I migrate from Hudson #1376 to Jenkins #1498.
I upgrade the plugins and clean the config files as proposed by the message views on "Manage Jenkins".

I monitor our JVM servers 24/7 and I see clearly the impact on the migration on the PermGen area. Same settings than Hudson, same plugins, same jobs and a huge difference in PermGen. (see the attached graph)

I upgrade Jenkins to #1500, the plugins, checked the configurations (and reboot Tomcat). No changes.

It's a critical problem since Hudson asked nearly nothing in maintenance.