CMS does not want to run concurrently

[mbien at fh-landshut.de]

Hello,

i tried to reduce full stops of an application running on JBoss by  
using the CMS on an 8 core system which is mostly idle. I thought it  
would be easy since the processing power is almost unused - it seems  
not ;)

I start JBoss (on 1.5u8, windows 2003) with
-server -Xms256m -Xmx800m -XX:+UseConcMarkSweepGC -XX:+UseParNewGC  
-XX:ParallelGCThreads=7

The used heap is between 300MB and 600MB after a few hours most of the  
time. The strange thing is that the CMS keeps triggering full  
collections (it even starts with full collections).

The other thing I don't understand is that the stops take around 5-8  
seconds which is from my experience far to high for those small heap  
sizes (correct me if i am wrong here).

any hints what could be wrong with this configuration?

regards,

michael


application start:
5.942: [GC 5.942: [ParNew: 16256K->0K(16320K), 0.0308416 secs]  
16256K->1415K(262080K), 0.0311875 secs]
8.352: [GC 8.353: [ParNew: 16320K->0K(16384K), 0.0407896 secs]  
17735K->3189K(262144K), 0.0410866 secs]
10.756: [GC 10.757: [ParNew: 16320K->0K(16384K), 0.0255010 secs]  
19509K->4290K(262144K), 0.0258390 secs]
12.108: [GC 12.108: [ParNew: 16320K->0K(16384K), 0.0229667 secs]  
20610K->5799K(262144K), 0.0232975 secs]
<cut>
29.500: [Full GC 29.500: [CMS: 24054K->23891K(245760K), 0.8014114  
secs] 26981K->23891K(262144K), [CMS Perm : 23221K->23174K(23296K)],  
0.8021225 secs]

after a few hours:
21372.393: [GC 21372.393: [ParNew: 15756K->0K(28608K), 0.0072765 secs]  
383481K->367869K(687748K), 0.0074989 secs]
21373.023: [GC 21373.023: [ParNew: 27810K->0K(28608K), 0.0325687 secs]  
395680K->381143K(687748K), 0.0327673 secs]
21373.461: [GC 21373.461: [ParNew: 28544K->0K(28608K), 0.0189914 secs]  
409687K->384469K(687748K), 0.0191790 secs]
21373.917: [GC 21373.917: [ParNew: 28544K->0K(28608K), 0.0059543 secs]  
413013K->384529K(687748K), 0.0061447 secs]
21375.508: [GC 21375.508: [ParNew: 26452K->0K(28608K), 0.0087766 secs]  
410981K->385283K(687748K), 0.0089809 secs]
21375.556: [Full GC 21375.556: [CMS: 385283K->323173K(659140K),  
4.7187553 secs] 392550K->323173K(687748K), [CMS Perm :  
46865K->46863K(65536K)], 4.7189925 secs]
21380.330: [Full GC 21380.330: [CMS: 323173K->322000K(659140K),  
4.6004591 secs] 325384K->322000K(687748K), [CMS Perm :  
46865K->46863K(65536K)], 4.6007203 secs]

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