Set JVM Options in JBuilder

First, you can view the current settings by running jbuilder executable with -info option, which dumps all JVM options and system properties before starting IDE. For JVM settings, look for output launcher.config.params

You can backup and edit install-dir/bin/jbuilder.config to modify the IDE JVM settings, among other things. The entries of interest are:

# Tune this VM to provide enough headroom to work on large
# applications
vmmemmin 32m
vmmemmax 75%
vmparam -XX:MaxPermSize=128m
I found these property names and value formats a little awkward. As their names suggests, vmmemmin 32m maps to JVM option -Xms32m, and vmmemmax 75% maps to JVM option -Xmx750m. The latter is confusing and seems to be a result of 1000m (physical memory) * 75%. It turns out you may also use the absolute amount like vmmemmax 750m.

But it seems I can't use the percentage format for initial heap size. For example, vmmemmin 12% would result in the wrong JVM option -Xmx120m.

If you want to specify additional JVM options, you need to add more vmparam entries (they are additive). For example, if you want to specify space for PermGen, in addition to heap size:
vmparam -XX:MaxPermSize=128m
vmparam -XX:PermSize=64m
Including them into the same entry value would result in syntax error. The following line is wrong:
vmparam -XX:MaxPermSize=128m -XX:PermSize=64m
Unrecognized VM option 'MaxPermSize=128m -XX:PermSize=64m'
Unable to create JVM.