The Relative Lack of SLI on AMD
Several years back we were introduced to the Lucid Hydra technology, and it seemed like an impressive multi-GPU implementation which could leverage the power of different video card combinations to improve performance over that of one card. Quite a few years have passed, and we have a handful of motherboards now supporting this technology. We take a look at the technology now implemented on the AMD side, and how it performs when using both AMD and NVIDIA based video cards.
Several years back we were introduced to the Lucid Hydra technology, and it seemed like an impressive multi-GPU implementation which could leverage the power of different video card combinations to improve performance over that of one card. Quite a few years have passed, and we have a handful of motherboards now supporting this technology. We take a look at the technology now implemented on the AMD side, and how it performs when using both AMD and NVIDIA based video cards.The first we heard of Lucid was a few years back when they showcased working silicon running multiple video cards together. Whether these were NVIDIA or ATI/AMD cards, Lucid had a way of allowing them to render a scene in a unique way, then composite the results to create a near seamless experience. It took some time before the first products hit the streets, and there is also quite a bit of controversy behind the actual implementation.
The primary rendering mode for both SLI and CrossFire is alternate frame rendering. Basically this allows each video card to process alternating frames, which theoretically can double performance. We have never seen true linear scaling in such situations, but it is not unheard of to reach 85% scaling or slightly more with the latest video cards on fast systems. Lucid does things a bit differently.
Block diagram of the Lucid Hydra chip and how it connects to the system.
The basis for the Lucid chip is a 300 MHz RISC processor embedded in their specialized PCI-E controller. The basic motherboard based chip is connected to the host chipset by up to 16 PCI-E lanes, and it then distributes up to 32 PCI-E lanes to other slots. Instead of just using AFR, Lucid actually breaks the scenes up, and sends the individual cards the work that needs to be done. It is not tiling, but rather it takes components out of the scene (eg. one card renders the wall, while the other renders the view outside the window in the wall). The image is then composited on the master card, and the output sent to the monitor.
Fairly advanced algorithms control this process, and the RISC core handles that particular workload. Theoretically this should allow two cards with different performance characteristics to still experience a speedup as compared to a single card. There are obviously limitations, and Lucid suggests using a card that is no less than 50% slower than the primary/faster card. Essentially a user should not be combining a HD 5550 with a HD 6970, and expect any kind of realistic performance improvement.
The Lucid 200 series of chip, usually located on finer motherboards throughout the market…
Hydra has been something of a relief for AMD. Since NVIDIA essentially pulled out of the chipset market the only SLI capable chipsets on the AMD side are the now relatively ancient 980a/780a/750a based motherboards from a handful of manufacturers. As we are moving toward greater SATA 6G and USB 3.0 adoption, these particular motherboards are becoming less and less appealing to users. If a user wants to run SLI on the AMD platform, their best bet for getting a more modern motherboard implementation is to purchase one which uses the Lucid HydraLogix technology. NVIDIA still has not given AMD license to include SLI support on AMD chipsets, unlike what NVIDIA has done for Intel with the last several generations of Core i7 based motherboards.
Hydra gets around the SLI license by not implementing SLI through the NVIDIA driver. Instead it uses its own custom algorithms to achieve multi-GPU scaling. As such, it is the only “legal” way to utilize SLI type configurations on the AMD platform utilizing recent motherboards. Running two NVIDIA cards on Hydra is called “N-Mode”. Hydra currently only works with Windows 7 and Vista. If a user wants to utilize both NVIDIA and AMD cards together, they must use Windows 7 as Vista does not support multiple video card drivers to be installed from different vendors.
The previously reviewed Asus Crosshair IV Extreme, which is the basis for this particular review.
I wanted to see how well the latest Hydra implementation works on the AMD platform, so with the support of Asus I was able to assemble a group of cards which represent performance from top to bottom. I have the Asus Crosshair IV Extreme motherboard, which implements the Hydra solution natively as the basis for the review.