PCIe Lanes - PCIe 8x vs 16x in SLI
Linus Tech Tips
·Linus Tech Tips
·2015-05-07
·
1,399 words · ~6 min read
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so the other day i was shopping for a state-of-the-art CPU for my gaming and
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productivity pc when i realized that Intel's latest 5820k and 5930k
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both have six processing cores with hyper threading both have 15 megs of
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cash they have a clock speed difference of only six percent and yet the
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difference in retail price is around 200
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oh there's the gotcha the 5820k only has 28
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pci express gen 3 lanes you know the electrical connections behind the slots
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that we use for graphics cards and other expansion cards compared to 40 on the
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5930k what a strange way for Intel to
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differentiate its mid-tier 2011 3 CPU from its entry-level one
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Corsair gaming RGB keyboards feature precision cherry mx RGB key switches for
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16.8 million color per key backlighting for virtually unlimited customization
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click now to learn more let's start with a brief explanation of what pci express
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or PCIe lanes are and what they're good
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for unlike previous expansion slots like agp and pci pci express has a physical
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size and an electrical operating mode
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that can be independent from each other power is provided by the pins near the
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key in the connector and the rest of the connector is used or not used to provide
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more or less bandwidth to the device that is plugged into it the more
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bandwidth the more lanes are said to be being used for communication the most
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common speeds are 1x 4x 8x and 16x so
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here's a cool fact the different bus width cards and slots are actually
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interoperable that means that installing a 1x card in a 16x slot is fine and you
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can even go the other way around in this video i cut out the back of a smaller
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slot and put a larger card into it all
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that happens is that both the slot and the card will now operate at the speed
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of the lowest link in the chain so that's what's going to happen if we
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plug our PCIe 16x graphics card into
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what appears to be a 16x slot but if the
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CPU in our system doesn't have enough pci express lanes to provide the full 16
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lanes your motherboard manual will give you more specifics with a slot by slot
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breakdown of how it's going to work but basically it'll simply interface with
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your pc at a lower speed usually 8x
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well that sounds horrible Linus won't cutting the link speed to my graphics
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card in half tank its performance not necessarily we're on the third
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generation of pci express with each new generation doubling the theoretical
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speed of the previous one that means a modern gen 3 PCIe 16x slot can
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communicate four times faster than an original gen 1 16x slot or another way
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of thinking about this is that a gen 3 4x slot is equivalent to a gen 2 8x or a
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gen 1 16x slot but while we already know that from
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countless articles that exist on the topic in the past that no previous
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generation of 8x slot has bottlenecked a then modern high-end graphics card with
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x99's positioning as the high-performance platform of choice for
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enthusiasts and for gamers i think it's fair to say that folks may be running
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not just one but two or even more
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graphics cards at a time and this is where concerns start to arise especially
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given that AMD and i suspect also NVIDIA is starting to do more inter-card
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communication over pci express when running in crossfire and again i suspect
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sli versus relying on those dedicated connectors on the top of the cards
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so let's introduce the test to eliminate the variable of the slightly different
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clock speed of the two cpus and since overclocking k-series chips is so easy
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these days we're gonna run both processors at four gigahertz a very
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conservative overclock that anyone should be able to attain for the rest of
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the bench we've got a Gigabyte x99 gaming five four-way sli compatible
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motherboard with 16 gigs of ddr4 2400 megahertz memory from adata thanks to
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both of them for providing the hardware for the test bench we're going to be using four NVIDIA GeForce gtx 980 4gb
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graphics cards and we're going to be running all this off of course our ax
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1500i power supply so that nothing melts during the benchmarking process and here
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are the numbers fraps was used to take average frame rates for each of our
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standard test runs and all games were run at 4k on pretty demanding settings
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to ensure that we were pushing the hardware as hard as possible as expected
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a single graphics card config is not affected in any way since it's running
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in 16x mode on either CPU but dual cards
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will be running at 8x8x on a 5820k and
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16x16x so an effective doubling on a 5930k so i guess as much as it might be
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a little bit more counterintuitive they both ran the same so there you go
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in three-way sli both cpus have to give up at least one 16x slot but again the
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results are not really affected in any way beyond sort of your standard
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variance from one run to another and then finally in four-way sli we see oh
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actually there's a difference here as you may or may not have noticed our 28
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lien CPU doesn't have any numbers for 4-way sli NVIDIA requires graphics cards
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to be plugged into an 8x slot at least for sli certification so only 40 lane
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cpus have support for this configuration however it should be noted that even at
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4k the benefit of a fourth card is fairly small and three-way remains our
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recommended max config even if you feel the need to spend over a thousand
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dollars on a bunch of graphics cards to put in your system it should be noted
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that AMD does not have the same limitation for crossfire so you could
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run four graphics cards if you really wanted to in that case but our
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recommendation remains the same anyhow
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so that's the story or at least most of it for gamers anyway compared to the
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mainstream z97 platform where it should be noted that even the highest end cpus
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have a mere 16 lanes Intel's enthusiast x99 platform in general and the core i7
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5820k CPU in particular is a great
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choice even for multi graphics card gamers if they want to reap the benefits
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of an extra two processing course ddr4 memory and higher overall memory
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capacity limits things do get a little bit dicey for
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non-gamers though when it comes to other pci expansions so make sure that you
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consult the manual of the motherboard you want before investing in this
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particular CPU because you may find that the m.2 high speed storage slots
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and in some cases even built on ports like SATA express and usb 3 headers
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might not work with too many pci express
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cards plugged in or even be available at all with a 28 lane CPU so if in doubt
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head over to the cpus and motherboard section of the Linus techtips.com forum
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where our helpful community will get you sorted out
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thanks for watching guys i hope this video put to rest some of the concerns
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about oh no the affordable one doesn't have enough pci express lanes i hope you
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all feel a little bit better like the video if you liked it dislike it if you
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disliked it leave a comment letting me know did you learn something today is
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this something you already knew was it nice to have it validated anyway also
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check out the link in the video description you can support us by buying a cool t-shirt like this one giving us a
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monthly contribution or that other thing right changing your amazon bookmarks one
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with our affiliate code so we get a small kickback whenever you buy cpus or
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graphics cards or whatever else thanks again for watching and as always don't
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forget to subscribe
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you