This message occurs when there is not enough memory to index the source root. This is usually caused by too low -Xmx setting (by default it is set to 20% of the System Memory). If this is not the case (the -Xmx is set to relatively high value) you may have encountered a bug.
In that case please report bug at Apache JIRA, choose 'ide - Performance' as component and if possible please attach the messages.log file and upload zipped heap dump. 1