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Klemen

PCIE 4.0 system usage of PCIe 3 card

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Posted · Original PosterOP

There should be none to zero differences because GPUs dont even use all the bandwidths from PCIe 3.0 (16x).

Even using PCIe 8x makes no difference at all.

The nice thing about this is, that there should be more PCIe-m2 slots, because 2x PCIe 4.0 is as fast as PCIe 3.0 4x.

 

The above is from reddit post 

 

I wanna ask if you have CPU with PCIe 4.0 and also motherboard that supports it but GPU that is one gen down - PCIe 3.0

Does that mean that the card automatically receives only max 8x PCIe lanes and uses only 4x lanes, unlike on 3.0PCIe system where it uses "16x lanes" but in the real world only 8x lanes? 
 

PCIE 4.0 doubles the "bandwidth" that's why i said 8x lanes and 4x lanes which in 3.0 terms are equal to 16x and 8x lanes or 16GB and 8GB and (4.0 PCIe 8x lanes = 16GB and 4x lanes 8GB)

so to my initial question, would the card then really only need 8/4x lanes on pcie 4.0 system while card remains 3.0pcie? 

 

"using PCIe 4.0 card on PCIe 4.0 system remains the same as PCIe 3.0 -at least the way it operates. Card would still require 16x lanes and used only 8x lanes, but the difference here would be only in double bandwidth bcuz whole pcie has been designed that way,

 

so pcie4 card with 4.0 system (mobo, cpu) would work the same way as having whole system with pcie3 (pcie3.0 mobo, cpu, gpu) speaking hypothetically that is. but it would work differently if you put 3.0 card in 4.0 system"

 

meaning 3.0 card wouldnt eat 16 lanes on 4.0 system but rather only 8 lanes (8lanes meaning (8*2GB=16GB) instead of 8*1GB which would be on pcie 3.0 and actaully it would be 16*1GB)

 

 

IK IT IS CONFUSING I'm sorry in ADVANCE

 

 

 

 

 

 

 

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The pci-e 4.0 bus gets downgraded to pci-e 3.0 when you insert a video card that can only use pci-e 3.0

 

pci-e 2.0 means 500 MB/s per lane, pci-e 3.0 means approx. 970 MB/s per lane, pci-e 4.0 means approx. 1940 MB/s per lane.  

 

So when you insert a pci-e 3.0 video card into a pci-e 4.0 slot, it will simply run at pci-e 3.0 x16, with a maximum available bandwidth of 16 x 970 MB/s or around 15.5 GB/s

 

The number of pci-e lanes doesn't change. 

 

In a lot of games the difference between pci-e 3.0 x8 (~7.5 GB/s) and pci-e 3.0 x16 ( ~15.5 GB/s) is let's say less than 3% fps ... something like 100fps vs 97 fps. 

With pci-e 4.0 video cards, it should be even less noticeable, as pci-e 4.0 x8 is same bandwidth as pci-e 3.0 x16. 

 

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If you have 16x 4.0 available lanes and 3.0 cards and you put 2 cards into the PC, they will both run at x8 3.0 speed so there is no benefit from the slots being 4.0.

 

If the cards are 4.0 they will run at x8 4.0 speed so there is benefit from the 4.0 link as each card is effectively at x16 3.0 bandwidth. 

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you're getting confused.

 

a PCIe3.0 16x card in a PCIe4.0 16x slot will run at PCIe3.0 16x

 

the x configuration doesn't change unless it's down to chipset or manual control for lane sharing, generation doesn't affect that

 

the effective "x" amount changes because the bandwidth per generation roughly doubles, but the actual number of lanes used does not change between generations. the number of lanes used only changes if it's manually specified or if the lanes are being split out to be shared between devices


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Posted · Original PosterOP
Just now, WereCat said:

If you have 16x 4.0 available lanes and 3.0 cards and you put 2 cards into the PC, they will both run at x8 3.0 speed so there is no benefit from the slots being 4.0.

 

If the cards are 4.0 they will run at x8 4.0 speed so there is benefit from the 4.0 link as each card is effectively at x16 3.0 bandwidth. 

aha okay, but in theory, the cards would only consume 4x lanes or 8GB or in terms of 3.0 speaking 8lanes each 

bcuz more than 8GB of bandwidth almost non of 3.0 card uses 

just like 3.0.. you have dedicated 16lanes but in reality it only uses 8lanes bcuz it doesnt need more than that 99% of the time

am i correct?

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Just now, Klemen said:

aha okay, but in theory, the cards would only consume 4x lanes or 8GB or in terms of 3.0 speaking 8lanes each 

bcuz more than 8GB of bandwidth almost non of 3.0 card uses 

just like 3.0.. you have dedicated 16lanes but in reality it only uses 8lanes bcuz it doesnt need more than that 99% of the time

am i correct?

No, the lane number doesn't change, the speed of each does.

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Posted · Original PosterOP
2 minutes ago, yaboistar said:

you're getting confused.

 

a PCIe3.0 16x card in a PCIe4.0 16x slot will run at PCIe3.0 16x

 

the x configuration doesn't change unless it's down to chipset or manual control for lane sharing, generation doesn't affect that

so if i put 4.0 NVMe then that nvme would run on 4.0 while gpu would run on 3.0?

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13 minutes ago, Klemen said:

Even using PCIe 8x makes no difference at all.

That's just wrong. There's plenty of games that will completely saturate PCIe 3.0 at 8x. It is true however, that there's not much point to PCIe 4.0 one way or another right now. Even the new 30-series cards, while PCIe 4.0, do not use more than PCIe 3.0 16x can provide.

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Posted · Original PosterOP
Just now, KaitouX said:

No, the lane number doesn't change, the speed of each does.

aha okay so only speed is affected while lanes  remain the same, basically almost nothing dramatically changes at all, right?

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Just now, Klemen said:

so if i put 4.0 NVMe then that nvme would run on 4.0 while gpu would run on 3.0?

providing the slot you're putting that 4.0 device into is a 4.0 slot, yes, that's correct


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you split by lanes, and then the lanes will run at the lowest pcie gen between two devices

 

you cant split by bandwidth unfortunately


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Posted · Original PosterOP
Just now, Chris Pratt said:

 Even the new 30-series cards, while PCIe 4.0, do not use more than PCIe 3.0 16x can provide.

meaning that equals to 8x on PCIe 4.0?

and the def of that sentence doesnt changes and still applies here

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Posted · Original PosterOP
Just now, yaboistar said:

providing the slot you're putting that 4.0 device into is a 4.0 slot, yes, that's correct

aha okay thx 

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Just now, Klemen said:

so if i put 4.0 NVMe then that nvme would run on 4.0 while gpu would run on 3.0?

The answer is it depends on how the pci-e lanes are routed between the cpu and slots and m.2 connectors. on some motherboards, the m2. connector may be downgraded to pci-e 3.0 speeds as well, but most of the cases it should stay at pci-e 4.0

 

Either way, a M.2 x4 pci-e 3.0 SSD  typically reaches up to 3.5 GB of transfer speed, and most m.2 pci-e 4.0 SSDs go up to 5.5 GB/s  - do you really think you'd notice the difference between 3.5 GB/s and 5.5 GB/s ?

A typical game will load a few hundred MB of data from drive when you load a game level, and that takes seconds to read from ssd... a pci-e 4.0 ssd won't make your games faster compared to a pci-e 3.0 ssd, most of the time is used processing the data that was read and converting it to the formats your video card understands and supports, and that's done in cpu and ram. 

 

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Posted · Original PosterOP
1 minute ago, Moonzy said:

you cant split by bandwidth unfortunately

yea, i didnt mean that

 

i meant if the cards would run similarly like you know if you have 1 card and compering it to two system one with 3.0 and another one with 4.0

3.0 would use 16x lanes

while 4.0 could only use 8x lanes, but the bandwidth would remain the same 

 

- speaking that "that gpu is both 4.0 and 3.0 at the same time, HYPOTHETICALLY speaking"

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Just now, Klemen said:

3.0 would use 16x lanes

while 4.0 could only use 8x lanes, but the bandwidth would remain the same

yes, then you would see no performance difference, since bandwidth is theoretically the same

 

and yes, if a GPU is capable of 4.0, it is also capable of 3.0, also 2.0, and 1.1


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26 minutes ago, Klemen said:

There should be none to zero differences because GPUs dont even use all the bandwidths from PCIe 3.0 (16x).

WE KNOW ...wtf


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Posted · Original PosterOP
20 minutes ago, mariushm said:

The answer is it depends on how the pci-e lanes are routed between the cpu and slots and m.2 connectors. on some motherboards, the m2. connector may be downgraded to pci-e 3.0 speeds as well, but most of the cases it should stay at pci-e 4.0

 

 

 

well i mean like, NVMe take lanes from  chipset to put that in prospective, i think comparing B550 and X570 motherboard's chipsets,

chipset on B550 uses 3.0 lanes while, X570 chipset uses 4.0 pcie lanes

if that's what you meant by "it depends" 

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Posted · Original PosterOP
36 minutes ago, mariushm said:

 

 

In a lot of games the difference between pci-e 3.0 x8 (~7.5 GB/s) and pci-e 3.0 x16 ( ~15.5 GB/s) is let's say less than 3% fps ... something like 100fps vs 97 fps. 

With pci-e 4.0 video cards, it should be even less noticeable, as pci-e 4.0 x8 is same bandwidth as pci-e 3.0 x16. 

 

Yea, i agree. but that means cards that are 4.0 and are in the system that supports 4.0 will be automatically put to 8x lanes

like 3.0 cards are set to 16x lanes? 

 

You know, i have 3.0 card and if i check bios or aka uefi, it says it's using "16x lanes"

does then using 4.0 card displays 8x lanes? (speaking 4.0 8x is the same as 3.0 16x ofc) 

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Unless you are doing 320Hz Gaming you have no need for PCI-e 4.0 on graphics.

If you are doing 8k, 4k or 1440p gaming you are unlikely to have enough GPU power to actually hit that level of FPS to trigger a bottleneck.

Remember it isn't 8k or 4k that is filling the pipeline its the calls for the FPS, the resolution is not a big issue.

 

a 3070,3080 or 3090 at 1080p gaming with a 320Hz screen may be able to bottleneck a 3.0 Slot and require a 4.0 pcie slot.

from memory 190fps was around the limit on the 2080ti where performance started to drop but there was enough GPU power to push it faster.

 

Maybe the 30 series will show us the limit, but realistically you need specific monitor and lower res to see his problem, most players are not in the 200+ area of FPS to see this problem yet...

 

if you are pumping out 100FPS and have something like a gsync 100Hz monitor pushing two m.2 drives at 3Gbps you'd likely have enough room to do that without any issues.

Even if you were gaming like that what game is going to be pulling down 6Gbps for more than a few seconds to load, its not going to be pulling down 6Gbps non stop not even Cyberpunk is going to be that big. you'd have to have a game that is 200GB or something that is drawing m.2 drive calls constantly, that type of action is going to be game developed with a loading screen more than likely.

 

This is likely an issue we'd see with old hardware in 2-3 years time with a modern title :)

 


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8 hours ago, Klemen said:

well i mean like, NVMe take lanes from  chipset to put that in prospective, i think comparing B550 and X570 motherboard's chipsets,

chipset on B550 uses 3.0 lanes while, X570 chipset uses 4.0 pcie lanes

if that's what you meant by "it depends" 

nvme doesn't take lanes ... it's just a specification, like a standard that defines how to connect and interact with storage that connects directly to cpu

 

Otherwise... no.

 

The processor has a built in pci-e controller, which creates 24 pci-e lanes. These 24 lanes are arranged like this:

* 16 pci-e lans go to video card slot, or the first two pci-e x16 (x8 electrically)

* 4 pci-e lanes go to the first m.2 conector

* 4 pci-e lanes are used to connect the chipset to cpu.

So actually 20 pci-e lanes  are acessible by users.

 

The chipset takes the 4 pci-e lanes and inside the chip there's a pci-e controller which creates more pci-e lanes : 8 pci-e 3.0 lanes for the B550 models, 8 pci-e 4.0 lanes for the x570 model. 

 

On both B550 and x570 the pci-e lanes going to pci-e x16 slot and m.2 connector are pci-e 4.0 , just the 4 lanes going to chipset are limited to pci-e 3.0.

 

However, I know at least one motherboard that contains that has 3 m.2 connector, all capable of pci-e 4.0

They do this by splitting the pci-e x16 from the video card into a pci-e x8 and 2  pci-e x4. So because they do this, if you insert a pci-e 3.0 in the two m.2 connectors that are involved, the pci-e x8 going to video card will he downgraded to pci-e 3.0 as well

 

 

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Hardware Unboxed did some testing on this today and i will attach the images from their youtube video.

 

4.02.PNG

4.01.PNG


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