JVM GC

-Xloggc:[filename] is used to log GC. verbose:gc output to stdout.
-XX:+PrintGCDetails is to display more GC details on stdout.

There are two primary measures of garbage collection performance. Throughput is the percentage of total time not spent in garbage collection, considered over long periods of time. Throughput includes time spent in allocation (but tuning for speed of allocation is generally not needed.) Pauses are the times when an application appears unresponsive because garbage collection is occurring.

Reference:
[1] Java theory and practice: A brief history of garbage collection
[2] Java theory and practice: Garbage collection in the HotSpot JVM
[3] Tuning Garbage Collection with the 5.0 Java[tm] Virtual Machine