I recently ran into a problem with an Apex application running on ORDS on Tomcat. The application has a page with a custom tabular form (built using the apex_item package). When this page is submitted, the form values are stored in the "g_fxx" arrays (g_f01, g_f02, etc).
The problem was that when the number rows (and therefore the number of elements in the arrays) got too big, the server would respond with a HTTP 500 - Internal Server Error message.
The first thing to do in this case is to check the Tomcat logs at /usr/share/tomcat7/latest/logs to see what the real error message is. Tomcat generates one log file per day, and the log files have the naming format catalina.yyyy-mm-dd.log.
Opening the relevant log file, I found several occurrences of this error message (truncated for brevity):
SEVERE: Java heap space
java.lang.OutOfMemoryError: Java heap space
So, Tomcat/ORDS was basically running out of memory when processing a page submission with dozens or hundreds of values (even though in my case the values were all contained in a few g_fxx arrays).
I googled a bit and found a recent post from the Apex forums that deals with the same issue. In my case I'm using ORDS 2.0.10, but it appears that the same issue can be experienced with ORDS 3.0.1.
Given that in this particular case my server only has 1 GB of memory, perhaps it's not that strange that the Java process would run out of memory (after all, the server is also running Oracle XE, Tomcat and Apache...).
To increase the memory available to Tomcat, you need to create a file called setenv.sh in the Tomcat bin directory, and make it executable.
# paste the below text and save
chmod +x /usr/share/tomcat7/latest/bin/setenv.sh
service tomcat restart
Here is the content I put in the setenv.sh file (based on this):
export CATALINA_OPTS="$CATALINA_OPTS -Xms128m"
export CATALINA_OPTS="$CATALINA_OPTS -Xmx512m"
export CATALINA_OPTS="$CATALINA_OPTS -server"
This sets the initial memory to 128MB and the maximum memory to 512MB (more info). The -server flag is to optimize execution when Java runs as a server (more info).
Remember to restart Tomcat for the new settings to take effect.