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Question on PCIe lane utilization

post #1 of 5
Thread Starter 
Hello,

I'm building a system with a 1080 ti video card, a 10gb/s quad port nic and a enteprise class raid controller card. The 10g nic and the raid controller have PCI 3.0 8 lane slots spec'd. When I'm gaming I'll be using a good amount of throughput on my video card but no utilization of my 10g nic and nearly none on my raid controller. When I'm backing up my server on the 10g nic (trunk) I'll be using my raid card and nic quite a bit but barely the video card. My question is is PCIe allocation fairly dynamic or do you need all the PCI lanes available at all times on all devices?

Not that it should matter but I will NOT be overclocking anything.
post #2 of 5
1- different motherboards = different lanes allocation
2- lane allocation is NOT dynamic : if a card is connected (used or unused) it will reserve some lanes
3- some motherboards, from UEFI, let you dis/enable some slots or change lanes allocation; you must read the manual
post #3 of 5
Thread Starter 
Thanks - might be worth waiting for the x299 after all.
post #4 of 5
Motherboards with PLX chip allow sharing of PCIe lanes between several (usually 2) PCIe slots. The idea is that both slots are not used at 100% at the same time. This would allow one to build a cheap Z270 system with many expansion cards in addition to GPU. It's also supposed to work for 3 and 4 way SLI for old NVidia or AMD GPUs. It's for people who are fine with less memory, 2 memory channel and fewer CPU cores that Z270 allows. For ordinary person using 1 or 2 GPUs it makes no sense to buy a board with PLX chip.

There are also X99 boards with PLX, Gigabyte Designare EX, ASRock Extreme 11 (this one has 2x PLX), but it doesn't usually make much sense with 40 lane CPU.

Without PLX, PCIe lanes are split between several slots and if only 1 is used then it gets all. If expansion card is inserted but is unused, the reserved bandwidth is wasted.

With Z270 one can still build a system with many expansion cards and use it for playing games, but you cannot do both without noticing. X99 is more advanced, there could be 2 M.2 drives, each having dedicated PCIe lanes to CPU, the system has 4 channel memory. So you could copy data between M.2 drives and still get decent speed, or the system could be used as server and for playing games at the same time. One also needs to look at cost. If there is no big price difference, X99 is better overall than Z270.

X99 despite being old, has big reserves. X299 will not bring a lot. CPUs will have 44 lanes instead of 40. Chipset will have more PCIe lanes (3.0, not 2.0), DMI will be 3.0. Hopefully they will not attach M.2 to chipset like in Z270. Waiting for X299 makes sense if one wants a system with Skylake CPU that will definitely beat that pesky Ryzen on the same clock frequency. smile.gif

But if you wait then AMD X390/X399 will probably also become available at less cost, offering similar features.
post #5 of 5
Thread Starter 
THanks
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