After installing Adobe Experience Manager 6.0 with SP2, there is increased memory usage and frequent GC events without any users working on the instance.
That behavior is expected, as the default topology mechanism does a heartbeat check every 30 seconds.
That check results in updating the content under /var/discovery/..., which is indexed by the default Lucene index (/oak:index/lucene). That async process is usually a bit heavy in memory, and can use 300-400 MB to load/update the index.
To avoid this effect, add an excludedPaths property to the Lucene index definition. This parameter is described in the Apache Jackrabbit Oak documentation here.
It requires that you use the recent Oak core bundle, which is available on package share. Check the list of available hot fixes for Adobe Experience Manager 6.0 here.