The Mysterious Ways of the PLX PEX 8747

[zliqdedo]

After reading a ton of technical data on PLX’s website and a few articles on the subject (including Ian Cutress’), I’m still at a bit of a loss. I would like to confirm my observations.

IVB supports 16 PCIe 3.0 lanes directly from the CPU for a total bandwidth of 16 GB/s, there’s nothing PLX can change about that. What their chip effectively does, as far as I can grasp, is manage available bandwidth in an intelligent way at the cost of latency which results in a small overhead (~2%). It increases lane count, but not actual bandwidth thus enabling tri-SLI on IVB. However, the bandwidth available to those three GPUs is still 16 GB/s, and not 32 GB/s.

Correct me if I’m wrong on any count. Please, respond only if you’re really informed.

 

[willomz]

You are correct.

 

[UaVaj]

until launch of gtx780.

still chewing over
z77+plx w/ IB/LGA-1155 16 lanes of PCIe 3.0 (16GB/s)
or
x79 w/ SB-E/IB-E/LGA-2011 32 lanes of PCIe 3.0 (32GB/s)

 

[zliqdedo]

I guess there's nothing more to it.

After a painful research, It appears that running a 3/4-GPU setup on IVB with a PLX chip is at least equal to an SB-E alternative. It should be even better with Haswell. It seems that 28nm GPUs can run adequately enough in 3/4-GPU setups at ~4 GB/s each. Of course, it goes without saying that such a setup would be used for the most GPU-demanding scenarios, such as: multi-monitor, 4K and S3D gaming.

It's interesting to see if next-gen GPUs require more bandwidth for these setups. Also, if Intel releases Haswell-E soon, that would change the landscape.

It's quite the niche, but I was interested to see how it can be properly addressed.

 

[UaVaj]

passing on 780 sli. sticking with 680 tri-sli. pickup an asus p8z77 ws.


you are right and you are wrong.

fact: a 680/7970 needs ~8x of pcie 3.0 for no bottleneck maximum performance.

for 680 tri sli or maybe 680 quad sli. perhap one can get away with Z77+PLX.
680 sli scaling is 85%. 680 tri-sli scaling is 80%. 680 quad-sli scalilng is 40%. could that be due to out of pcie bandwidth or poor sli profiles. imho - it is the prior.

for 780/titan tri-sli or 780/titan quad-sli. it is time to step up to X79.


Upgrading from IVB to Haswell is a joke. so until IVB-E debut this fall. that will be a real upgrade.