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PCIe Lane Bottlenecking?

Go to solution Solved by YoungBlade,

With that motherboard, you will not limit the PCIe bandwidth by installing another card.

 

Some motherboards, typically with two x16 slots, will reduce the bandwidth from x16 to x8 if you install a card in each - splitting the bandwidth across two. And some boards don't allow certain slots to be used simultaneously, or disable things like SATA connections that share slot bandwidth. Whether this happens or not will be specified in the manual.

Hello all!

 

Recently, I built my PC. I'm not quite familiar with PCIe lanes and to what extent they bottleneck each other.

 

My motherboard is a MSI A520M-A, so it only has two PCIe 3.0 slots (not including M.2,) one x1 and the other x16. I know that plugging in other hardware into PCIe means that the data speeds are shared and can bottleneck your other hardware. Last night, I installed a PCIe x1 to mini PCIe adapter for a Wi-Fi/Bluetooth card for my PC. My graphics card, currently an RX 270, is in the x16 slot. Later this month, I will be upgrading to a RTX 3060. There is no M.2 SSD installed.

 

I realize that I should expect this to possibly negatively affect my graphics card performance, but with only 2 PCIe slots (excluding M.2,) will a just a Wi-Fi card installed to the PCIe x1 slot affect my graphics card performance to such a degree that I probably shouldn't have my Wi-Fi card installed? Or would it be a negligible bottleneck that leaving it installed ultimately does not matter?

 

Thank you for your help!

 

Edit: CPU is Ryzen 7 5700X if that matters.

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11 minutes ago, DAPOPBEFASTONYOASS said:

will a just a Wi-Fi card installed to the PCIe x1 slot affect my graphics card performance to such a degree that I probably shouldn't have my Wi-Fi card installed?

Nope, the x16 slot is all 16 lanes dedicated to the CPU, nothing is going to change that. 

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7 minutes ago, DAPOPBEFASTONYOASS said:

I know that plugging in other hardware into PCIe means that the data speeds are shared and can bottleneck your other hardware

Not true.

 

7 minutes ago, DAPOPBEFASTONYOASS said:

I realize that I should expect this to possibly negatively affect my graphics card performance

It won't.

 

Sometimes lanes are allocated between slots (lane to either one or the other slot, not bandwidth shared), but never will an x1 (chipset) slot effect your x16 (CPU) slot.

 

The motherboard manual will detail (sometimes well, sometimes horribly) how the PCIe lanes are shared/allocated in different configurations.

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                           ┌─────────────── Office/Rack ────────────────────────────────────────────────────────────────────────────┐
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(500Mbps↑/500Mbps↓)                             UniFi CloudKey Gen2 (PoE) ─┴─ Veda (IPMI)           ╠═ Veda-NAS (HW Passthrough NIC)
╔═══════════════════════════════════════════════════════════════════════════════════════════════════╩═ Narrative (Asus USB 2.5G NIC)
║ ┌────── Closet ──────┐   ┌─────────────── Bedroom ──────────────────────────────────────────────────────┐
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   (PoE)                 │                        ╠═ Narrative (Cable Matters USB-PD 2.5G Ethernet Dongle)
                         │                        ╚═ Jesta Cannon*
                         │ ┌─────────────── Media Center ──────────────────────────────────┐
Notes:                   └─ UniFi Switch 8 ─────────┬─ UniFi Access Point nanoHD (PoE)
═══ is Multi-Gigabit                                ├─ Sony Playstation 4 
─── is Gigabit                                      ├─ Pioneer VSX-S520
* = cable passed to Bedroom from Media Center       ├─ Sony XR65A80K (Google TV)
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With that motherboard, you will not limit the PCIe bandwidth by installing another card.

 

Some motherboards, typically with two x16 slots, will reduce the bandwidth from x16 to x8 if you install a card in each - splitting the bandwidth across two. And some boards don't allow certain slots to be used simultaneously, or disable things like SATA connections that share slot bandwidth. Whether this happens or not will be specified in the manual.

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15 minutes ago, DAPOPBEFASTONYOASS said:

Hello all!

 

Recently, I built my PC. I'm not quite familiar with PCIe lanes and to what extent they bottleneck each other.

 

My motherboard is a MSI A520M-A, so it only has two PCIe 3.0 slots (not including M.2,) one x1 and the other x16. I know that plugging in other hardware into PCIe means that the data speeds are shared and can bottleneck your other hardware. Last night, I installed a PCIe x1 to mini PCIe adapter for a Wi-Fi/Bluetooth card for my PC. My graphics card, currently an RX 270, is in the x16 slot. Later this month, I will be upgrading to a RTX 3060. There is no M.2 SSD installed.

 

I realize that I should expect this to possibly negatively affect my graphics card performance, but with only 2 PCIe slots (excluding M.2,) will a just a Wi-Fi card installed to the PCIe x1 slot affect my graphics card performance to such a degree that I probably shouldn't have my Wi-Fi card installed? Or would it be a negligible bottleneck that leaving it installed ultimately does not matter?

 

Thank you for your help!

 

Edit: CPU is Ryzen 7 5700X if that matters.

The 5700x has the I/O die (previously referred to as the North Bridge) which is the CPU's primary peripheral connectivity. The 'chipset' (the South Bridge, it used to be called a chip-set for a reason) only has a 4x lane connectivity to the CPU that it shares with any device plugged into it.

 

Every motherboard can be different, but usually follow the same formula, where the top PCIe 16x and primary M.2 slot will usually be wired to the I/O die's (CPU's) PCIe connectivity, so its not shared bandwidth.

 

Back in the day, like my Z97 ROG Impact board, would drop the PCIe 16x slot to 8x if using the M.2 slot, because PCIe bandwidth was a lot more limited. Now a days, having 20x dedicated PCIe lanes is common, also considering its PCIe 4.0 or 5.0 which double or quadruple the bandwidth of PCIe 3.0 respectively.

 

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25 minutes ago, AbydosOne said:

Not true.

 

It won't.

 

Sometimes lanes are allocated between slots (lane to either one or the other slot, not bandwidth shared), but never will an x1 (chipset) slot effect your x16 (CPU) slot.

 

The motherboard manual will detail (sometimes well, sometimes horribly) how the PCIe lanes are shared/allocated in different configurations.

I should re-word to I believe from what I heard...thanks for the reply and the help!

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Wow guys, thanks for providing helpful information. While I'm well versed in quite a few things tech, sometimes I still get tripped up on an question that I have. You all provided great answers and while I wish I could mark all answers as a solution (as they the solution, as well as very helpful and insightful to how things work,) I had to pick the most straightforward answer should anybody else have the exact same question as I did.

 

Thanks again guys!

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