Every WebLogic admin at anytime he/she encounter this JVM issues. Few of them are resolved by tuning the JVM memory arguments values for the server instances. Some times you need to debug what is happening for your JVM. Some situation requires you to modify the GC algorithms. To know more about these in detail go through this page and attachments.
In a multi-server WebLogic domain, monitoring the JVM statistics showing frequent full garbage collections (GC) on one of the server instances. The best suggested workaround is Increasing the young object space in the heap memory would greatly reduce the frequency of Full GCs.
Garbage collection logs using -verbosegc provide help in gathering garbage collection details from Java Heap memory.
You can have a look on the great shell script to display current, used spaces for given WebLogic server instance in the JVM Eden, Old, Perm sizes. This script is designed by retrieved with jstat command line utility with little trick with awk.