P965 chipset : Confused in lanes

[bonanza]

With P965 chipset :
Can I get full x8 bandwidth for RAID x8 card at 2nd PCIex16 slot and still get x16 bandwidth for video card ?

Please notice that the second card is a RAID controller instead of vga card, so it's NOT in crossfire/sli mode.


Thanks.

 

[Heidfirst]

no

 

[bigsnyder]

I believe most P965 boards with two "x16" slots only provide x4 to the second slot.
Thats my understanding anyway.

 

[bonanza]

Thank you all for replying.

I thought the x4 bandwidth on second slot apply only when it's in crossfire mode. I was wrong.

Ok, when the 2nd slot filled with a NON-VGA card (and run in x4), would it make the bandwidth on primary slot (VGA) down to x8 ?

 

[bigsnyder]

The first slot is a true x16 slot. I don't think PCI-E lanes can be rerouted to shift bandwidth, they
are hardwired.

 

[lopri]

Here is a quote from latest P35 review article.

Originally posted by: Gary Key
Considering the performance of the P35 chipset when set up correctly, we would love to see a manufacturer utilize a different PCIe controller chip setup and bring 8x8 CrossFire capability to this chipset. If abit or DFI is listening, please take a run at it as this type of engineering is something you excelled at previously. If not, we anxiously await the new X38 chipset even though the 975X obviously still has a lot of life left in it.

PCI-E lanes can be (and should be, IMO) manually adjusted. It's just that motherboard manufacturers don't utilize this functionality. I really don't understand this because we could do this like 3 years ago when SLI was first introduced. (x16 or x8/x8)

My personal opinion is that all PCI-E slots should be x16 length (physically) and users should be able to control how the electrical lanes are configured.

 

[lopri]

OK I found this. It's really sad that this is considered such a 'feature' for marketing. And for whatever reason, I have this growing dislike on heatpipe-messed motherboards.

http://plusd.itmedia.co.jp/pcuser/articles/0705/26/news003.html

 

[Peter]

Whether or not any given chipset's PCIE lane setup can be configured, that is entirely up to the chipset's design. Some are, some aren't. Consumer grade chipsets typically aren't, and the 965 appears to be no exception - and besides, it doesn't even have the 24 lanes the OP would need to begin with.

This chipset has the following:

On the northbridge, there is one PCIE-16 slot for graphics, splittable into twin PCIE-8 for SLI if supported.

The southbridge typically offers six lanes, configurable to 4-1-1 or 1-1-1-1-1-1.

This is what the mainboard designer has to make do with. No miracles.

More flexible configurations are found in NVidia chipsets, as well as in anyone's workstation/server chipsets.

 

[lopri]

I think it's rather got to do with mobo manufacturers' indifference (they just assume everyone will use one graphics card and leave it at that) or inability (can't figure it out in a timely/appropriate manner). The proof is that many of them actually do play around with the X4 bandwidth stemming from SB. (You can find out quite a few different manipulation of those X4 lanes)

It is true that X16 bandwidth is mainly for video cards, but like AT reviews commented several times, the P965/X35 are supporting CrossFire. X8 bandwidth wouldn't cripple the video cards, but X4 certainly does. And there are increasing amount of PCI-E peripherals on the market today. (lots of RAID controllers are X4/X8) On top of that, there are people who rather run their graphics cards on the bottom PCI-E x8 slot. Imagine a typical enthusiast's setup. Everything that heats up are concentrated in upper-left corner of the board. (PSU/CPU/VRM/NB/GPU) It's a severe imbalance, IMO. I myself use the bottom PEG on my 680i SLI board, and while the performance loss is little to none, the thermal advantage is huge. It will also hasten many peripherals' transition to PCI-E (from 32-bit PCI) if many boards allow flexible PCI-E configuration.

 

[Peter]

Well, like I said, on this particular family of chipsets, the north bridge lets you do 16 or 8-8, and the south bridge lets you do 4-1-1 or 1-1-1-1-1-1 split. Nothing else is available.

Now, mainboard makers have to make a choice when they want to support twin graphics slots. Run them both off the north bridge, and choke them both equally? Or leave the 1st one at full 16x, and choke the 2nd one twice as much by running it on a 4x link?

The former solution is better for pure graphics use; the second one lets you alternatively run non-graphics peripherals in the 2nd "graphics" slot without compromising graphics performance.

Want SLI _and_ high bandwidth peripherals? Get a workstation chipset. These have plenty more PCIE ports and lanes to play around with.

 

[bonanza]

Originally posted by: bigsnyder
The first slot is a true x16 slot. I don't think PCI-E lanes can be rerouted to shift bandwidth, they
are hardwired.

I asked this because on 975x the primary slot will down to x8 in crossfire mode (will it also down to x8 in non-crossfire mode ??).

 

[renethx]

Originally posted by: Peter
Well, like I said, on this particular family of chipsets, the north bridge lets you do 16 or 8-8, and the south bridge lets you do 4-1-1 or 1-1-1-1-1-1 split. Nothing else is available.

P965 MCH

• One PCI Express x16 link. Split 8-8 is not supported (at least officially) and there is no mobo supporting 8-8. The possible second PCI Express x16 is always connected to SB.

ICH8

• Total 6 PCI Express lanes: 4-1-1 or 2-1-1-1-1 (ASUS P5B-E Plus!) or 1-1-1-1-1-1. (But 1 lane is used for IDE controller and 1 lane for Gb LAN controller in almost every mobo. So actually only 4 lanes are available.)

Originally posted by: bonanza
I asked this because on 975x the primary slot will down to x8 in crossfire mode (will it also down to x8 in non-crossfire mode ??).

Yes, the first PCI Express x16 slot runs only at x8 mode if you install any PCI Epxress device in the second PCI Express x16 slot.

 

[lopri]

Originally posted by: Peter
Whether or not any given chipset's PCIE lane setup can be configured, that is entirely up to the chipset's design.

False. Although now I understand what you're trying to say. Yes, the real blame should be on Intel, but as you can see from the link above (ASUS CrossLink?) the PCI-E lanes configuration isn't entirely up to the specific chipset.

 

[Peter]

You're a bit quick in saying "false" there.

ASUS are putting a so-called PCI Express Switch device behind the core chipset's capabilities. These take an uplink of arbitrary width, and create /new/ downlink ports of arbitrary width. This isn't some magic trick, let alone an ASUS invention, just an extra chip that adds the functionality the chipset doesn't bring.

Like, (and in this case probably exactly this one), you'd be using a 32-lane six-port PCIE switch in 16-up 8-8-down configuration, connected to a chipset that runs a single 16-lane port.

http://www.plxtech.com/pdf/product_briefs/PEX8533_Product_Brief_v1.2_11Jan07.pdf

These are pretty expensive, and consume quite some power and board space too. You get the same level of flexibility, without the extra board space but not without the extra cost and power, straight from a workstation chipset.

 

[Peter]

renethx: Ah well then, so the 965 doesn't even /support/ the 8-8 split.