Is NVLink BETTER than SLI??
Linus Tech Tips
·Linus Tech Tips
·2019-05-06
·
1,641 words · ~8 min read
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When NVIDIA released SLI,
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it enabled gamers to enjoy next generation levels
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of quality today.
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Assuming of course,
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that they could afford literally twice
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as many graphics cards.
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But as monitor resolutions have grown,
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the scalable part of the scalable link interface,
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which has been with us for over a decade,
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hasn't been able to keep up in spite of fancy,
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high bandwidth bridges like this one.
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Meanwhile, over on the professional side of things,
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NVIDIA has been pushing
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a newer inter-GPU communication protocol called NVLink.
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This essentially turns SLI up to 11.
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But why would you, the general consumer, care about that?
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Well, because NVLink is coming to consumers
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with the GeForce RTX series.
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So it is time then to ask the big question.
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Does it make gaming
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better?
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Whew, that's a lot of hardware.
1:06
Speaking of big questions,
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So one of the first things you'll notice
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about a card equipped with NVLink
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is just how big the connector fingers are
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compared to traditional SLI.
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They are more than three times as wide with way,
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way more pins.
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Like seriously, a single NVLink finger
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is wider than the entire SLI connector setup.
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It almost even looks like they're little
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PCI Express connectors, which,
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as we're about to see, isn't by accident.
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So the way that SLI works is actually a lot like,
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oh, here, I have a good prep for this.
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It's actually a lot like the older SCSI and IDE.
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One card functions as the master in the relationship
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and the other one as a slave,
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or in the case of multiple other cards,
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they would all then be slaves.
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So that means that because the master alone
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is directing the workload for those slave cards
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with at best two gigabytes per second of bandwidth
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using one of NVIDIA's high bandwidth bridges,
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you've got enough for the render results
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to be returned to the master
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and honestly, not a whole lot more.
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This is the reason why you can't simply add together
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the memory of your SLI graphics cards,
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taking two 11 gig cards and saying,
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well, I've got 22 gigs of RAM now.
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And the same is true for Team Red's Crossfire.
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By contrast, NVLink is bi-directional
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and it's configured as a mesh,
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which means that no one card is the master,
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and there are no slaves.
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Think of it more like if you were plugging computers
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into a router or a switch.
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So this, along with the extra pins
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and newer signaling protocol,
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gives these cards a lot more bandwidth,
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more than even PCI Express,
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at a total of up to 160 to 300 gigabytes per second.
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That kind of speed lets them pool resources,
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that kind of speed lets them pool resources,
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that kind of speed lets them pool resources,
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in a way that allows access to each card's memory
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and CUDA cores as though they were a single card.
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And that's perfect for the scientific
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and high-end render stations
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that NVIDIA has traditionally targeted with NVLink.
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Now, you might be thinking to yourself,
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awesome, NVLink is coming to GeForce RTX cards.
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We're gonna get those benefits.
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I'm doubling my pre-order.
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Hold your horses there, Tom.
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Yeah, it's awesome.
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But the number of links provided on RTX
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is relatively minimal,
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and the RTX cards only support SLI over the NVLink bus.
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So there will be no fancy resource pooling going on here.
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So our plan today then is to take our Quadro GP100s
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and run them both in compute mode,
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which actually disables the graphics engine,
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like we couldn't plug a display
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into these things right now if we tried,
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and in what's called SLI mode,
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to look at their gaming performance.
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Yes, yes, I know.
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This card isn't intended for gaming,
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but if you look closely at the spec of it,
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it's got HBM2 memory, yes, and more of it,
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but it's otherwise actually very similar to the GTX 1080 Ti.
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So this is probably as close as we will ever get
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to an apples-to-apples comparison between SLI and NVLink.
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Since Pascal is likely to be the only generation of products
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where both of these technologies are present.
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First up, some pre-flight tweaks
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to get everything working though.
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We needed a Quadro SLI-certified motherboard,
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so our ASUS X299 Deluxe with a Core i9-7900X
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worked nicely for this.
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And to look at NVLink's non-gaming performance,
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we needed to configure both cards
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in Tesla compute cluster mode,
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which we can check by going ahead and running...
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this command in the Windows PowerShell.
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So you can see right here,
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links one to three, or zero to three, excuse me,
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or one to, whatever the point is,
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they're all running, and that's good.
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Unfortunately, many of our benchmarks
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actually didn't cooperate very well
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with this particular setup,
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though the latest experimental Blender build managed it,
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and, whew!
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The results pretty much speak for themselves.
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Three and a half minutes for Gooseberry?
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20 seconds for BMW?
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In spite of these tests
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not being particularly memory intensive,
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we are seeing a clear advantage here.
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As for Luxmark's lower OpenCL performance scaling,
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that suggests that CUDA is a necessary ingredient
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if we wanna take full advantage of NVLink.
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Big surprise, of course.
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Not all there is to it, though.
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Remember how NVLink allows us to utilize
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all of the available memory on our cards
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as though they were one big card?
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Well, because of that,
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we can now work with much larger data sets
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than would have been possible on smaller configurations.
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And trust us, we tried on smaller configurations.
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You can see here, even our twinned GP100s
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couldn't handle this particular workload.
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So, it's time to bring out the big guns.
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Our GV100s with their new NVLink bridges
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will give us a total of 64 gigs of HBM2 memory.
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That's more than the system memory
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of even many workstations.
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And there it is.
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Our GV100s handle this just fine.
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So, that's super impressive and extremely useful
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for people with huge data sets.
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But the real thing we were after here was
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evaluating the SLI mode that is coming with the RTX series.
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So, and here we go.
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So, in a massive surprise to no one,
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the GV100s are the fastest solution on the block, for now.
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In SLI, even at 4K Ultra,
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the average frame rate never dipped below 60, which is huge.
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Nothing else can even come close to claiming that.
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What's more interesting, though,
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is when we look at the scaling figures side by side.
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So, our GP100s here,
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these guys seem to scale better than the GV100s
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in gaming and productivity,
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giving them the best scaling overall,
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which may suggest some kind of CPU bottleneck.
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As for the GTX 1080 Ti,
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well, there's huge gains to be made in gaming,
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but not as much in productivity.
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So, as you might expect with anything new,
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I think SLI doesn't scale the same way for everything,
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but it does look to be a pretty decent improvement
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over traditional SLI,
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about 10 to 23% better by our measure,
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with the potential to dramatically improve
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undesirable behavior like micro-stuttering as well,
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or even enable more than two-way SLI
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with decent scaling in the future.
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That is, depending on how much NVIDIA decides to neuter it
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compared to its professional-grade cousin.
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You, too, never know.
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You never know with those guys.
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I mean, one thing we discovered
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in the course of our testing for this video
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is that the new NVLink bridges here
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don't work with the old NVLink cards, even the pro ones.
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So, NVIDIA told us something about
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consumer NVLink bridges having fewer pins,
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or more importantly, a slightly different pin-out,
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but could they have made it work?
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I don't know.
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I don't know with those guys.
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Either way, NVLink has lots of potential
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and looks like a significant hardware upgrade
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that should only improve
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as the drivers themselves continue to improve.
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So maybe, just maybe, SLI isn't dead.
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Yet.
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Maybe.
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10:24
So thanks for watching, guys.
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If this video sucked, you know what to do.
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But if it was awesome, get subscribed,
10:28
hit that like button, or check out the NVLink.
10:32
Oh, Lordy, that's awful.
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Anthony, come on.
10:35
To where to buy the stuff we featured
10:37
in the video description.
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Also, NVLink.
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NVLinked.
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In the description is our merch store,
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which has cool shirts, and our community forum,
10:43
which you should totally join.
10:48
She made me do it twice.
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I mean, it's really, it's my fault.
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I'll read anything on the teleprompter.