help, jvm performance tuning

[essa liu]

I am doing jvm performance tuning now on our server,and I have encountered some problems.
First the environment is
jre: IBM 1.3.1
OS:linux
Operation: add document from disk to our server(which will load the files to another dir that control by our server),multi-thread
The problem:
The gc information printed out indicate no heap shrink or heap expand,in GC cycle,action=1 or action=2 is taken.
I tried to set -Xminf to 0.2 and -Xmaxf to 0.5 in multi-thread scenario, sometimes the performance is improved but unfortunately sometimes the settings degrade the performance.
PS,I will not change our codes,and the work I do is to tun the performance outside(just jvm settings).
Anyone has did similar work or can give me some advice? I am only a beginner on performance tunning and I need your help and suggestion.
Thanks a lot.

[srungta srungta]

You might find it helpful to read the three papers at:
http://www-106.ibm.com/developerworks/ibm/library/i-garbage1/
cheers