Continuous Full GC even though Old Generation is not filled up.

Question

Every few days we see continuous Full GC (more than 1000 FullGC's) and then the application comes to normal...the verbose GC log is as below .. when it is doing Full GC, the old generation is not filled up.. but what is prompting this Full Gc??
Environment/Settings
JDK 1.6
Memory Settings -Xms256M -Xmx2048M -XX:MaxPermSize=512M
Attaching to process ID 25118, please wait... Debugger attached successfully. Server compiler detected. JVM version is 14.3-b01
using thread-local object allocation. Parallel GC with 8 thread(s)
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 2147483648 (2048.0MB)
NewSize = 2686976 (2.5625MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 21757952 (20.75MB)
MaxPermSize = 536870912 (512.0MB)
Partial Verbose GC logs, when the continuous Full GC starts.
=====================================
404006.016: [GC [PSYoungGen: 242756K->10659K(249408K)] 609285K->378416K(825536K), 0.0244300 secs] [Times: user=0.09 sys=0.00, real=0.03 secs]
404126.211: [GC [PSYoungGen: 239715K->11421K(250624K)] 607472K->379300K(826752K), 0.0314540 secs] [Times: user=0.08 sys=0.01, real=0.03 secs]
404498.907: [GC [PSYoungGen: 241181K->15113K(250816K)] 609060K->383750K(826944K), 0.0567160 secs] [Times: user=0.11 sys=0.00, real=0.06 secs]
404559.340: [GC [PSYoungGen: 81552K->9784K(250688K)] 450190K->378571K(826816K), 0.0255340 secs] [Times: user=0.08 sys=0.01, real=0.03 secs]
404559.366: [Full GC (System) [PSYoungGen: 9784K->0K(250688K)] [PSOldGen: 368787K->225993K(576128K)] 378571K->225993K(826816K) [PSPermGen: 181648K->181648K(187968K)], 1.4180830 secs] [Times: user=1.42 sys=0.00, real=1.42 secs]
404560.791: [GC [PSYoungGen: 1155K->64K(251072K)] 227148K->226057K(827200K), 0.0141730 secs] [Times: user=0.03 sys=0.02, real=0.01 secs]
404560.806: [Full GC (System) [PSYoungGen: 64K->0K(251072K)] [PSOldGen: 225993K->202050K(576128K)] 226057K->202050K(827200K) [PSPermGen: 181648K->178996K(185216K)], 1.5066450 secs] [Times: user=1.50 sys=0.00, real=1.51 secs]
404562.329: [GC [PSYoungGen: 1351K->272K(252032K)] 203401K->202322K(828160K), 0.0129750 secs] [Times: user=0.03 sys=0.01, real=0.01 secs]
404562.342: [Full GC (System) [PSYoungGen: 272K->0K(252032K)] [PSOldGen: 202050K->202125K(576128K)] 202322K->202125K(828160K) [PSPermGen: 178996K->178996K(184512K)], 1.3184310 secs] [Times: user=1.32 sys=0.00, real=1.32 secs]
404563.674: [GC [PSYoungGen: 1146K->128K(253312K)] 203271K->202253K(829440K), 0.0106520 secs] [Times: user=0.02 sys=0.01, real=0.01 secs]
404563.685: [Full GC (System) [PSYoungGen: 128K->0K(253312K)] [PSOldGen: 202125K->202126K(576128K)] 202253K->202126K(829440K) [PSPermGen: 178997K->178997K(184000K)], 1.3197180 secs] [Times: user=1.32 sys=0.00, real=1.32 secs]
404565.025: [GC [PSYoungGen: 1146K->64K(254976K)] 203272K->202190K(831104K), 0.0152030 secs] [Times: user=0.03 sys=0.01, real=0.01 secs]
404565.041: [Full GC (System) [PSYoungGen: 64K->0K(254976K)] [PSOldGen: 202126K->202126K(576128K)] 202190K->202126K(831104K) [PSPermGen: 178997K->178997K(183296K)], 1.3072740 secs] [Times: user=1.31 sys=0.00, real=1.31 secs]
404566.355: [GC [PSYoungGen: 1151K->160K(255360K)] 203278K->202286K(831488K), 0.0107640 secs] [Times: user=0.02 sys=0.02, real=0.01 secs]

Answer 1

I would try Java 6 update 23 which has JVM build 19.0-b09
A number of things can trigger more full GC than you need, such as a lack of survivor space.
However, given it is come in bursts it could be something like running out of direct memory. e.g. when openning some new files or reconnecting via NIO to a server. When you run low on direct memory, the JVM triggers a Full GC as this is the only way to force these objects to be collected.
RMI can also call full GCs, but I largely turn this off, so I don't have much experience with its performance quirks.