PCIe Lanes Clarification

[enome]

New to LTT forums.. I wasn't clear on which topic this would fall under.

 

I have been building PCs mindlessly for years, but now this has my attention

I wanted some clarification on the PCIe lanes on a typical i7 with Max of 16 PCIe lanes

im still having some trouble understanding the concept even though I had hours of googling and watching videos

 

So taking 6700k as an example, the PCIe lanes are 16/0/0, or 8/8/0, or 8/4/4.

I plan to have a single GTX 1080 card (ASUS STRIX) and an old sound card (X-Fi Titanium Fatal1ty - PCIe 2.0 x1)

I will be using these on a mATX board (Maximus VIII Gene) and this board has two 16x and one 4x slots.

 

If I were to follow the intel PCIe lanes, by adding the sound card I would be running 8x on my graphics card regardless.

So would it make sense for me to do 8/8 and put the soundcard on the x16 lane?

From the diggings I have done, the soundcard won't be affected at all because it can't fully utilize the entire bandwidth 8x can offer. 

However, I read somewhere that because the sound card is PCIe 2.0 x1 so it can still allow the graphics card to run 16/0/0.

 

So what really does this come down to?

Running the graphics card in 8x, does it even impact the performance?

[thekeemo]

the gpu will run at x16 and 8x has a minimal impact on performance

[megaant4]

3 minutes ago, enome said:

-snip-

 

Really it doesn't matter that much. You are correct though, only other 16x PCIe cards have to share the 16x bandwidth. If you have a 16x card it won't be affected by the 1x sound card. 

 

Also, currently graphics cards aren't even able to utilise the whole 16x slot, they barely can max out 8x.. The performance difference isn't noticeable.

[enome]

5 minutes ago, thekeemo said:

the gpu will run at x16 and 8x has a minimal impact on performance

 

3 minutes ago, megaant4 said:

 

Really it doesn't matter that much. You are correct though, only other 16x PCIe cards have to share the 16x bandwidth. If you have a 16x card it won't be affected by the 1x sound card. 

 

Also, currently graphics cards aren't even able to utilise the whole 16x slot, they barely can max out 8x.. The performance difference isn't noticeable.

thanks guys for the clarification and quick replies

[zMeul]

since it's a Z170 chipset it may switch that 1st x16 PCIe to a x8

you absolutely need to test it, check BIOS too

I'm not sure the board has a PCIe lane aggregator hub; if so, you might want to put that sound card in the x4 slot

 

[xentropa]

There are two "sets" of pcie lanes.

 

One set of PCIe lanes connect directly to the CPU.  There are 16 of them, and the pcie slots that use those lanes might be a different color.  Your graphics cards will use these lanes.

 

There are an additional 24 PCIe lanes that are connected to the Z170 PCH and then to the CPU via DMI or QPI (forgot which one, too lazy to wiki).  Those lanes share bandwidth with, sata, usb, network, etc.

 

Most likely your sound card will use those 24 PCIe lanes connected to the PCH, not the 16 lanes connected to the CPU.

[Sakkura]

1 minute ago, xentropa said:

There are two "sets" of pcie lanes.

 

One set of PCIe lanes connect directly to the CPU.  There are 16 of them, and the pcie slots that use those lanes might be a different color.  Your graphics cards will use these lanes.

 

There are an additional 24 PCIe lanes that are connected to the Z170 PCH and then to the CPU via DMI or QPI (forgot which one, too lazy to wiki).  Those lanes share bandwidth with, sata, usb, network, etc.

 

Most likely your sound card will use those 24 PCIe lanes connected to the PCH, not the 16 lanes connected to the CPU.

This is the right answer.

 

Though technically there are only 20 lanes on the Z170 chipset. But whatever, it's plenty for a sound card.

[zMeul]

6 minutes ago, xentropa said:

Most likely your sound card will use those 24 PCIe lanes connected to the PCH, not the 16 lanes connected to the CPU.

depends on the board implementation

since it's a Z170 and not a H170, those to x16 slots share lanes directly from the CPU - if the mobo does not have PCIe lanes aggregator hub, the moment the board detects that sound card, it will switch to 2*8x opperation

if it was a H170, that 2nd x16 slot would've absolutely worked of the PCH

[zMeul]

@enome + @xentropa

here's the manual: http://dlcdnet.asus.com/pub/ASUS/mb/LGA1151/MAXIMUS_VIII_GENE/E10484_MAXIMUS_VIII_GENE_UM_WEB.pdf

take a look at page 1-19:

[enome]

@zMeul

thanks for the indepth clarification and finding the PCIe info on the MOBO manual

so if I understood correctly, if both x16/x8 slots are occupied the MOBO will switch to x8 mode for both slots

while the x4 will be running under the Z170 PCH

 

So would it be better for the sound card to be fitted on the x4 slot?
I understand that there isn't much of performance impact of using x8 on graphics card

but from what I read so far, ideally its better off using x16 on the graphics card and sound card on the Z170 PCH?

 

@xentropa& @Sakkura

thanks for the new information, learning a lot from this

[zMeul]

Just now, enome said:

thanks for the indepth clarification and finding the PCIe info on the MOBO manual

so if I understood correctly, if both x16/x8 slots are occupied the MOBO will switch to x8 mode for both slots

while the x4 will be running under the Z170 PCH

 

So would it be better for the sound card to be fitted on the x4 slot?
I understand that there isn't much of performance impact of using x8 on graphics card

but from what I read so far, ideally its better off using x16 on the graphics card and sound card on the Z170 PCH?

yes, it would be better to use the sound card of the x4 slot, that's connected to the Z170 chipset

 

alternatively, if you need all those slots you should look for a H170 alternative: example ASUS H170M-PLUS