The PLX Chip and 2-Way SLI

[borntofly]

Hey Guys,


I have read everything on Anandtech and elsewhere that I could find on the PLX PEX 8747 chip. I have been researching components for a Z77-based gaming build, which will be a 2-Way SLI setup with dual 680s.


Apparently, from what I can determine, the current crop of Z77 boards which do have the PLX chip (such as the ASUS Maximus V Formula) implement it such that we have one PCIe 3.0 slot running all 16 lanes and a second 3.0 slot running 8 lanes.


My question is this: Given that the two graphics cards will communicate with each other via the SLI bridge, how important to framerates is the use of two 16-lane PCIe 3.0 slots (a' la an X79 board)? My supposition is that in the real world having the second graphics card in an 8-lane slot will exhibit no appreciable degradation in performance compared to a dual 16-lane setup, but I have no benchmarking or other data to support this.


I am therefore soliciting your knowledge and opinions.


NOTE: If you guys know of a Z77 motherboard which does implements PLX with two 16-lane slots, please post on that as well, as it would likely render this thread mute.


Thank you so much! You guys are the best.

 

[blackened23]

I've used both the P8Z77 deluxe and premium motherboards, the first which uses CPU lanes and is dual x8 and the latter which is dual x16 with the PLX chip.

I can tell you the PLX chip adds a significant amount of latency, enough so to completely negate any benefit of dual x16; in fact my benchmark scores in 3dmark 11 with dual msi lightning 680s were quite a bit worse with the dual x16 in the premium motherboard (using the PLX chip) whereas the CPU lanes with dual x8 on the z77 deluxe scored better in benchmarks and provided better framerates in games.

Bottom line:

The PLX chip is only beneficial for quad SLI. In dual SLI scenarios, it is worse in every way. I've done quite a few benchmarks to come to this conclusion myself, and there is mention of the PLX chip introducing latency on many tech sites.

My suggestion is to avoid any motherboard which uses the PLX chip, the only benefit it provides is allowing QUAD sli. If you aren't using Quad SLI with a PLX chip, you are losing performance and doing your system a disservice. Dual SLI with the PLX chip will perform worse than dual x8 on a "normal" motherboard.

 

[blackened23]

I should add that some motherboards with the PLX chip will disable the PLX chip unless you're using quad SLI. However, 99 out of 100 of them do not.

I seem to recall the maximus V being one of the boards that disables the PLX chip with dual SLI, but i'm not 100% sure. Don't count on this though - most motherboards use the PLX chip for everything. My suggestion again is to avoid any motherboard with the PLX chip unless you're using quad sli.

 

[rgallant]

"Apparently, from what I can determine, the current crop of Z77 boards which do have the PLX chip (such as the ASUS Maximus V Formula) implement it such that we have one PCIe 3.0 slot running all 16 lanes and a second 3.0 slot running 8 lanes."

-the MVF is not a full plx chip from what I know , it only splits the 2nd and third pic-e slots for tri cross fire @ x? + x4 + x4.
-sli 2 cards run @ x8+x8 , 3 card sli is not supported on the MVF with x4+x4+x4.

-the MVE with the full plx chip and mb lanes bypasses[more added chips] the plx chip for 2 card sli @ x8 + x8 for better performance as blackened23 has stated.

 

[borntofly]

Wow, OK that is some really helpful info, blackened23 and rgallant. I had read about the latency, but it was all theoretical stuff - no real experiential data.


rgallant, I was hoping that the MVF would work for my scenario, and it sounds like it may. I will see what I can conclusively verify on the MVF disabling or not using PLX with only two cards installed.


I guess if I wanted dual 16-slots I'd have to go with X79, right? Raises the real world question again: Would dual x16+x16 in an X79 really buy many framerates over x8+x8 in Ivy Bridge?


Thanks again, guys.

 

[blackened23]

I think hardOCP has done direct x8/x16 comparisons in the past and found negligible differences, i'm not sure if that has changed.

Gaming benchmarks are pretty much even between 3770k and 3930k, so I don't think there's a compelling reason to get x79 unless you use highly threaded applications (ie not many games do this). X79 is the best method of getting true dual x16 pci express, but I haven't seen evidence to indicate it being better performance wise than Z77 with dual x8 for games(someone please correct me if i'm wrong)

 

[borntofly]

Thanks, blacked23. I found those HardOCP links and will check them out now. Just got back from under the house where I ran 50 feet of Toslink (two 25' coupled together) from my computer to my TV. No signal loss that I could dectect. Already had the HDMI connected, so now I can game on the big screen (wireless mouse and keyboard work pretty darn well) and watch blu-rays ripped to my 2TB hard drives.

But back to the point: I agree, no need to go X79 for gaming, at least not for me. Just need to do a bit more research on which ASUS mobo to grab.

 

[borntofly]

I have just reviewed the full technical documentation on the PLX website for both the PLX 8606 and the PLX 8747 chips. I now feel fairly confident of the following regarding the ASUS Maximus V Formula:

1) That the board will run one graphics card with 16 PCIe 3.0 lanes and will run two cards with x8 / x8 PCIe 3.0 lanes, connecting directly to the CPU's 16 native PCIe 3.0 lanes.

2) The board does NOTcontain a PLX PEX 8747, which is used on many Z77 boards to enable three-way SLI or four-way SLI/Crossfire.

3) The board DOES implement a PLX PEX 8608 to utilize it's lane mutliplexing methodology to "double" the CPU's 8 native PCIe 2.0 lanes to ensure that most if not all of the board's PCIe 2.0 devices (e.g., SATA and USB 3.0) will remain operable during simultaneous operations.

I have also reviewed gaming benchmark testing (Tom's Hardware) between PCIe 3.0 vs. 2.0, which revealed only an average of a 5% increase with 3.0 across a variety of graphically intensive games.

Additionally, I have reviewed gaming benchmark testing (HardOCP) between 16 x 16 lane SLI vs 8 x 8 lane SLI, which revealed no real world dicernable differences between the two setups.

I will therefore be purchasing a Maximus V Formula, along with an Intel 3770k, 16 gigs of Corsair Dominator Platinum 2400, a second water cooled GTX680, a second Samsung 840 Pro 512 gig SSD, plus USB 3.0 peripherals (SATA docking station and flash drive). I will apply a nice, healthy 24/7 overclock and I look forward to using the built-in WiFi, Bluethooth, SupremeFX sound, as well as the other ROG exclusive software and hardware innovations.

 

[borntofly]

OK?

 

[blackened23]

Nice, good info. That board looks very nice, post your build here once you're done with it.