How many PCIE lanes does this board have?

[p1esk]

Asus P9X79-E WS board claims to support 4 full speed (x16) PCIE cards (x16/x16/x16/x16 configuration). I think this is misleading.

4x16 slots require 64 PCIE lanes, however any single CPU only supports 40 lanes (which means at best x16/x16/x8 configurations). See the diagram here:

Asus used two PCIE switch chips (PLX PEX8747) to allow any of the 4 slots to communicate at x16 speeds, however, the 40 lanes CPU limitation is still in effect, so my understanding is that only 2 of those cards can talk to CPU at the same time at x16 speed.

I wonder, are there any performance gains provided by this additional multiplexing of x16 slots? For example, would there be any difference in performance if I plug 4 Titans into this board, or into the regular P9X79 WS (which can only run them in x8/x8/x8/x8 configuration). What about 3 Titans?

Anyone can explain?

 

[Eximo]

Most video cards won't saturate an 8x PCIe lane, so the quad configuration is fine.

You are right that the bottleneck on some of the X79 boards is the CPU. If all four cards were able to throw all of their data to or from the CPU simultaneously it would probably have to wait a few cycles to get it all through. Shouldn't be much of an issue in everyday circumstances, probably just benchmarking.

 

[babernet_1]

I've studied this for quite a while. Here's what I have found:

The Plex chips really only help when you have three or four graphics cards. They are slower for one, and even a bit slower for two even though they can allow a full 16X for both cards (not at the same time though).

Graphics cards are getting faster and faster. Now cards CAN max out a A PCI-E 3.0 X 8 interface per Maximum PC's latest findings. So if your card is running at X4 (as they would if you have a 3 or 4 card SLI/CF) then the PLEX does help despite it's slight latency.

Sooooooooooo, if you have three or four fast cards you could use that PLEX chip.

Edit, sorry, I just saw that you are talking about a X79 or 3960 type intel chip. I believe it could put out 16X 8x 8X 8X. Since we can see that the PLEX can really only help with the X4 issue found in triple or quad SLI/CF setups, I don't think it really helps for X79 issues except for perhaps some professional applications, but not for gaming.

 

[Eximo]

For a second there I was like, wait a second...

 

[babernet_1]

For a second there I was like, wait a second...

Yep. I have never given much of a look at the X79 type motherboards/3960 intel before to this detail.

It appears the Intel chips actually only put out two PCI-E 3.0 X16 channels for a total of X32. They have one mux for X16 or X8 X8 action. It looks like the motherboards can do this:

X16
X16 X16
X16 X8 X8
X8 X8 X8 X8

Note, only some boards have the extra mux for the fourth channel.

 

[p1esk]

A related question: on Asus P9X79 WS board (non E) there are 4 x16 slots that share 32 lanes, plus 2 additional x16 slots which share total of 8 lanes (x4 each).

If I populate the first 4 slots with Titans (for CUDA computing), and plug Quadro K600 card for display output, will K600 be able to drive two 2560x1440 monitors given the x4 PCIE 2.0 bandwidth in that slot?

 

[babernet_1]

A related question: on Asus P9X79 WS board (non E) there are 4 x16 slots that share 32 lanes, plus 2 additional x16 slots which share total of 8 lanes (x4 each).

If I populate the first 4 slots with Titans (for CUDA computing), and plug Quadro K600 card for display output, will K600 be able to drive two 2560x1440 monitors given the x4 PCIE 2.0 bandwidth in that slot?

As long as you are not gaming, you should do fine.

 

[p1esk]

Can mixing PCIE 3.0 and 2.0 cards on the same board cause issues? I really don't want K600 to bring Titans down to 2.0 level.

What do you think?