PCI-Express Performance and the NVIDIA WarningWritten on July 21, 2011 by William George
Puget Systems has been in the business of building computers for 11 years now, and we know what we are doing when it comes to assembling top-notch custom computers. It is a bit insulting, then, when a parts manufacturer puts out a warning which appears - on the surface - to indicate something we do is resulting in anything other than the highest performance possible. Yet here I am, to let you know about just such a notice that nVidia's latest driver software is giving when using their graphics cards in certain configurations.
To give a little background, PCI-Express has been the connection of choice for video cards over the last several years; in fact, the technology is on its second generation now and is rapidly approaching a third generation. PCI-E slots also come in a variety of sizes, with each being named after the number of "lanes" it has for data in communicating with the rest of the computer. PCI-E x16 is the largest, followed by x8, x4 and x1. Smaller cards are designed to also fit in larger slots and work fine, and similarly a larger card can run at slower speeds if installed in a slot where it fits physically but a reduced number of lanes are available. Graphics cards receive a lot of data from other parts of the system, especially when playing games and the like, so they are built to utilize the full x16 size slots.
All computing platforms have a limited number of PCI-Express lanes, though, so you can’t simply fill a motherboard with several full x16 speed slots. With the current Intel Sandy Bridge processors, and the accompanying P67 and Z68 chipsets, there are only enough lanes for one full-speed slot. When dual video cards are desired, then, motherboard makers split those so that there are two full-size slots but they only function at x8 speeds if both are populated with cards. And just so we are all on the same page, nVidia has certified many such motherboards for SLI operation.
According to benchmarks conducted by Toms Hardware, a GeForce GTX 570 video card only performs 2% slower on a x8 connection compared to x16 - hardly noticeable without actually running tests to compare. And yet, with nVidia’s latest drivers - on a SLI approved motherboard, where the only option when running two cards is to use PCI-E x8 mode - the following warning pops up in the system tray:
So despite following the proper procedure for this motherboard, and it having been given nVidia’s blessing (in the form of SLI certification), they warn that using x8 speed slots is "low performance". I would get that if the difference between x16 and x8 speeds was significant - say 10 or 20%. But 2% slower is officially going to be called "low performance"? Really nVidia? That seems like it will just confuse customers, even those who went out of their way to research SLI options ahead of time - and I’m disappointed. At least the warning only seems to come up once... per user... each time new drivers are installed...