Why bother with three- and four-GPU rendering?
Current state of play. What's the point?CrossFireX covers all of ATI's multi-GPU shenanigans under one branding, and naturally includes two-, three, and four-way GPU support.
Thinking of graphics cards in the traditional sense (X2 excluded, then), where one GPU is placed on to a single PCB and then connected to the system via a PCIe interface, three- and four-way CrossFire, at the very least, requires chipset support for that number of cards. Further, ensuring that performance is not compromised by a lack of chipset-related bandwidth, AMD's 790FX, part of the Spider platform, provides a total of 32 PCIe 2.0 lanes for graphics, which can be auto-split into four x8.
A slew of empirical benchmark data indicates that well-profiled games' high-resolution performance (non-CPU-limited scenarios) can be practically doubled in a two-way CrossFire or SLI setup, given the same image-quality levels - meaning two Radeon HD 3870s can produce practically double the frame-rate as a single card's.
The exact degree of improvement is down to a number of factors, including just how well the engine queues up frames and what impediments the API places on multi-GPU rendering. Both ATI and NVIDIA tend to agree that 1.75x scaling of a single-card's performance is validation enough for a multi-GPU setup. The thinking behind CrossFire and SLI is that you can buy a particular graphics card - usually at the top-end of your budget - and multi-GPU-supporting motherboard now, and then add a second card to boost performance, if needs be, when funds permit.
Troubles in paradise: why effective three- and four-GPU scaling is worthwhile but problematic.
Adding an identical third and fourth card into the system inevitably results in diminishing scaling as extraneous factors come into play, and that's why three- and four-way CrossFireX, available solely with Radeon HD 3800-series cards with dual CrossFire connectors - is ostensibly aimed at the enthusiast, where pure performance is paramount, because it represents the pinnacle of graphics rendering in a desktop environment. If you want the absolute highest-performing ATI or NVIDIA graphics-card sub-system, then either four-GPU CrossFireX or three-GPU SLI is the way to go.
An important point to note is that three- and four-way CrossFireX - and, remember, that combining two Radeon HD 3870 X2s results in four-GPU CrossFire on two cards - is limited to Windows Vista only. Windows XP, the gamers' OS of choice, we reckon, introduces limitations on just how far the CrossFire engine and API can queue the frames required for processing, leading to poorer-than-expected scaling. That's why CrossFireX, in Windows XP form, will be limited to the tried-and-trusted two GPUs.
Carrying one from this, certain games' engines, notably Crysis, impose limitations on just how far you can render ahead and pool data, leading to sub-optimal three- and four-GPU scaling. Yes, you still receive marginally more performance than a dual-GPU CrossFire setup, but it no longer becomes a compelling case. Do rememberthat another limitation comes in the form of no additional scaling, over and above two GPUs, with OpenGL titles.
Multi-GPU scaling works very well with DirectX9 code but is rather more difficult to program for in a DX10 environment, we were told.
Reiterating an important and salient point, scaling works best with higher resolutions and tasty I.Q., where the performance is bound by the graphics sub-system.
ATI's research team has found that, whilst technically possible, it's practically pointless to scale above four GPUs: the returns simply aren't there.
The hardware for extravagant CrossFire is already in place right now with cards and 790FX boards available from all and sundry, so the remaining part of the jigsaw, the tri- and quad-Fire CrossFireX driver, is all that's missing from ATI. It will be officially released on March 4th, in CATALYST 8.3, to coincide with the CeBIT trade show.
