Just don't get this lanes thing

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RedRider14

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Just don't get this lanes thing

PostWed Jun 23, 2021 10:12 am

Hi
I am hoping someone can shed some light on my query. The PC store that builds my workstations doesn't seem to able to answer it definitively.

My motherboard is a Z390 and as you can see from my signature, I am running a 2070Super GPU and nvme drive.
I want to add a PCIe Thunderbolt card to the board (to run a BM MiniMonitor) but I am concerned that by adding it, it will share some of my existing PCI lanes, thereby slowing down my Davinci Resolve workflow.

Could anyone shed some light on this for me please.
Regards RED
Windows10 Home
Core i7 9700K 3.6 GHz
64 GB RAM
nVidia RTX 2070 Super GPU Studio Driver 462.59
Media drive Samsung 970 EVO 1TB SSD
Dual 32" monitors
Intensity Shuttle to 46" panel
DR Studio 17.2.2
Desktop Video 12.1
CEO Post Facility 40 years
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Kays Alatrakchi

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Re: Just don't get this lanes thing

PostSat Jun 26, 2021 4:25 pm

I'll give it my best try (I'm not an expert on hardware architecture but I think I understand it enough).

So think of PCI lanes as data pipelines that can function both individually or together for data to be streamed through. Regardless of the bandwidth max (determined by the various generations of PCI, like PCI 1.0, 2.0, 3.0 (what you have) and 4.0 (the latest gen), multiple lanes act as multipliers if a specific device needs that much more data.

The biggest PCI lane hogs are GPU's, so that 2070 Super can suck up to 16 lanes if it wants to. It typically doesn't need to and you'd never know the difference if it was only utilizing 8 lanes.

To further complicate things, PCI lanes can be provided by both the CPU and MotherBoard. Typically CPU lanes are used for GPU's, while Motherboard lanes are used for nVME drives and some minor PCI slots like those 1X and 4X.

As more lanes are used by GPU's, the number gets divided, so for instance a single 2070 can be placed in the top PCI slot and utilize 16X speeds (as I mentioned, overkill in most situations), however if you add a second GPU, then they would both max out at 8X. Add a third GPU and you might be down to 4X for each card (which might slow down the GPU's for some operations, but is still plenty fast for most).

In your particular case, I wouldn't worry about it. You have a single GPU and a single nVME drive (likely using your motherboard PCI lanes), so adding that Thunderbolt card will still keep you with plenty of speed, and even if your GPU might go down to 8X from 16X, you'll never notice it in Resolve.

As a matter of fact, I would even say that you have room to expand and add a second GPU, and even a second nVME drive (if your motherboard has a slot for it) without Resolve taking any performance hits.

In the future, if you decide to build a new workstation and are concerned about running out of PCI lanes, you might consider switching to either the higher end Intel CPU's like the 10980XE or XEON line, or alternatively switch to AMD Threadrippers which support considerably more PCI lanes and in the AMD case even support the latest PCI 4.0 generation which is even faster (hence requiring less lanes per card).
>>Kays Alatrakchi
Filmmaker based in Los Angeles, CA
http://moviesbykays.com
Resolve 17.2, Mac OS X 11.4 (Big Sur), i9 10980XE 18-core, 64Gb DDR4 RAM, RX 5700 XT GPU, Decklink Mini 4K Monitor, LG C9, USB 3.1 RAID
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RedRider14

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Re: Just don't get this lanes thing

PostSun Jun 27, 2021 1:37 am

I can't thank you enough Kays, for your excellent explanation.
Thank you for taking the time to reach out.
New workstation will be coming in about 6 months I think.
Kind regards Steve
Windows10 Home
Core i7 9700K 3.6 GHz
64 GB RAM
nVidia RTX 2070 Super GPU Studio Driver 462.59
Media drive Samsung 970 EVO 1TB SSD
Dual 32" monitors
Intensity Shuttle to 46" panel
DR Studio 17.2.2
Desktop Video 12.1
CEO Post Facility 40 years
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Steve Fishwick

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Re: Just don't get this lanes thing

PostSun Jun 27, 2021 11:16 am

Great description. I'm no techy guru on these things either but doesn't pci lane usage depend on the bus? for example most laptops (except MacBook Pros with 2 TB3 buses/4 ports) with 2 TB3 ports would have 1 bus of 4 lanes and if you load up 2 TB3 devices they would share those 4 lanes on that one bus and then sub-divide like you say, but that will not subdivide/affect the 16 x lanes for the gpu, since that's on a separate bus altogether?
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John Paines

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Re: Just don't get this lanes thing

PostSun Jun 27, 2021 4:20 pm

As I recall, Puget Systems tested GPUs designed for 16x running at 8x. There was no difference in performance. You run into other processing bottlenecks and hardware limitations long before hitting that one.

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