Single GPU & PCI-e 8x or 16x needed?

[phatstar]

Hi, Crew-
I'm thinking the Gigabyte GA-P55A-UD6 is my new motherboard of choice for my i7 860.

A reviewer of this motherboard over at Newegg posited that a single GPU (in this case a Zotac GTX 275 Amp!) suffers a HUGE decrease in performance when limited to an 8x PCI-e channel instead of the usual 16x channel; and that Toms has the evidence to back it up. Where is this review as I can't seem to find it? (and anything at anandtech.com or anywhere else would be useful, too).

[n.b. on the UD6 the main PCI-e is limited to 8x on the 16x slot when USB 3.0 & Sata 6Gb/s is enabled]

Thanks!
-ps

 

[brockh]

USB 3.0 and Sata 6Gb/s aren't worth the tradeoff regardless. 😛 Most reviews show they only provide a minimal performance increase unless you need the extra speed for something very intensive and specific, like workstations I suppose. Is there a special reason you need to have those features at the expense of the full PCI-e slot?

 

[phatstar]

Well, correct me where I'm wrong, but I always understood that the bandwidth in the PCI-e 2.0 lanes vastly outpaced the throughput of the GPUs (except maybe the top-ended GPUs). If this is true, then I can have my cake & eat it too by having Sata 6Gb/s, USB3.0, as well as enough PCI-e bandwidth for a single GPU when playing games. Of course, if 8x in the 16x 2.0 slot isn't enough then I'll have to pick my poisons (and USB 3.0 & Sata 6Gb/s can be disabled in the BIOS thus bringing the board back to full 16x and 8x in the two 16x 2.0 slots, respectively).

I'm ready to upgrade to a Core i7 and don't wish to wait any longer (but don't want to feel an urgency to upgrade once USB 3.0 & Sata 6Gb/s start to saturate the market).

So, anyone have an article or two?
Thx!

 

[phatstar]

Answering my own question, it appears 8x (2.0) from 16x debilitates a GPU performance roughly 2%:
http://www.techpowerup.com/reviews/AMD/HD_5870_PCI-Express_Scaling/25.html

(original article: http://www.techpowerup.com/reviews/AMD/HD_5870_PCI-Express_Scaling/1.html )

In fact, it appears 4x (2.0) is totally sufficient to get the majority out of the PCI-e lane.

Anyone have any evidence to suggest 16x (2.0) is necessary for a single GPU system versus 8x?

Thx!

 

[phatstar]

A bit older of a review, but 'confirmed':
http://www.tomshardware.com/reviews/pci-express-2.0,1915.html

Anyone see anything more recent?

 

[phatstar]

http://www.anandtech.com/mb/showdoc.aspx?i=3649&p=1

(showing the 8x scores are similar to the same CPUs running at 16x)

The evidence appears overwhelming, at least at this stage in GPU development. No?