology operates, we don’t have accurate info about it. Nvidia seems to have followed ATI in abandoning all rendering modes other than AFR. SLI technology used to support SFR mode in which each GPU would render one half of the frame, and a Quad SLI system could use a hybrid SFR on AFR mode, in which each pair of GPUs would render one frame using SFR and the frames were output alternately in AFR mode. The use of SFR is currently obstructed by the spread of sophisticated rendering techniques, so the transition to AFR looks reasonable. This mode doesn’t require meticulous software optimizations and can often do without them altogether. However, some reviewers don’t think that SFR mode is forgotten completely like Scissor and SuperTiling modes of ATI CrossFireX.

Among the drawbacks typical of AFR, the delayed response of the game to the user’s actions can be mentioned. This is going to be almost imperceptible with two GPUs, though. With four GPUs, the delay may be conspicuous, affecting your playing comfort. We’ll check out this problem below.