Long young generation GC?

[Hi Guava]

The third young generation GC took 439.2720750 secs but the user and
real time are only 0.08 seconds. What does it mean?

72667.213: [GC 72667.213: [ParNew: 38336K->4224K(38336K), 1.2473840 secs]
3443948K->3420569K(6549376K), 1.2474290 secs] [Times: user=0.64 sys=1.10,
real=1.25 secs]
72680.531: [GC 72680.532: [ParNew: 38336K->4221K(38336K), 0.2916570 secs]
3008948K->2979033K(6549376K), 0.2916710 secs] [Times: user=0.26 sys=0.03,
real=0.29 secs]
72681.425: [GC 72681.426: [ParNew: 38325K->4224K(38336K), 439.2720750
secs] 3013053K->2979055K(6549376K), 439.2720750 secs] [Times: user=0.08
sys=0.00, real=0.08 secs]

Environment:
Java HotSpot(TM) 64-Bit Server VM Version 1.6.0_20-b02
Linux Version 2.6.18-128.1.1.el5 on amd64
-Xms6400m
-Xmx6400m
-Xss256k
-XX:+UseConcMarkSweepGC
-XX:+PrintGCDetails
-XX:+PrintGCTimeStamps
-XX:+UseCompressedOops