Question Which DIMM slots should I populate?

[hashy]

I have an ASRock x99m Killer 3.1 and there are two pairs of memory slots on each side of the processor. I don't know if I should use slot 1+2, 3+4, 1+3 or 2+4 for dual channel.

 

[SkyNetRising]

That is quad channel motherboard. For optimal performance you have to use them all.
For dual channel mode you can use any 2 of them (doesn't matter which ones).