How many PCIE lanes does this board have?

[2plesk]

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?

 

[glugglug]

I think the switch chips allow the cards to communicate with each other at x16 speed, regardless of connectivity to the CPU?... but the total bandwidth between the CPU and all the cards would be limited to the number of lanes connecting the bridge to CPU itself.

Unfortunately it seems like Intel only wants to include a good number of lanes on chipsets for -E chips.

 

[ShintaiDK]

Its simply under provisioning. And you will never get those 64 PCIe lanes. As you write yourself, the CPU supports up to 40. You will only get full speed, if you dont use all the lane bandwidth elsewhere.

 

[2plesk]

@glugglug No, the switch chips serve as muxes to connect multiple cards to CPU. Just look at the diagram. Also, I'm not sure if direct communication between cards is possible via PCIE bus. They might have to go through CPU. But that doesn't matter, because we use SLI bridge for that.

@ShintaiDK I think you meant "over-provisioning".
My question was: is there any performance advantage of such pseudo "16x16x16x16" setup vs regular 8x8x8x8.
I think there could be if one or two of the cards for some reason happen to do more intensive transfers than the rest. I wonder if that ever happens.