For a System.gc() the call to ConcurrentMark::abort() can be excessively long.
Cut-and-pasted from a log (some parts left off). I added timers to get these
numbers.

285.569: #0: [Full GC (System.gc()), 6.3754329 secs]
285.569: #0: [Abort concurrent mark (System.gc()), 5.4316610 secs]
285.569: #0: [_nextMarkBitMap->clearAll() (System.gc()) , 2.6187331 secs]
288.188: #0: [clear_all_count_data() (System.gc()) , 2.8120570 secs]