Ripping hair out my head trying to get PCIe graphics cards to run at full x16 speed

Kandsmerlin

Junior Member
May 6, 2019
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#1
Okay, the title says it all. I don't even know where to begin with this one. I initially started out with a asus z270e mb with a i7-7700k cpu. I installed an rtx 2080 ti into the top slot and it did once report as 16x. ever since then, it's only ever reported as 8x. So my first thought is that it must be something in the bios. I cleared the cmos, rebooted into the bios and the uefi still reports that the card is running at 8x instead of 16x speed. So then I thought that it might be a defective 2080 ti card. I replaced it with a 1080 ti card and it too only reported as 8x speed. So then I changed sides and swapped the 1080 ti card out for the amd vega 64. Again, the uefi still reported the card as running at 8x speeds.

This is when I then suspected that I might have a defective motherboard and/or cpu. So... I ordered a whole new motherboard and cpu. I'm now rocking an asus z390e with a i9-9900k cpu. I was sure this would fix the issue I was having... WRONG! After installing my 2080 ti once again. I noticed that the uefi persisted in reporting 8x instead of 16x speed. Now, keep in mind, this is on a new system, new cpu, no bios settings changed, installation and yet the pcie lanes on TWO SEPARATE builds will not operate at full 16x speeds.

Now, before you ask me which slot the graphics card is in, i'll tell you right now, it's only ever gone into the top most x16 slot (the one closest to the cpu)

What the *#&@ am I doing wrong here? Could it be that I have two defective cpu's and/or motherboards? (what's the likelihood of that?)

Could it be that the graphics card is recognizing that my monitor is only capable of 1920x1080 and therefore is reporting to the cpu that it only needs 8x lanes instead of the full x16? (that might be a bit ridiculous but I just dunno anymore.)

Please help one very frustrated individual grow his hair back by stopping him from pulling out more.

Thanks. :/
 

Stuka87

Diamond Member
Dec 10, 2010
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#2
Testing has shown that unless you are running like, PCIe 1.0, there is no performance difference between 8x and 16x. With you having a 7700k, it means you have PCIe 3.0, so there is nothing to gain by going from 8x to 16x. Hell, even 4x (on PCIe 3.0) is nearly fast enough for the majority of cards out there with only minor performance drops.

Is there a specific reason you want it to be 16x? Many 16x physical slots are only 8x electrical, as the other 8x lanes are directed to another PCIe slot.
 

Kandsmerlin

Junior Member
May 6, 2019
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#3
Yeah, it's a peace-of-mind issue. I wanna know that I don't somehow have defective hardware somewhere in my rig or rather... rigs (plural.) I know there's not much of a diff between graphics hardware running at 8x and graphics hardware running at 16x. Again, it's just very concerning to me that I can't seem to get anything running at 16x speeds on two separate motherboards using two separate cpu's… and while using separate gpu's.
 

Stuka87

Diamond Member
Dec 10, 2010
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#4
Chance are both boards are only wired for 8x, which is pretty common. Especially on Intel boards are they limit the number of PCIe lanes on lower end (as in not Xeon) motherboards.
 

Kandsmerlin

Junior Member
May 6, 2019
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#5
The Asus ROG STRIX line of motherboards are not low-end by any stretch of the imagination. I can see that the electrical contacts within the x16 slot are fully populated. unlike the middle "x16" slot where the electrical connectors only populate to about two thirds the entire length of the x16 slot which tells me that the middle slot is in fact x8 but that's not the one I'm plugging my card into. I'm plugging it into an actual x16 slot but am only getting x8 speeds.

I did mention before that I did originally get x16 out of the slot once before... but never again.

I did also discover something new... I plugged an old GTX 770 into the x16 slot and it appears at full x16 speed. So I can rule out my motherboard as being the problem. I'm leaning towards the 1080ti... the vega 64, and the 2080 ti all being defective. (as hard as it is to fathom and as improbable as it may be.)
 
Aug 25, 2001
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#6
Do you have an NVMe SSD? Those need lanes too.
 

Kandsmerlin

Junior Member
May 6, 2019
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#7
yes, i'm using a NVMe… but it's using lanes supplied by the PCH and not the CPU so I should technically still have at least 16 lanes available from the CPU
 

Stuka87

Diamond Member
Dec 10, 2010
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#8
Here is a exert from the Asus Z390 motherboard article on Asus' website:

Since Coffee Lake processors only make 16 PCIe lanes available to devices, we add a PLX bridge chip to split them evenly. Each of the physical x16 slots can connect to the chip with an eight-lane link sufficient for four-way SLI certification. Although the pathway to the CPU is still only 16 lanes wide, the bridge reduces traffic by passing communication between the cards directly, without involving the processor.

PCI Express lanes branching off the chipset are largely allocated to high-speed storage interfaces. Dual M.2 slots lurk under heatsinks and support longer 110-mm drives with higher capacities, while dual U.2 ports provide a cabled interface for enterprise-grade SSDs that incorporate their own cooling. Only one of the U.2 and M.2 slots share a PCIe link, so you can run a trio of four-lane NVMe drives simultaneously. Optional Pike kits add even more storage via eight-port SAS 12G RAID cards.
Which goes along with what I said above. Intel limits the number of PCIe lanes for anything that isn't a Xeon system. Intel artificially limits lots of things across their CPU lineup, this is one of them.
 

Kandsmerlin

Junior Member
May 6, 2019
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#9
No, that isn't right... I did get 16x speed on the z270 once... but never again... that one, i'm chalking up to a zapped motherboard.. =/

on the z390... I CAN GET A FULL 16x but the only card that does it for me is the gtx 770 (due to lack of having other cards to try.) it's only if I plug in the other gpu's I have... (keep in mind, i'm only pluging one in at a time. no crossfire or sli funny biz) then i'll only get the 8x speed... (the 1080ti, the vega 64, and the 2080 ti) are apparently all bad. I can't do anything bout the vega 64 or the 1080 ti unfortunately but i'm going to rma the 2080 ti.
 

Campy

Senior member
Jun 25, 2010
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#10
You should absolutely get x16 lanes on the first x16 slot, but only if the second x16 slot is not in use. Assuming you have no other PCIe cards, boot into Windows and use GPU-Z to check your PCIe link speed. On the first tab in the program see where it says "bus interface", it should tell you your current link speed. Remember to click the "?" to the right, and start the render test to put some load on the GPU to get an accurate reading.
 
Jan 12, 2013
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#11
Just thought I'd share my experience, as this happened to me on my last rig! I ran an ASRocks Extreme 6 with a 3770k and was never able to get my GPU running at x16.

When I put it in the one nearest the CPU for some reason it ran at x4 0_o

I eventually shifted it along one and GPUZ then said it was running at x8...

Never worked this out to be honest! Given there's no difference I just gave up worrying about it...
 

PrincessFrosty

Platinum Member
Feb 13, 2008
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#12
To mirror what has been said here, that real world benchmarks of high end video cards dont show any measurable difference between 8x and 16x PCI-E slots, GPUs just do not transfer that much data across the PCI-E bus, probably the most saturated they ever get is when actually filling vRAM during loading periods and that's limited by the drive speed way before the PCI-E bus (outside of very exceptional circumstances with many RAID NVMe drives)

But to address the question, one thing I'd do is read your motherboard manual and check to see what speeds the PCI-E slots support because typically on most motherboards different ports support different speeds and it's dependent on what you have installed in other PCI-E ports, for example some x16 capable ports are forced to x8 when other PCI-E are in use. Essentially there's only so many PCI-E lanes supported by the CPU itself and so if you're running other PCI-E cards you might not have enough spare for x16

If you do in fact have enough spare then it's likely just a setting in the BIOS that allows you to limit or specifically set the PCI-e port speed, you may need to enable advanced settings in the BIOS to unhide that stuff, again refer to your mobo manual on that. If you no longer have the manual you can typically find them online on the manufactureres website in pdf form.
 

owdi

Junior Member
Dec 13, 2009
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#13
Run a 3d benchmark in the background then check how many pcie lanes are in use. I believe pcie link state power management can reduce pcie lanes to save power.

I ran my GTX 1070 at PCIe 2.0 x8 for a year before I noticed it while messing around in System Information Viewer. BIOS update on my old motherboard fixes it, but I couldn't tell in the few games I play.
 
Jun 11, 2019
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#15
Can you run the gpuz in built bench and post a screenshot
 

EXCellR8

Diamond Member
Sep 1, 2010
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#16
Your motherboard probably won't allow it. Maybe?

Like they said. You should just do it at 8x instead of 16x pcie
OP wouldn't have created this thread if they were content with doing that... is the issue.
 

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