CrossfireX and PCI lanes

[cmsbfent]

Hey folks,

As many are, I'm trying to decide whether a skylake upgrade is worth my pennies, so just trying to clear something up.

Basically, my current system is a 2600k running 3x r290s. All in all, it's a fairly capable system, albeit with an old CPU probably starting to show its age.

The motherboard is an asus maximus extreme IV, and this is where I get a little lost..

I 'think' it has 16 PCI lanes, which I believe don't really do me any favours running 3 cards. For example, on GTAV, running two cards actually gives better performance than running three. On catalyst, I have the following options for crossfire:

3 GPUs (6 0 12)
2 GPUs (6 12)
2 GPUs (6 0)

When using 3 cards, I get fps dips into the mid 40s, and when using 2 they only go into the mid 50s. It's a much smoother experience. Settings wise, generally all very high, at a resolution of 1440p. Not as this makes much of a difference as similar drops occur with a variety of lower settings making me think there is more to it, and a serious bottleneck is occurring somewhere.

So yeah, as well as the improvements in CPU speed of skylake, will the added lanes be an added bonus?

I'm almost certain an upgrade is coming in the darker months, but just trying to justify it. With DDR4 and another SSD, it's gonna cost a few pennies : p

Thanks

 

[rtho782]

Assuming it's a Z series board, that can split the CPU PCIe lanes, you have....

16 lanes from the CPU split into two x8 groups, going to card 1 and 2. You're on Sandy Bridge so it's PCIe 2.0, and equivalent to PCIe 3.0 x4 per GPU.

Then, your CPU connects to the PCH (the motherboard chipset) via DMI2.0 This is effectively PCIe 2.0 x4 (or 3.0 x2).

The PCH hosts your SATA ports, your LAN, your audio, your USB, etc, and also has a couple of PCIe ports of it's own. Best case is PCIe 2.0 x4 for your GPU here, equivalent to 3.0 x2 but going via the PCH, fighting for bandwidth with SATA, USB etc as detailed.

As it's R9 290x, it's XDMA crossfire and relies on consistent low latency for frame synchronisation.

Personally, I wouldn't say even Skylake is enough. You gain PCIe 3.0 for GPUs 1 and 2, the DMI link is doubled in speed, but you are depending on finding a board that turns the 20 CPU lanes into x8 x8 x4 slots, rather than x8/x8/ x4-m.2.

You need either a motherboard with a PLX switch, so for your current CPU something like the Z68X-UD7, others exist for the z77, z87, and z97 era, or you need Haswell-E.

 

[pastymuncher]

His board is P67 and uses a NF200 chipset to deliver 3 way crossfire/SLI at 8x/16x/16x (slots 1, 2 and 4). Interestingly, the recommendation for two cards is slots 1 and 3 which use the intel controller at 8x/8x. There must be a lot of overhead when that NF200 chip comes into play.

 

[cmsbfent]

Thanks for the replies.

I suppose it won't be long till reviews start appearing, then I can make a decision between Haswell-E and Skylake.

My current setup certainly feels crippled when all cards are in play though.

Cheers

 

[pastymuncher]

It's probably due to the NF200 chipset hence why they recommend using slots 1 and 3 for dual cards so that they only run off the Intel controller. If it makes 16x/16x slower than 8x/8x it makes you wonder why they even used the damn thing in the first place.

 

[cmsbfent]

Thanks

So after further reading, the 'perfect' match would be a chip with 40 lanes?

As my catalyst show a 6 0 12 configuration when three cards are being used, does that mean that the middle card is running a lot slower than it could be, or do the numbers not really equate to much (as is the case with a lot of these types of things)

Cheers

 

[pastymuncher]

Going by the manufacturers suggestion of using the Intel controller and running a pair of cards at 8x/8x compared to the 16x/16x available on the NF200 controller I would say that the cards in slots 2 and 4 are not performing as they should due to the overheads from the NF200 chip.