The PermGen is a concept that has been deprecated in Java 8.
The new equivalent is the meta space. In theory, there is no need to limit that space, but I found that there is a hard limit on the opennms init script that clamps the meta space to 256MB:
I think that number is too low, as I found some meta space problems on a customer site.
If we're planning to keep that limitation, we should increase the number to at least 512MB.