Question Question about number and type of PCI ports ?

goguvarra

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Hello,
I wanted to know and understand the following.
1) What's the difference between the CPU supported PCIe lanes and the chipset ones?
I guess the speed, right? The chipset speed depends on the speed of the connection between the chipset and the CPU.

2) So for example the 11gen intel i9s --> https://ark.intel.com/content/www/us/en/ark/products/212325/intel-core-i9-11900k-processor-16m-cache-up-to-5-30-ghz.html - supports 20 pcie lanes at PCI 4 speeds. That means I can have a graphics card on the x16 slot and a x4 slot to fill up with whatever I want, let s say an M2 SSD.
The Intel Z590 chipset --> https://ark.intel.com/content/www/us/en/ark/products/196612/intel-z590-chipset.html - supports an additional 24 pci lanes and the total bus speed is 8 GB/s DMI.
So actually you can have at most 2x4 PCIe lanes. What's with the 24 PCI lanes in the specs?

3) So where do these modern motherboards get all their PCI slots from? It seems that there are more than what the specs can support.
For example this Gigabyte card -->https://www.gigabyte.com/us/Motherboard/Z590-GAMING-X-rev-10/sp#sp
seems to have:
CPU PCI lanes:
22 lanes - in the Expansion slots category
4 lanes - for an M2 SSD

Chipset PCI lanes:
8 lanes - from 2 x M2 SSD

Not to mention the 6 x SATA ports and all the USD ports, but I'm not sure if these also share the PCI bandwidth.
To top it all if you want Thunderbolt connectivity it also takes about 4 lanes of PCI to reach its true potential. where do you get these ports from?

So, where s the catch? How can the CPU have 26 lanes when it only supports 20 as it is written in the specs. What am I missing?
I'm trying to understand all this since i'm planning to buy a new system.
I swear 10 years ago I don't remember asking myself all these questions :)).
thank you.
 
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its kinda similar to network switches, but u can also compare it to USB
lets say u have usb 3.0, which has 5Gbit with single port, now u use usb hub with 4 ports, so now instead of single port u get four 5Gbit ports
same goes with chipset...its like that USB hub...tons of devices connected, but limited to cpu <-> chipset lanes speed
 

goguvarra

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now u use usb hub with 4 ports, so now instead of single port u get four 5Gbit ports
You are saying you get 4 ports but each draw from that 5Gbit bandwidth, right? They can't all be 5Gbit individually.

So for my intel Z590 chipset --> https://ark.intel.com/content/www/us/en/ark/products/196612/intel-z590-chipset.html it would mean that it shares the 8GB/s bandwidth between all those 24 lanes. But realistically you would never find all these ports on a motherboard because they would be very slow.

But still there are more pci lanes than are supported on the product specs of the CPU and the chipset in my examples above, something doesn't add up. That gigabyte card has 34 lanes of PCI, where do they all come from? :)
 
You are saying you get 4 ports but each draw from that 5Gbit bandwidth, right? They can't all be 5Gbit individually.

So for my intel Z590 chipset --> https://ark.intel.com/content/www/us/en/ark/products/196612/intel-z590-chipset.html it would mean that it shares the 8GB/s bandwidth between all those 24 lanes. But realistically you would never find all these ports on a motherboard because they would be very slow.

But still there are more pci lanes than are supported on the product specs of the CPU and the chipset in my examples above, something doesn't add up. That gigabyte card has 34 lanes of PCI, where do they all come from? :)
20 from cpu https://ark.intel.com/content/www/us/en/ark/products/212325/intel-core-i9-11900k-processor-16m-cache-up-to-5-30-ghz.html
24 from chipset https://ark.intel.com/content/www/us/en/ark/products/196612/intel-z590-chipset.html
44 lanes is maximum what i9 gen11 combined with z590 chipset have
 

goguvarra

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hmm...yes you are right, and my calculations were correct it seems. :)
But still, how can you have the necessary speed on every lane, especially the chipset lanes. The totla bandwidth available is 8GB. if you split that onto 24 lanes it would mean each lane gets about 333 MB/s. That 's not even PCI 2.0 speed. What am I missing?
 
8GB/s is chipset bandwith limit no matter how many lanes are used
each lane can have its maximum speed (1GB/s) as long chipset 8GB/s doesnt becomes bottleneck, its not like every device will need that much speed at same time, but ye, once u hit 8 GB/s limit, some devices will be bottlenecked by chipset speed limit
 

goguvarra

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Yes, that's what I was thinking.
But if you were to use all of the 24 lanes that the chipset provides then each lane will have at most 333MB/s which is much lower than what PCI 3 can provide.
So probably you won't use more than 8 lanes as in your example above, so as to maintain 1GB/s per lane which is the PCI 3 standard.
Or maybe you use more and you don't really have PCI 3 speeds, because as you said not every device needs that much speed anyways. In my example in the first post, that Gigabyte motherboard couldn't supply PCI 3 speeds to all of its ports, there s too many of them.
 
u can fully utilise two 3.0 x4 m2 drives at full speed (until they run out of theirs ram cache :p)
well anywy, if you want to turn your pc into NAS server, u still have 40GB/s from CPU and 8GB/s from chipset, that should be still plenty enough for heavy network and data traffic
 

goguvarra

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yes, two PCI 3 x 8 lanes sound just about right :). But what do you mean until they run out of cache, how much speed does this type of drive lose when that happens. I'm guessing where talking about NVMe drives right?
Sorry to jump to a bit different topic but i'm trying to catch up a bit on all these new techs and i'm curious.
thanks for your answers :)
 
yes, two PCI 3 x 8 lanes sound just about right :). But what do you mean until they run out of cache, how much speed does this type of drive lose when that happens. I'm guessing where talking about NVMe drives right?
Sorry to jump to a bit different topic but i'm trying to catch up a bit on all these new techs and i'm curious.
thanks for your answers :)
hmm that depends on drive used....but 3.5GB/s isnt drive speed, but ram cache speed, nand isnt that fast..smt like 500MB/s can be expected
 

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