will Z170 mobo, 6700K cpu allow for dual 980Ti SLI w/ PCIe 3.0 x4 M.2 SSD

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ozyiceman

ozyiceman

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okay, so i'm going to be migrating from haswell to skylake...yes, i know the performance boost is little to none; i'm building a computer from my haswell rig for someone else so i figured i'd just move to skylake...i'd go haswell-e, but the ambient temperature in this corner of the room is already 10+ degrees hotter than the thermostat is set to with my 4790K OC'd to 4.8GHz, and I would probably melt with a 140W CPU...ANYWAYS; I want to know if the new build I'm going with will have enough PCIe lanes to support dual SLI with a M.2 PCIe 3.0 x4 SSD (I'm pretty sure the answer is yes, but I want to be sure before I order everything)

Relevant parts to be used:

ASUS ROG MAXIMUS VIII HERO Mobo

Intel Core i7 6700K Processor

Samsung SM951 M.2 PCIe 3.0 x4 SSD

and 2x MSI GTX 980Ti Gaming 6G Graphics Cards in Dual SLI Configuration

 
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wildfire707
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wildfire707

wildfire707

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Yes, you can do that. Keep in mind that you will have no remaining PCIE lanes left. The maximum number of lanes that you have with the Socket 1151 processors is 20, and the minimum lanes per graphics card for SLI is 8. So with two graphics cards using a total of 16, the remaining 4 lanes will go to the M2 port that you will be using.

There are ways to expand the number of available PCIE lanes using a multiplexer (lane manager), but ASUS does not currently carry any motherboards using this technology for the Socket 1151 processors. They do have a few available on the older Z97 chipset for Socket 1150 processors.

Just a word of warning - don't plan on plugging in anything else to PCIE slots or SATA Express ports on that system.

On another note, I would tend to use the new Samsung 950 Pro instead of the SM951. It uses less power, so it is less likely to overheat. I have seen people have problems with the SM951 in ultrabooks because of thermal throttling.
 
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junkeymonkey

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well I would assume it depends on how the boards slots are wired to the chip and chipset ??

look for reviews that cover this

example

http://www.hardwaresecrets.com/asrock-fatal1ty-z170-gaming-k6-motherboard/2/

http://www.hardwaresecrets.com/gigabyte-z170x-gaming-7-motherboard/2/
 
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ozyiceman

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don't the M.2 ports generally utilize the chipset pcie lanes instead of the cpu's?

my understanding is that the skylake processor itself has 16 PCIe lanes, which are utilized by the PCIe slots...in the case of graphics cards, 1 gfx card by itself will use all 16 lanes, while a dual SLI config will split the 16 lanes between the 2 @ 8x apiece, while the 20 lanes you're referencing are the Z170 chipset's own PCIe lanes, which theoretically should even support multiple pcie 3.0 x4 drives in addition to the SLI setup...the total number of PCIe lanes on the setup i will be using should be 36 between the processor and the chipset, with this particular setup utilizing 16 cpu + 4 chipset pcie lanes for a total of 20, leaving 16 to be utilized in whatever way i see fit

also, assuming i have 36 pcie lanes to work with, will the sli only utilize the cpu's lanes at 2x x8, leaving me with 20, or will it split it 1x x16 from cpu and 1x x16 from PCH, taking 32 and leaving me with just the 4 for the SSD?
 
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wildfire707

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In older setups this was true. With newer boards, it can be either. I just checked out the motherboard manual for the ASUS ROG MAXIMUS VIII HERO that you plan to get, and the PCIE x1 and last PCIE x16 slots use lanes provided by the PCH. The first two PCIE x16 slots use lanes provided by the CPU. The others ports (M2 / SATA express) are not specified. So it looks like I am probably incorrect about the other peripherals and SATA express ports.

The only note provided by ASUS is "When the M.2 Socket 3 is operating in SATA mode, SATA ports 1,2 will be disabled."

You plan on running it in PCIE mode, so there is no problem.

The only other note is that it says if you use a PCIE x4 card in the last PCIE x16 slot, the PCIE x1 slots will be disabled and the last PCIE x16 slot will run in x4 mode. Their wording is "When a PCIEx4 card is inserted into the PCIEX4_3 slot, PCIEX1_2 and PCIEX1_3 will be disabled and the PCIEX4 card will run in X4 mode".
 
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ozyiceman

ozyiceman

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sorry, i edited my post and tacked this on before i saw that you replied:

also, assuming i have 36 pcie lanes to work with, will the sli only utilize the cpu's lanes at 2x x8, leaving me with 20, or will it split it 1x x16 from cpu and 1x x16 from PCH, taking 32 and leaving me with just the 4 for the SSD?
Click to expand...

so now that we've established that it will work, i just need to know if i'm going to be left with 20 lanes or 4...from what i've heard there's no significant dropoff from dual x16 to dual x8 as of yet, but most of what i read about it was written before DX12, so, with gfx cards becoming more prominent in the future for overall performance than the cpu, will dual x16 be more practical than it is at present? if not, i would probably opt to set the 2 x16 slots to x8 in lieu of anything i might want to stick in the pcie ports

EDIT: I re-read your post, and, from this tidbit:

PCIE x1 and last PCIE x16 slots use lanes provided by the PCH
Click to expand...

I would assume that putting the graphics cards in slots 1 and 2 i would get 2x x8 and if i put them in 1 and 3 or 2 and 3, i would get 2x x16, so if i had something i just absolutely had to stick in a pcie port, i could just switch from pcie slots 2/3 to 1/2 and get the extra 16 lanes back

as for this:

On another note, I would tend to use the new Samsung 950 Pro instead of the SM951
Click to expand...

damn, price is comparable to the OEM price for the 951 w/ better read/write speeds and lower power consumption...will be released on amazon tomorrow so looks like i'll be pre-ordering this right now :D
 
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wildfire707

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The first two PCIE x16 slots use bandwidth provided by the CPU. They can run as x16 and x0 or as x8 and x8 mode. The third PCIE x16 slot uses bandwidth provided by the PCH and can run as x1 or x4 mode, but if it is run in x4 mode the two PCIE x1 slots are disabled.

That means that for NVIDIA graphics cards you can only run them in the first two PCIE x16 slots, as NVIDIA requires 8 lanes of bandwidth for SLI operation. This should work fine.

For AMD graphics cards, you can use 3 card CrossFire since they require at least 4 lanes of bandwidth per card and that would be running them in x8 x8 x4 mode.

As far as the SSD goes: I haven't tried out the Samsung 950 Pro myself yet, but it should be an excellent unit.

Good luck!
 
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ozyiceman

ozyiceman

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what? why would an x16 slot only run in x1 or x4 with an x16 card in it? i fail to see why i can't put 1 card in slot 2; using the full 16 lanes from the cpu, and 1 card in slot 3, using 16 of the 20 available PCH lanes...from what you stated here:

The only other note is that it says if you use a PCIE x4 card in the last PCIE x16 slot, the PCIE x1 slots will be disabled and the last PCIE x16 slot will run in x4 mode. Their wording is "When a PCIEx4 card is inserted into the PCIEX4_3 slot, PCIEX1_2 and PCIEX1_3 will be disabled and the PCIEX4 card will run in X4 mode".
Click to expand...

...the only reason for the 3rd slot to be running in x4 would be if i were plugging an x4 device into it, which would not be the case...unless i'm missing something incredibly obvious, the only reason for the sli in slots 2 and 3 or 1 and 3 to not run correctly in dual x16 mode would be if SLI required both cards to be utilizing PCIe lanes from the same source, i.e. both from the cpu or both from the PCH, which i have never seen explicitly stated anywhere

just to reiterate and make sure there's no miscommunication here: there are 16 lanes from the CPU and an additional 20 lanes from the PCH...some of what you're saying would make sense if you thought there were only 20 lanes total (16 from cpu + 4 from PCH) but this is not the case...given the information you provided and the fact that i have 36 PCIe lanes to work with (not 20,) it should be entirely possible to have dual SLI in x16 slots 2 and 3, using 32 out of the possible 36 lanes (16 from CPU +16 from PCH,) leaving 4 for the M.2 x4 SSD as there should be absolutely no reason for a x16 slot to be running at x4 with 16 or more lanes available and a x16 card plugged into it
 
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wildfire707

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Sorry, my explanation was for graphics cards. With the Intel chipsets all graphics cards get all of their PCI Express lanes from the CPU. You are right in the fact that the last PCIE x16 slot can get more than four lanes from the PCH, but it looks like ASUS is making it simpler by limiting the last PCIE x16 slot to x4 speeds. This way it can be used by any card and not cause problems with the motherboard BIOS.

There is an overview of the Z170 chipset at:

http://www.tomshardware.com/reviews/skylake-intel-core-i7-6700k-core-i5-6600k,4252-2.html

It points out that in triple video card format, the PCIE lane configuration is x8 x4 x4 - all from CPU lanes. Also, keep in mind that the 20 PCIE lanes available to the PCI are split among all on board peripherals, like extra SATA controllers, firewire, wireless, and audio.

Since basically all motherboard manufacturers make motherboards for both Intel and AMD processors, they tend to not use the built in Intel networking and audio solutions. High end motherboards tend to have at least one additional SATA controller. All of this means that you are normally lucky if a PCH controlled PCIE x16 slot has 8 lanes available for the mainstream (Z170 or Z97) motherboards.

The CPU likely controls video cards directly to avoid synchronization problems between video cards - since they can be the most demanding devices plugged in.
 
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ozyiceman

ozyiceman

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ah, so all the extra bells and whistles like the usb 3.1 controller are using the PCH lanes? i was afraid of that...oh well, as previously stated, no significant dropoff in performance from dual x16 to dual x8 has been noted yet, at least for gaming...thanks for the clarification :)
 
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