What does x4 bandwidth mean on a PCIe 2.0 x16 slot?

By The Teej
in Hardware Hangout

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Grand Master

The Teej

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Hey guys. I'm building my first ever PC, and as I was looking through the motherboards, I've noticed quite a lot of them state they have a PCIe 2.0 x16 slot, and then they have a PCIe 2.0 x16 slot, but with x4 bandwidth. The problem is, I don't know what they mean by x4 bandwidth. Here's the mobo in question:

http://www.scan.co.uk/products/gigabyte-ga-ph67a-ud3-b3-intel-h67-s-1155-pci-e-20-%28x16%29-ddr3-1333-sata-6gb-s-raid-atx

Basically, I'd like to use it for Crossfire support. I'm not looking into getting a second card straight away, but would like to get a mobo with Crossfire support so I can future proof myself when I want to get some extra graphics horsepower. It does say on the spec sheet it supports Crossfire, but I was worried that the x4 bandwith would limit the Crossfire capabilities in someway. I've tried googling for an explanation but haven't gotten very far. So, if someone could explain to me what that means, I'd be real appreciative.

Grand Master

TheLegendOfMart

Posted
Posted

Pretty much, its a 16x slot that runs at 4x electrically which means its going to have 1/4 the bandwidth. You are going to lose a lot of performance its not going to be much faster than a single card.

(I think) Its similar to my motherboard in that with 1 graphics card in it is x16 but with two in crossfire, the second card will only be 4x while the other is 16x. Making x8 overall for both in crossfire.

It doesnt work like that, the 4x slot is going to severely cripple one card which is going to affect the performance of the 16x slot card trying to keep in sync at the very least you are going to get massive microstutter.

Grand Master

The Teej

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So PCIe x16 only has 6.4GBPS bandwith? Surely that means there's a severe bottleneck with graphics cards like the 5870, a la the graphics card I'm probably going to get? Seen as the 5870 has a memory bandwith of 153GBPS? Or am I missing something completely?

(Why do I get the feeling that the more I learn the less I actually know? :laugh:)

Grand Master

Blackhearted

Posted
Posted

So PCIe x16 only has 6.4GBPS bandwidth? Surely that means there's a severe bottleneck with graphics cards like the 5870, a la the graphics card I'm probably going to get? Seen as the 5870 has a memory bandwidth of 153GBPS? Or am I missing something completely?

(Why do I get the feeling that the more I learn the less I actually know? :laugh:)

Those two bandwidth numbers are entirely different things.

That 153GB/s memory bandwidth on your video card is the internal bandwidth of the card. Meaning from the GPU to the ram on the card itself and vice versa. Where as The PCI-E bandwidth is the rate at which the card can communicate with other devices in the system(such as the CPU, or the 2nd card in sli/xfire).

Grand Master

The Teej

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Those two bandwidth numbers are entirely different things.

That 153GB/s memory bandwidth on your video card is the internal bandwidth of the card. Meaning from the GPU to the ram on the card itself and vice versa. Where as The PCI-E bandwidth is the rate at which the card can communicate with other devices in the system(such as the CPU, or the 2nd card in sli/xfire).

Oh, right, yeah, that makes sense. I guess I was having a blonde moment :blush:

Grand Master

Vice

Posted
Posted

If you really want to crossfire then pay the extra money for a motherboard which supports x16/x16 crossfire (something i wish I'd known before I bought my crappy x16/x4 motherboard :p)

Testing has shown that x16, x8 and x4 only makes a 1% difference in performance.

Even putting a HD 6990 in x16 or x4 resulted in a 1% performance loss. And that is the fastest card on the market. That 1% was within testing error range as the x4 link in testing was connected to the South Bridge and not the CPU's built in PCIe controller, this added latency.

What people need to realise is that there is not enough data going over the PCIe link to make a difference. The graphics cards want low latency to the CPU and system memory they don't need high bandwidth as all that graphics data is being stored on the GPU itself in its memory and processed by the on-card GPU. Very little PCIe activity. An x4 link still has 16Gb/ps (2GB/ps) of bandwidth. Do you seriously think the GPU is transferring 16Gb of data per second with the CPU and System Memory? - No.

I don't know where this misinformation started but it has been debunked over and over and over again.

Grand Master

Vice

Posted
Posted

Do you have a source on that? I would be pretty intrested to read through.

Just one source. I've however done the testing myself and saw a 0.5-1% difference between all 3 configurations. In short it makes no difference and again 16Gb/ps people that is the full x4 bandwidth. :laugh:

Grand Master

Vice

Posted
Posted

So if I stick another Radeon HD 5850 in my system (16x/4x), it will be not much less performance than in 16x/16x?

Could I put a 5830 in? Or does it have to be the exact same card?

For ATi's Crossfire the cards do not have to be identical but they do need to be close to each other. Here is AMD's Crossfire compatibility chart to help you understand which cards will work together in crossfire and yes x16 and x4 will work fine and you wont be able to discern any performance difference between x16 and x4.

http://i.imgur.com/Db0uz.jpg

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