PCI Express slots designed for graphics generally have 16 lanes. If you are running in SLI, until recently you had to make do with 8 lanes for each card. Despite this dramatic reduction in bandwidth, there was a negligible performance decrease. In actuality, we’ve only just started to notice an improvement moving away from AGP 8x.
Despite this, nVidia feels that we still need more bandwidth and that LinkBoost is the answer. As part of the nForce 5 specification (i.e enforced as a necessary inclusion in the BIOS), as soon as a compatible card is inserted in to the machine (currently only the 7900 GTX), the PCI-E bus and the link between the MCP and SPP should be increased by 25 per cent. This would make the graphics slots the equivalent of having 20 lanes instead of 16.
We decided to put this to the test using two default clocked 7900 GTX graphics cards running in SLI. Without any interference on our part, LinkBoost was automatically activated. We ran a slightly cut down version of our normal game tests (what we in the office like to call Spode Mark) at 1,280 x 1,024 and 2,048 x 1,536 with both 4x FSAA and 8x AF on. The results for this are shown at the end and as expected they clearly show no difference at all when LinkBoost is on or off.
LinkBoost would be beneficial on the 570 SLI chipset where there are only eight lanes per card, but even this difference may well be negligible. Similarly, Quad SLI would possibly notice an improvement as two GPUs are sharing a single x16 slot. Unfortunately, neither 570 SLI, Quad SLI or its up and coming replacement will support LinkBoost. However, there is nothing stopping the savvy user from overclocking the PCI-E bus themselves and reaping any performance benefit there may be to have.
All in all, LinkBoost is nothing more than marketing leverage to make the 590 SLI chipset stand out and certainly nothing to cry about missing.