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Sakkura

Skylake & Haswell-E PCIe lane misconception

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On 10/6/2015 at 5:19 AM, Sakkura said:

Okay, so watching the 5820K vs 6700K showdown video on Vessel, I came across a common misconception about the PCIe lanes on Skylake and Haswell-E unfortunately being perpetuated by @Slick.

 

The Core i7-6700K, and all the other Skylake CPUs, offers 16 PCIe 3.0 lanes. The chipset offers additional PCIe lanes separately from that. For the Z170 chipset, that's 20 PCIe 3.0 lanes, a huge boost over Z97 which had just 8 PCIe 2.0 lanes. Luke was just saying 20 lanes, which is not really correct; there is a kernel of truth to it though, because while the chipset offers all those lanes, it's still only connected to the CPU by a DMI 3.0 link that's equivalent to 4 lanes of PCIe 3.0 (in addition to the 16 lanes directly from the CPU). Still, you can hook up lots of PCIe 3.0 SSDs to the chipset just fine without affecting lanes for the GPU(s). Just don't expect RAID0 to give you like 10GB/s combined bandwidth.

 

As for the Core i7-5820K, it offers the well-known 28 PCIe 3.0 lanes directly from the CPU. But again, the chipset offers additional PCIe lanes. In this case, X99 is far inferior to Z170, because it only offers 8 PCIe 2.0 lanes (just like Z97).

 

Here are the block diagrams showing what I explained above:

 

Hidden Content

 

Do note that the lower-end chipsets, eg. H110, cut down on the PCIe connectivity on offer.

It is also correct to add that for example the motherboard Z97X-Gaming G1 WIFI-BK, was possibly the most expensive Z97 motherboard on the market because it added the chip PLX PEX8747, which by multiplexing gave us 16 lines in two PCI Express x16 (x16) / x16) or 8 lines in four PCI Express x 16, that is, (x8 / x8 / x8 / x8) was the equivalent to having a CPU of 32 PCI lines, when they really had 16.

There are motherboards that used smaller PLX chips to increase the PCI Express connectivity of the motherboard chipset eg the Asrock P67 Extreme4Gen3.

These issues are very important, but few people talk about this.

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All these lanes are confusing.  I built 2 systems 5+ years ago

 

i7 3930k w/ Asus Sabertooth x79 = 40 Lanes Support

i7 4960x w/ Asus x79-Deluxe = 40 Lanes Support

 

This was back in 2012, I'm not sure why Intel screwed around and really crippled stuff since then. Both Motherboards were advertised as PCIE 3.0 ready, but they are not really as both CPU are rated 5 GT/s DMI2 and PCIE 3.0 is 8 GT.   5 > 8


CPU i7 4960x Ivy Bridge Extreme | 64GB Quad DDR-3 RAM | MBD Asus x79-Deluxe | RTX 2080 ti FE 11GB |
Thermaltake 850w PWS | ASUS ROG 27" IPS 1440p | | Win 7 pro x64 |

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On 1/30/2019 at 1:36 PM, THraShArD said:

All these lanes are confusing.  I built 2 systems 5+ years ago

 

i7 3930k w/ Asus Sabertooth x79 = 40 Lanes Support

i7 4960x w/ Asus x79-Deluxe = 40 Lanes Support

 

This was back in 2012, I'm not sure why Intel screwed around and really crippled stuff since then. Both Motherboards were advertised as PCIE 3.0 ready, but they are not really as both CPU are rated 5 GT/s DMI2 and PCIE 3.0 is 8 GT.   5 > 8

 

X79 still had DMI 2.0 not 3.0

 

First gen X79 boards were PCI-E 3.0 compatible but were never certified. Nvidia did put out a patch to make cards recognized at 3.0 not 2.0

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4 minutes ago, dhbartlett12 said:

So, what I'm gathering is that PCIe lanes are additive between the motherboard chipset and CPU. Correct?

and is it the same for memory channels?

Yes the CPU provides a certain amount of fast PCIe connections and the Chipset add more lanes but those are much slower and can't be used for graphics for example...they can be used for storage devices though...but they aren't as fast...to know how it works you have to check for your specific platform and motherboard because depending on the chipset it used and how the board is wired it can differ from board to board. 

 

Memory channels has nothing to do with it, it's something else, and it's very simple...most platforms that are your standard ''mainstream users'' platform support dual chanel (two channels) where as the HEDT (high end desktop (X79, X99 for example) and server platforms support quad-channels or more.


| CPU: Core i7-8700K @ 5.0ghz - 1.3v  Motherboard: Asus ROG STRIX Z370-E GAMING  CPU Cooler: Corsair H100i V2 |
| GPU: MSI GTX 1080Ti Gaming X Trio 2ghz OC  RAM: 16GB T-Force Delta RGB 3000mhz |
| Displays: Acer Predator XB270HU 1440p Gsync 144hz IPS Gaming monitor | Oculus Rift S

 

Read: My opinions on VR in it's current state, should YOU buy into it?

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Posted · Original PosterOP
7 hours ago, Ebony Falcon said:

Isn’t it time this got unpinned ? 

Yeah probably.

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