Java garbage collection long pause with high 'sys' time

Issue

• A Java garbage collection is sometimes taking much longer than expected.
• Most young collections are taking hundreds of seconds, but sometimes they take 20 seconds with very high sys time. For example:

1234.830: [GC [PSYoungGen: 987488K->15360K(998400K)] 3452660K->2585790K(10470400K), 19.4339220 secs] [Times: user=0.55 sys=74.94, real=20.43 secs]