GC with long period (3.5sec)

Question

Hi
I am facing a very citical problem. We develop an JSP/Tomcat 4.1.x framework and use jdk 1.3.1
small introduction:
Additional we use RMI and a SAX-Parser to analyse the JSP-Sites. We use JDBC to connect Oracle 9 database server with a connection pool.
I use the jvm parameters -Xms and -Xmx of 128 MB
I try to explain my problem:
Sometimes one request to a JSP take 10 to 12 seconds or more. Sometimes it don't need so long. So i had the idea to implement a small performance class which trace me some critical sections in my code. But if i add up all the traced time, i get always 1.2 to 1.6 seconds.
What happened?
I use the -verbosegc option of the JVM and see that the gc works often with normal duration. But sometimes the duration is between 3.5 and 4.5 seconds. And this sometimes three times per request. (~12 sec)
I think this is to unusal to believe it due to the long duaration.
The heap size is throughout between 30 and 40 MB. (80 MB are free!)
What can be the reason for?
Are there objects which hold references to each other and the gc can release them? Other opinions?
I start with reusing Objects like StringBuffer and others. I use a profiler to see the most instantiated classes to make them if possible reusable. I this the right way?
I shall be highly oblidged if anybody can help.
best egards
Wolfgang

Answer 1

First thing to try (and this is just a guess, but empirically true) is to be sure to null-out everything that you are done with inside a method. I did a test once against 2 similar pieces of code (do 1 million times....) The gc was super-slow when it had to try to figure out which objects were dead.
This is more true with arrays. myArray = null. In collections, be sure to use the remove() method when you don't need something.
Simply put, clean up after yourself in the methods, and then the GC shouldn't spike-up like that.