You did not mention which level of JVM you are using. I assuming Sun JVM 1.4.x, and if so here are some suggestions.
First, did you specify a young generation size? Typically, you will want the young generation to be about 1/3 the size of theh eap. Assuming your 100MB heap size, the command line would be "-Xmx100m -Xms100m -XX:NewSize=33M -XX:MaxNewSize=33M". Proper young generation sizing can lead to a significant performance boost.
Have you monitored garbage collection using -verbose:gc (or any of the other commandl ines)? See http://wiki.jboss.org/wiki/Wiki.jsp?page=SPECjAppServer2002TuningComponentsJVMMonitoring for more information. Such monitoring will usually let you know if you have properly sized the heap.
Finally, any possibility of adding more memory to your servers? If you upped their emmeory to 1GB, you could run a 600MB heap which could give you a significant performance boost.
We are using Java 1.5 for our JVM. I tried setting the new young size as you recommended, and things take 5-10seconds longer.
As of right now, there are no plans for upgrading the server memory to 1GB. With all the applications already running on the server, 100mb is all that can be dedicated to jboss. This is actually a bump from the 50mb we previously had. Perhaps Jboss 4.0 is simply designed to run with more resources than we can provide for good performance.
Based on the slowdown you mentioned, the new size of 33M is large, you are probably getting full garbage collections each time. Try playing with other sizes such as 15M, 20M or 25M. You often have to play with several sizes before finding the one that fits best for your situation.
Also, try setting -verbose:gc to get some garbage collection statstics. Those statistics will tell you how often you get a full collection versus a regular collection. I guess I should also ask if you are using the default garbage colleciton algorithm or if you are specifying an alternate one on the command line.
I agree with your last statement that 4.0.2 probably uses more in the way of memeory resources that 3.0.0 does. Running with -verbose:gc on both would indicate how much of an increase there was.
We have seen a problem where some images of our web pages have returned, and others are blank with NO red X. We have made a change to the server.xml in the tomcat directory. We had:
We went to:
Are the changes listed above the correct path to take to troubleshoot the problem of blank images?