Understanding CPU Bandwidth

[expeditionist]

Hi guys

I wanted to add a 512GB Samsung 950 Pro M.2 to my system spec (see below) but I've been told that I would need to remove my 4 Port USB 3.0 Card due to my CPU bandwidth - to ensure that my CPU can still work properly with all PCI channels on the motherboard.

So I decided not to add the 512GB Samsung 950 Pro M.2 to my system because I don't want to remove my 4 Port USB 3.0 Card.

Now I want to add other components and I don't know how to calculate in general whether or not the available CPU bandwidth is a problem or not for other components.

I have to confess that I've read up on this and I'm confused. Could anybody explain to me the calculations necessary in easy language maybe?

Thanks

Intel Core i7 5820K Haswell-E (Overclocked up to 4.2 GHz)
Asus X99-S Intel X99 Motherboard
32GB Micron DDR4 2133MHz Memory (4 x 8GB)
2 x MSI NVIDIA GeForce GTX 970 4GB Graphics Cards
256GB Samsung 850ProSSD
256GB Crucial MX550 SSD
2 x 3TB Seagate 7200rpm Hard Disk - 6Gbps - RAID 1
4TB WD Black Hard Disk
Corsair Hydro Series H100i Liquid CPU Cooler
Akasa AK-ICR-14 Internal USB 3.0 Card Reader
4 Port USB 3.0 PCI-E Card
Corsair RM 750 PSU
Blu-Ray ReWriter
Fractal Design R4 Case
Windows 8.1 Pro

 

[geofelt]

I see no links to your system spec.

Regardless, I doubt that you would need to remove anything.

Then, I do not find the 950PRO to be a particularly worthwhile upgrade.
I changed from a 500gb 850 PRO and did not feel that performance changed much. Virus scans were faster, but that w as about all.

 

[expeditionist]

system spec updated - I'd like to understand the topic and calculation more please if I can - thanks?

 

[ulysses35]

What other items are you planning to add to your system ? M2 SSD's use PCI-E lanes, and the X99 chipset has up to 40 lanes available... obviously your graphics card uses some of these as will any other PCI-E cards you add.

I would suggest checking the motherboard manual to see how many PCI-E lanes are allocated to each graphics card slot, and then the M2 slot, finally your USB card... if both graphics cars use 16 lanes each, I think the M2 is another 4 PCI-E lanes.... leaving just 4 lanes for the USB card = total of 40.

 

[expeditionist]

Thanks - that's genius and makes some sort of sense to me now I think. If I understand correctly, being told that it is a CPU Bandwidth issue has diverted me from understanding that what I really need to be looking at is the motherboard and the PCI-E slots. At least I understand now that I'm free to add more SATA drives without impact.

The X99-S motherboard manual states 40-LANE CPU, 5 x PCI-E x16 slots with slot 5 sharing bandwidth with M.2 x4 - so your calculations make sense in common with what I was told before. The graphics cards are in slots 1 and 3 using 32 lanes, the M.2 drive would use 4 lanes, so there's only 4 lanes left for the USB card which is in PCI-E x16 slot 4.

How many lanes does the USB 3.0 4 port expansion card in PCI-E x16 slot 4 use? It's some sort of unbranded generic card I think so I have no info on it.

What I don't understand now is that I'm already using 3 x PCI-E x16 slots with the 2 graphics cards and the USB card and 3x16 is 48 so why am I not already over the 40 lanes maximum?

 

[jahmenmyst]

I would like to know the answer to what you want know myself. I'm rebuilding my Gaming PC and of course, also need to know how to sort out the motherboards bandwidth limitations found in the Specs. There are a lot of motherboards to choose from, but in the end they all have the same variation of shared band width between all the slots & plug-in's (PCIe x16,USB, SATA, M.2, MU) they all share bandwidth. Something I need to know and also know how to calculate in order to know what CPU, GPU, SSD, etc. to buy that will all be compatible. This thread was posted over 2.5 years ago and I'd like to think motherboards have gotten way better since then in having more bandwidth options.

 

[MasterMadBones]

We can't know for sure how many lanes the USB card uses, but we can look at the maximum bandwidth of both USB 3.0 and PCIe 3.0.
USB 3.0 delivers op to 5 Gb/s of bandwidth and PCIe 3.0 delivers just shy of 8 Gb/s per lane. This means that the card will not need more than 4 lanes of bandwidth. Maybe it uses 8 or 16 lanes by default. That being said, you should not experience any slowdown, unless the USB card forces 8 lanes and steals a couple from one of your GPUs. That normally shouldn't be an issue since PCIe slots are usually prioritized from the top to the bottom.

You shouldn't be too worried if it does steal some lanes from a GPU - even modern high-end GPUs don't even get close to saturating a the PCIe bus. Moving from x16 down to x8 results in a frame rate loss of less than 1% on average on a GTX 1080.