PCIE Lanes

[Guest]

I am getting a GTX 1070. I would like to know if using it in a 8x slot will give noticeably lesser performance compared to a 16x slot.

[r4tch3t]

No you will be fine, remember that pretty much all sli configurations run at 8x. As far as I remember gpus barely saturate a pic-e 3.0 x4.

[Guest]

1 minute ago, r4tch3t said:

No you will be fine, remember that pretty much all sli configurations run at 8x. As far as I remember gpus barely saturate a pic-e 3.0 x4.

If they barely use 4x why are they all 16x cards?

[Windows7ge]

You're fine. GPU's can't even saturate the full bandwidth of a PCI_ex16 2.0 slot let alone 3.0

[Windows7ge]

3 minutes ago, Emma Nieuwenhuis said:

If they barely use 4x why are they all 16x cards?

That is a good question of which I cannot answer. But then I could ask a similar question. Why do motherboards have PCI_ex16 physical slots but electrically they're x8 or even x4?...It's a conspiracy. 

[stealth80]

i believe its like 2 - 3% absolute max

[Guest]

1 minute ago, Windows7ge said:

That is a good question of which I cannot answer. But then I could ask a similar question. Why do motherboards have PCI_ex16 physical slots but electrically they're x8 or even x4?...It's a conspiracy. 

The motherboards have 16x slots that aren't wired for it so that they can hold 16x cards.

[stealth80]

http://www.gamersnexus.net/guides/2488-pci-e-3-x8-vs-x16-performance-impact-on-gpus

[Windows7ge]

1 minute ago, Emma Nieuwenhuis said:

The motherboards have 16x slots that aren't wired for it so that they can hold 16x cards.

Then that brings us back to your last question. Why are they all x16 cards if they can't utilize the bandwidth?

[Drak3]

6 minutes ago, Emma Nieuwenhuis said:

If they barely use 4x why are they all 16x cards?

@Windows7ge

 

Because GPUs need to be backwards compatible with PCIe 1.0, which they can now saturate close to the full 1.0 x16 speeds. It also helps provide assurance that future cards can have enough bandwidth moving forward.

[Electronics Wizardy]

13 minutes ago, Emma Nieuwenhuis said:

If they barely use 4x why are they all 16x cards?

Power

[Windows7ge]

4 minutes ago, Electronics Wizardy said:

Power

To my knowledge that is an incorrect statement. The small section of pins that are sectioned off from the rest are for power the rest is just for data,

[Zyndo]

12 minutes ago, Emma Nieuwenhuis said:

If they barely use 4x why are they all 16x cards?

Nobody here got the answer correct. the reason they have x16 slots and bandwidths on motherboards and GPU's is because GPU's (and other PCIe devices) can do a lot more than what more users have them for. most gaming workloads will not come even remotely close to  using those lanes.... but some high end compute programs and applications will definitely need to make use of it. PCIe lanes are largely used to communicate between components in a computer. in a gaming workload, the majority of the information is one way (from CPU to GPU) and there is a relatively small amount of it. Other, more hardcore, uses of your hardware can have tons of communication between the GPU and CPU that goes both ways and deals with tons of calculations and whatnot (don't ask me what they are, i don't know what they are, only that they exist)

You also have to remember that the PCIe 3.0 standard is going to be used for a great many years before it is replaced by 4.0... and they need to build enough performance headroom into the hardware so that people don't run into PCIe limitations before 4.0 comes out.

Basically, PCIe lanes are about intercommunication. MOST workloads that most people use will not come close to needing the full 16 lanes for a GPU, but that doesn't mean there are not specific use-cases where they are definitely needed.

[Drak3]

2 minutes ago, Zyndo said:

Nobody here got the answer correct. the reason they have x16 slots and bandwidths on motherboards and GPU's is because GPU's (and other PCIe devices) can do a lot more than what more users have them for. most gaming workloads will not come even remotely close to  using those lanes.... but some high end compute programs and applications will definitely need to make use of it. PCIe lanes are largely used to communicate between components in a computer. in a gaming workload, the majority of the information is one way (from CPU to GPU) and there is a relatively small amount of it. Other, more hardcore, uses of your hardware can have tons of communication between the GPU and CPU that goes both ways and deals with tons of calculations and whatnot (don't ask me what they are, i don't know what they are, only that they exist)

You also have to remember that the PCIe 3.0 standard is going to be used for a great many years before it is replaced by 4.0... and they need to build enough performance headroom into the hardware so that people don't run into PCIe limitations before 4.0 comes out.

Basically, PCIe lanes are about intercommunication. MOST workloads that most people use will not come close to needing the full 16 lanes for a GPU, but that doesn't mean there are not specific use-cases where they are definitely needed.

That'd make more sense if we were talking purely high end cards of recent years, and even then, saturating more than PCIe 3.0 x8 won't happen in most use case scenarios without needing cards along the like of the P6000 or Tesla cards. It'd make more sense to just use x8 slots on most cards if your answer was 100% correct.

 

It's a question without a single, real answer.

 

Between maximizing backwards and forwards compatibility easily (use a standard that is way overkill, and keep it mainstream and allow current systems to upgrade regardless, vs using the bare minimum slot and needing the next size up the next generation), power delivery (spec does actually provide set power limits to x1, x4, x8, and x16 slots and devices, 10W, 25W. 25W, 25W respectively, with the exception of up to 75W exclusively for GPUs and 25W for high power x1 devices), and even physically supporting the card (GPUs are rather heavy due to their heat sinks, and could potentially torque an x8 slot), we get viable answers.

[Drak3]

24 minutes ago, Windows7ge said:

To my knowledge that is an incorrect statement. The small section of pins that are sectioned off from the rest are for power the rest is just for data,

The PCIe spec specifies that:

x1 devices can use 10W off the slot, 25W if configured as 'high power'

x4, x8, and x16 devices can use 25W off the slot

x16 GPUs can use 75W off the slot, a special exception due to how power hungry these devices are.

[Windows7ge]

3 minutes ago, Drak3 said:

The PCIe spec specifies that:

x1 devices can use 10W off the slot, 25W if configured as 'high power'

x4, x8, and x16 devices can use 25W off the slot

x16 GPUs can use 75W off the slot, a special exception due to how power hungry these devices are.

OK. Informative, but it doesn't really deny my previous statement. I was told the first small section of pins is where the card can draw up to 75 watts the remainder of the slot is for data. I'm just saying what I've been told.

[Drak3]

1 minute ago, Windows7ge said:

OK. Informative, but it doesn't really deny my previous statement. I was told the first small section of pins is where the card can draw up to 75 watts the remainder of the slot is for data. I'm just saying what I've been told.

Except those cards need the data pins to validate that they can draw more than 25W, hence why the 75W exception has only been made for GPUs.

[Windows7ge]

2 minutes ago, Drak3 said:

Except those cards need the data pins to validate that they can draw more than 25W, hence why the 75W exception has only been made for GPUs.

Wait...OK, I think I'm starting to understand what you're saying. Based off of how many PCI_e lanes that card uses will affect how much power the motherboard will supply?

[Drak3]

1 minute ago, Windows7ge said:

Wait...OK, I think I'm starting to understand what you're saying. Based off of how many PCI_e lanes that card uses will affect how much power the motherboard will supply?

Sort of, there isn't exactly any mechanism in mainboards to support it. The devices just lose PCIe certification.

[Windows7ge]

1 minute ago, Drak3 said:

Sort of, there isn't exactly any mechanism in mainboards to support it. The devices just lose PCIe certification.

I was gonna say, on one of my GPU's I taped off half the pins (x16 to x8) because I ran out of usable PCI_e lanes. I have the GPU under high load all the time but I've had no malfunctions as if it were being suffocated for power in anyway so I think at x8 it's still pulling all the power it desires.