TECH TALK Nvidia’s RTX Multi-GPU Shifts to NVLink

THE DUST FROM THE TURING launch has set­tled, and we’re left with the most ex­pen­sive con­sumer GPUs in Nvidia’s his­tory. The RTX 2080 Ti takes over where the Ti­tan Xp left off, with a de­cent in­crease in per­for­mance. Most of us would be happy with that, but

Maximum PC - - QUICKSTART - Jarred Wal­ton

SLI has a long his­tory, go­ing back to 3dfx’s orig­i­nal SLI (Scan-Line In­ter­leave), where two Voodoo2 cards could ren­der al­ter­nate lines. Nvidia ac­quired 3dfx’s as­sets in 2004, and be­gan of­fer­ing a new SLI (Scal­able Link In­ter­face) with the GeForce 6800 fam­ily. In­stead of al­ter­nat­ing ren­dered lines, Nvidia’s GPUs used al­ter­nate frame ren­der­ing (AFR). It was a sig­nif­i­cant change in func­tion­al­ity, and has led to a host of is­sues over the years.

SLI sys­tems—and AMD Cross­Fire PCs—ren­der mul­ti­ple frames sort of si­mul­ta­ne­ously. The driv­ers need to bal­ance things so that frame pac­ing is rel­a­tively con­sis­tent, or you get mi­cros­tut­ter and other prob­lems. Ideally, with two GPUs, GPU1 starts the first frame, and GPU2 starts work on the next when GPU1 is half­way fin­ished with its frame.

Nvidia later in­tro­duced three-way and four-way SLI, but the dif­fi­culty of main­tain­ing con­sis­tent frame pac­ing, the ad­di­tional la­ten­cies, and lim­ited scal­ing in games made them niche so­lu­tions that were more trou­ble than they were worth. Nvidia killed off three-way and four-way SLI with Pas­cal. With Turing and the RTX 2080, two-way SLI is the only sup­ported mode, but there’s a sec­ond change.

Pas­cal GPUs in­tro­duced the HB SLI bridge, which in­creased SLI link band­width from about 1GB/s to 4GB/s. That was nec­es­sary to bet­ter sup­port 4K60 res­o­lu­tions, but it still wasn’t enough—not with 4K144 HDR and 8K dis­plays now avail­able. So, Nvidia moved to an NVLink con­nec­tor for the RTX 2080 and 2080 Ti. The 2080 gets a sin­gle NVLink chan­nel that’s good for 50GB/s, while the 2080 Ti has two and 100GB/s. Thanks to the higher band­width, there’s no dif­fer­ence be­tween run­ning dual GPUs off x16 PCIe con­nec­tions ver­sus dual x8 con­nec­tions, which is good, as the Core i7-8700K and i9-9900K end up be­ing faster in games than any X299 en­thu­si­ast plat­form CPU.

But how does RTX 2080 Ti SLI per­form? Re­sults vary. 3DMark shows nearly per­fect scal­ing in Time Spy Ex­treme and Fire Strike Ul­tra. GTA5 ,Rise of the Tomb Raider, Strange Bri­gade, and The Witcher3 also show ex­cel­lent scal­ing of 80 per­cent or more at 4K. Other games, such as Bat­tle­field 1,F12 018, Far Cry5, Rain­bow Six Siege,Shadow of the Tomb Raider, and Shadow of War, aren’t so im­pres­sive, but man­age 30–50 per­cent scal­ing. Then there are games that show neg­a­tive scal­ing:

As­sas­sin’ s Creed, Call of Duty, Di­vin­ity Orig­i­nal Sin, Hell blade, Hit man, Mon­ster Hunter World,

Prey, Wolfen­stein, and more. As a $2,500 propo­si­tion ($1,200 per 2080 Ti and $79 for the NVLink adapter), this isn’t for the faint of wal­let. It’s also sober­ing to think that even 4K isn’t enough to max out scal­ing from SLI. Be­yond that, we’re years away from 8K gam­ing be­ing a vi­able choice. That’s all for the best as far as I’m con­cerned. When SLI or Cross­Fire work well, they’re im­pres­sive, but far too of­ten, you run into in­com­pat­i­bil­i­ties or glitches. I’d rather spend my time play­ing games in­stead of futz­ing around with SLI pro­files. Jarred Wal­ton has been a PC and gam­ing en­thu­si­ast for over 30 years.

RTX 2080 Ti cards in SLI use Nvidia’s high-speed NVLink bridge.

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