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reason for sporadic long full GC pause times?!?....

 
 
m.ahaus@usu.de
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      07-28-2006
Hello folks,

in our java business application our customer realize sporadic very
long pauses caused by full gc's.
Normaly the app runs smoothly with acceptable gc- and
full-gc-pausetimes. (gc=0.0xx sec / fullgc <=2 sec)

But sometimes during 8 hour work the occurs full gc's with 90-150
seconds (!)
which our customer complains about - of course.

In the first step I thought that our JVM-settings were wrong:
-Xms450m -Xmx450m -XX:NewSize=32m -XX:MaxNewSize=32m
-XX:SurvivorRatio=8 -classpath....
--> long full gc pauses occured sporadicaly.

I thought that the Newsize is too small -> wich implies the too big
tenured space -> which could be the reason for too longs full gc's. I
decided to use -NewRatio=4 Parameter.
And I also lowered the max. Heap size to Xmx350m, because the gcviewer
told me that the memory usage was only about 290 MB.

With the new settings
-Xms350m -Xmx350m -XX:NewRatio=4 -XX:SurvivorRatio=8 -classpath....
our customer had the same experience that sometimes the full gc's
pausetime took more than 100 sec's.
Additionally we/they relaized that the full gc freed not enough memory
so that the full gc frequency raised up to a point where no minor gc
were done - only full gc's !

Due to that I raised Xms and Xmx
-Xms512m -Xmx512m -XX:NewRatio=4 -XX:SurvivorRatio=8 -classpath....
which enables the JVM to free more memory at full gc (already tested),
but I fear this will not help to solve the sporadic problem with the
long full gc pause times at customer side.

Does anybody have an idea what could be the reason for the
extraordinary long full gc pause times?

Info btw:
customer PC's all have 1 GB RAM and the throughput of the application
was in any case >= 99,8% !
The problem with the long full gc pause times was never reproduced in
our lab / dev.-deptmnt.

Thanks in advance

Martin

 
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