I have an Intel Atom D2700 (Synology NAS DS412+) with 4GB RAM running kernel 3.2.30 x86_64. This unit has a single DIMM slot.
One thing I, and other's have found, is that when adding a 4GB DIMM versus a 2GB DIMM, the unit experiences significantly higher CPU usage when under load (for example, 'heavy' Java applications like Minecraft servers, or Plex transcoding, etc). Many users have found that when they drop back to 2GB all of these high load issues disappear.
Is this something specific to Linux that may cause this? Or is this an issue with the Atom itself?